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Thread: Engine Compression

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    Engine Compression

    For the past few years my compression has read in the low to mid 60's (over 80 psi when measured cold). Two months ago at my last annual it was again in the low 60's....all four cylinders. I had the plane into another shop for unrelated repairs and they wanted to do a top end overhaul cause they said they measured the compression and it was at 37-45-50-61......? How could three cylinders have changed so much in two months? Comments? I am going to get it checked a second time by another shop. [/b]

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    SJ's Avatar
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    Supercruiserman,

    I would definitely get it checked again. It is amazing the variation that can happen on compression with different techniques.

    There is another post out here about compressions, and some other ways of testing it, you might try the "search" function and see what you come up with. Somebody else will probably chime in here too.

    sj

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    irishfield's Avatar
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    Re: Engine Compression

    Quote Originally Posted by supercruiserman
    For the past few years my compression has read in the low to mid 60's (over 80 psi when measured cold). Two months ago at my last annual it was again in the low 60's....all four cylinders. I had the plane into another shop for unrelated repairs and they wanted to do a top end overhaul cause they said they measured the compression and it was at 37-45-50-61......? How could three cylinders have changed so much in two months? Comments? I am going to get it checked a second time by another shop. [/b]
    Were they afraid to rock the prop a nudge each way to get the valves to actually seat? Other question would be did the unrelated repairs give them any reason to be doing a compression test in the first place and are they taking you to the cleaners? Mind you low 60's is already getting questionable I get concerned if any of mine show below 75 and start looking for problems.

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    Steve Pierce's Avatar
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    Under 70 in a Lycoming is cause for concern in my book. Compression gauges get dropped and other stuff. I send mine out for calibration once a year. They replace both indicators. Where is the air coming from. Hissing out the exhaust- exhaust valve, air filter- intake valve or crankcase (dip stick or breather)- rings. I would ask some questions for sure.
    Steve Pierce

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    What engine are we talking about. If it's the O 235 Those valves are supposed to be adjusted every 100 hours because they are solid lifters. The valve will close right up if not adjusted and then they are held open because no more clearance and even worse compression results if its hot. Check the valve clearance after its warmed up and compare it to the overhaul manual. Be present at the next compression check. Do it according to lycoming's procedure and see what you have. If still all low then find out why. Kind a weird three are that low. Once you find where its leaking to -perform some of the customary home remedies like oil in the cylinders,snaping the valves in the closed position with a block of wood and hammer. If the first shop new what they were doing sounds like it's time for some new or rebuilt cylinders. And if that's the case she's going to fly with a lot of new power. Tell us what you find.

  6. #6
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    Quote Originally Posted by FORTYSIX12
    What engine are we talking about. If it's the O 235 Those valves are supposed to be adjusted every 100 hours because they are solid lifters. The valve will close right up if not adjusted and then they are held open because no more clearance and even worse compression results if its hot. Check the valve clearance after its warmed up and compare it to the overhaul manual. Be present at the next compression check. Do it according to lycoming's procedure and see what you have. If still all low then find out why. Kind a weird three are that low. Once you find where its leaking to -perform some of the customary home remedies like oil in the cylinders,snaping the valves in the closed position with a block of wood and hammer. If the first shop new what they were doing sounds like it's time for some new or rebuilt cylinders. And if that's the case she's going to fly with a lot of new power. Tell us what you find.
    Thanks for the reply. The engine is the 125 HP lycoming. Someone at the airport today said I really shouldnt be running the 100LL and they recommended I try some non-oxygenated fuel with some Marvel Mystery oil in the oil and in the gas to see if it cant clean out the carbon that may be holding the valves open....? could be worth a try.
    Supercruiserman

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    cubdrvr's Avatar
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    Is there any guidance that specifies technique for doing a compression test? I was told that you maneuver the prop back and forth until you get the highest reading.
    Also agree with Pierce on the above post with Lyc's. Usually if it's a serious compression issue your bird will let you know.
    "Sometimes a Cigar is just a Cigar"

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    stewartb's Avatar
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    Don’t both major manufacturers have sevice instructions for differential compression testing?

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    frequent_flyer's Avatar
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    Quote Originally Posted by cubdrvr View Post
    Is there any guidance that specifies technique for doing a compression test?
    Lycoming has issued instructions that include a specification of the required tester orifice size. See Service Instruction No. 1191A.

    "To assure that the piston rings are seated, the propeller is moved slightly back and forth with a rocking motionwhile air pressure is applied; thus providing a more accurate reading. Meanwhile, a second person adjusts theair supply pressure to 80 psi, indicated on the supply pressure gage of the differential compression device. Then,observation of the engine cylinder pressure gage will give an indication of the condition of the parts in the combustion chamber of the cylinder."

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    Kodiakmack's Avatar
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    Is cylinder replacement really the first move if you have low compression? If I’m paying someone to do work, I’d hope they would offer some other simple remedies to attempt to bring the compressions back up before doing open heart surgery.
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    Bill Rusk's Avatar
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    Food for thought on Compression Test.......


    On another note I am well into my annual condition inspection. This year I had a leaking exhaust valve. Staking it helped and a borescope inspection indicated crud on the valve seat. We were able to lap it in situ (in place) which cleaned up the seat and restored the compression. I then went out and got my own borescope so I could inspect all valves and cylinders. I got the Vividia 980 model. In the process of looking at the internet I found this by Mike Busch via AOPA and thought it had some really interesting stuff that I was very surprised by.........

    One pervasive old wives’ tale has it that compression readings in the high 70s are excellent, in the low 70s are good, in the high 60s are marginal, in the low 60s are poor, and anything below 60/80 is unairworthy. Another widely accepted old wives’ tale is that an engine with compressions in the low 60s is a “tired engine” that will not put out full rated horsepower. Both are dead wrong.
    More than three decades ago, Continental Motors issued a service bulletin (M84-15) debunking the first of these superstitions by establishing a new go/no-go criterion for compression tests: the master orifice tool. Mechanics who followed this guidance were astonished to find that compression readings in the low- to mid-40s were deemed acceptable by Continental.
    This 1984 guidance was based on a series of engineering studies performed using an IO-550 engine mounted in the dynamometer test cell at the Continental factory in Mobile, Alabama. Those studies revealed that when the compression ring gaps on the IO-550’s pistons were filed oversize intentionally to reduce the compression of all six cylinders to 40/80, there was no measurable loss of horsepower output (although there was an increase in oil consumption). This effectively debunked the “tired engine” old wives’ tale.
    Enter the borescope
    Nineteen years later, Continental threw mechanics another curveball by issuing Service Bulletin SB03-3 (which superseded M84-15), directing that a borescope inspection of each cylinder be performed at each annual and 100-hour inspection, and any other time that a compression test is done. It further made it clear that the borescope, not the compression tester, was to be the gold standard for assessing the airworthiness of a cylinder. It directed that if a cylinder flunks a compression test but the borescope reveals no obvious cause for the low compression, then the engine is to be flown for at least 45 minutes and the compression test be redone. Only if a cylinder flunks its compression test twice in a row (with at least 45 minutes of flying in between) is it deemed unairworthy.
    Continental’s SB03-3 was pretty shocking to mechanics when it was first published in March 2003. In those days, few GA maintenance shops owned a borescope (unless they did a lot of turbine work), and there was no training available to mechanics on how to use one to inspect a piston aircraft engine cylinder. Most A&P schools still don’t teach anything about how to use borescopes in piston engine maintenance.
    The service bulletin recommended using a low-cost rigid optical borescope—the Lenox Autoscope, which was so named because it was designed for automotive use, and at more than $2,000 was one-tenth the cost of the fiber-optic borescopes being used for turbine engine hot-section inspections. Still, lots of mechanics and small GA maintenance shops were not amused by being told that they had to shell out two large to buy one of these instruments. Fourteen years later, some A&Ps still don’t own a borescope.I was an early adopter of borescopy. Having gone through the painful experience of pulling cylinders because of low compression readings, only to find nothing physically wrong with them, I was anxious to adopt this more enlightened way of evaluating cylinder condition. I borrowed a Lenox Autoscope from a shop on my field and began inspecting the 12 cylinders on my Cessna 310. It was an eye-opening experience, almost as if I could climb inside each combustion chamber—or at least stuff one eyeball inside. Over the years, the compression test has proved untrustworthy and prone to false positives, resulting in tens of thousands of cylinders being removed unnecessarily (including a few of mine). That’s why the SB03-3 guidance calls for any disqualifying compression test that is not corroborated by borescope evidence be retested after flying for at least 45 minutes. That’s excellent advice. I’ve seen many cases where a cylinder that flunked the first compression test easily passed the second one. In one notable case involving a Cirrus SR22, a cylinder that tested at 38/80 (and that the shop doing the annual wanted to yank) wound up measuring 72/80 on the retest after a one-hour flight.
    SB03-3 did not go so far as to recommend that borescope inspections should replace the venerable compression test. Continental couldn’t do that, because the requirement to perform a compression test is written into the FARs (Part 43, Appendix D). But SB03-3 did all it could to convey that Continental is no big fan of the compression test for determining cylinder airworthiness. (A senior Continental executive once confessed to me that if they could’ve dropped the compression test altogether, they would have.)


    Hope this helps

    Bill
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    stewartb's Avatar
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    Back when Continental couldn’t build a cylinder that could maintain compression they changed the compression standards, which most everyone figured was to slow down their warranty claims. Whether that was true or not they didn’t waste any time revising their honing procedure. Go figure. https://www.aircraftspruce.com/catal...cebulletin.pdf

    Lycoming instructions are pretty clear about sagging compressions. https://www.lycoming.com/sites/defau...ompression.pdf
    Last edited by stewartb; 08-11-2022 at 08:11 AM.

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    Kodiakmack's Avatar
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    Quote Originally Posted by stewartb View Post
    Back when Continental couldn’t build a cylinder that could maintain compression they changed the compression standards, which most everyone figured was to slow down their warranty claims. Whether that was true or not they didn’t waste any time revising their honing procedure. Go figure. https://www.aircraftspruce.com/catal...cebulletin.pdf

    Lycoming instructions are pretty clear about sagging compressions. https://www.lycoming.com/sites/defau...ompression.pdf
    I still think the lycoming SB here is overkill in that it doesn’t allow for the fact that “sagging compressions” can be un-sagged through various means.
    Last edited by Kodiakmack; 08-11-2022 at 02:18 PM.
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    cubdrvr's Avatar
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    I should have stated my question more clearly. Is there a MANDATORY requirement for testing and acceptable compression limits? Are SI's mandatory? Does the FAA deem them mandatory?
    I would have replaced many cylinders without cause if that is the case.
    "Sometimes a Cigar is just a Cigar"

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    Quote Originally Posted by Kodiakmack View Post
    I still think the lycoming SB here is overkill and that it doesn’t allow for the fact that “sagging compressions” can be in-sagged through various means.
    Not sure how clear that is. Two words come into play, Should and Shall. If it sags below 60 replacement "should" be considered. If you can pull some tricks and get the numbers back up its not forbidden to do so.

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    Quote Originally Posted by cubdrvr View Post
    I should have stated my question more clearly. Is there a MANDATORY requirement for testing and acceptable compression limits? Are SI's mandatory? Does the FAA deem them mandatory?
    I would have replaced many cylinders without cause if that is the case.
    Not a lawyer and didn't even stay at that motel, but my understanding is that a comression test is mandatory at every annual inspection. There is no mandatory pass/fail condition. Simply record the results in the engine log. However, the inspector is required to decide if the aircraft is airworthy and inspector's judgement may be guided by an SI.

    For part 91 operation SI are only mandatory if they are called out by an AD. I don't operate under 135 or 121 and don't know if the rules are different.

    Not all IA read or follow the same rule book. My IA still does not accept that he should use the small orifice on my PA-28 O-360. We now use my compression tester not his. It saves a discussion.

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    (d) Each person performing an annual or 100-hour inspection shall inspect (where applicable) components of the engine and nacelle group as follows:
    (1) ------


    (2) ------


    (3) Internal engine - for cylinder compression and for metal particles or foreign matter on screens and sump drain plugs. If there is weak cylinder compression, for improper internal condition and improper internal tolerances.

    This is from Part 43, Appendix C.

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    stewartb's Avatar
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    The most coherent answer I've seen- https://aviation.stackexchange.com/q...with-less-than

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    This 1984 guidance was based on a series of engineering studies performed using an IO-550 engine mounted in the dynamometer test cell at the Continental factory in Mobile, Alabama. Those studies revealed that when the compression ring gaps on the IO-550’s pistons were filed oversize intentionally to reduce the compression of all six cylinders to 40/80, there was no measurable loss of horsepower output (although there was an increase in oil consumption). This effectively debunked the “tired engine” old wives’ tale.
    Good old Mike Busch. It isn't always worn rings when the compressions are low which in turn does actually effect power. Having flown the same airplane and engine before and after I can say that what Mike says about making power with low compression is not always the case.
    Nineteen years later, Continental threw mechanics another curveball by issuing Service Bulletin SB03-3 (which superseded M84-15), directing that a borescope inspection of each cylinder be performed at each annual and 100-hour inspection, and any other time that a compression test is done. It further made it clear that the borescope, not the compression tester, was to be the gold standard for assessing the airworthiness of a cylinder. It directed that if a cylinder flunks a compression test but the borescope reveals no obvious cause for the low compression, then the engine is to be flown for at least 45 minutes and the compression test be redone. Only if a cylinder flunks its compression test twice in a row (with at least 45 minutes of flying in between) is it deemed unairworthy.
    Some of us were using borescopes long before Continental recommended it. Some of us also didn't automatically pull a cylinder because it was low on compression. The compression test, borescope inspections, oil screen and filter inspections and oil analysis, reading spark plugs are all tools we use to determine the health of an engine. Mike Busch likes to lump all mechanics together and claim the sky is falling. Sure there are some bad ones but there are some good ones as well. He told me I was thinned skin. I told him it isn't good for his business plan to say anything good about mechanics. Continental or Lycoming can say whatever they want but the law for us operating part 91 are the FARs. Is it wise to go by just the FARs, probably not but that is the minimum. The rest is recommendations by the manufacturers. The real world that I work in lies somewhere in between.
    Steve Pierce

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    frequent_flyer's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    Good old Mike Busch. It isn't always worn rings when the compressions are low which in turn does actually effect power. Having flown the same airplane and engine before and after I can say that what Mike says about making power with low compression is not always the case.
    I thought Mike Busch was reporting the results of a specific Continental test. It's a while since I read the complete article but I don't remember him saying that no cause of low compression will result in a power reduction. He has written quite extensively about checking valves and seats and described lapping valves before they develop serious problems.

    I find his writing a refreshing change from the doomsayers who believe an engine becomes un-airworthy the minute it reaches TBO.

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    Steve Pierce's Avatar
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    Bill posted a direct quote from Busch's article. He has some good information but I take offense to his categorization of mechanics and have told him so. He writes for AOPA and EAA but it is really an Infomercial for Savy Aviator. I asked him why he never wrote an article about an aircraft owner breaking down in bufu Egypt and there being a mechanic there with the knowledge, tools and parts to get the person going. He said I was thin skinned. I said it doesn't promote your business. He decides, we have enough decision. Owners and mechanics have to trust each other and work together.
    Steve Pierce

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    cubdrvr's Avatar
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    A long time mechanic I've worked with has told owners to find another mechanic when informed that Savy was involved with the inspection.
    Said it was a big waste of time........too many back and forths regarding decision making.
    "Sometimes a Cigar is just a Cigar"
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    Quote Originally Posted by ;281546
    For the past few years my compression has read in the low to mid 60's (over 80 psi when measured cold). Two months ago at my last annual it was again in the low 60's....all four cylinders. I had the plane into another shop for unrelated repairs and they wanted to do a top end overhaul cause they said they measured the compression and it was at 37-45-50-61......? How could three cylinders have changed so much in two months? Comments? I am going to get it checked a second time by another shop. [/b]
    OP,
    have you had it looked at by another shop? I think the question of whether an immediate topping is necessary.
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    cubdrvr's Avatar
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    Quote Originally Posted by stewartb View Post
    The most coherent answer I've seen- https://aviation.stackexchange.com/q...with-less-than
    "Manufacturers Instructions take precedent over all"........so are the SI's mandatory?
    AC 43 says "cylinder MUST be removed and inspected" with under 60/80.
    His paragraph then states "if compression is below 60/80 IN GENERAL then yes it needs to be removed" and goes on to say that AC's are guidelines and not regulatory.

    Confusing with no definitive answers................
    "Sometimes a Cigar is just a Cigar"

  25. #25
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    Per 43.13-1B

    This advisory circular (AC) contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when there are no manufacturer repair or maintenance instructions. This data generally pertains to minor repairs. The repairs identified in this AC may only be used as a basis for FAA approval for major repairs. The repair data may also be used as approved data, and the AC chapter, page, and paragraph listed in block 8 of FAA form 337 when:

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    Bill Rusk's Avatar
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    For what it is worth I was just throwing the Busch article out there as food for thought.

    I am one of those folks that always read, and heard, that the compression test was the "be all and end all". If the engine didn't pass then the whole world was going to end tomorrow. The engine would surely fail on the next flight, it was totally unsafe, you MUST overhaul the whole engine immediately etc etc.

    It never really made sense to me so I thought it was interesting to hear that the whole world was not going to end tomorrow just because one cylinder was below 60.

    I like Steve Pierce comment......"The compression test, borescope inspections, oil screen and filter inspections and oil analysis, reading spark plugs are all tools we use to determine the health of an engine".

    Look at the total package and make an informed decision.

    Bill
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    Quote Originally Posted by Steve Pierce View Post
    Bill posted a direct quote from Busch's article. He has some good information but I take offense to his categorization of mechanics and have told him so. He writes for AOPA and EAA but it is really an Infomercial for Savy Aviator. I asked him why he never wrote an article about an aircraft owner breaking down in bufu Egypt and there being a mechanic there with the knowledge, tools and parts to get the person going. He said I was thin skinned. I said it doesn't promote your business. He decides, we have enough decision. Owners and mechanics have to trust each other and work together.
    Weird when all mechanics share the same opinion of him.


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  28. #28
    RaisedByWolves's Avatar
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    The compression is giving you a quick idea of the cyl. And then you can figure out what is leaking and why.


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    I've not seen any mention of where the compression is leaking on these "failing" cylinders. When I run a compression test, I am more concerned about what I feel and what I hear than the actual numbers on the gauge. Exhaust valves can often times be either staked or lapped to restore compression as Bill found with his engine. Rings are often times fouled with lead/carbon deposits. I find that adding some MMO to the oil and flying it for 5 hours often times cures leaky rings. I'm not real big on tearing down an engine unless other options are not realistic. Where the compression is leaking is pretty critical information to understanding the problem when diagnosing an engine.

    In answer to the original question, even though they should be the same, often times there are significant differences between compression gauges using the same size oriface. There are also differences between mechanics as to how hard they are willing to work at it to seat the rings during a compression test. I've got two gauges, both with a .040" oriface. One I call my happy owner gauge, and the other I refer to my needs an overhaul gauge.

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    As an owner I've wondered why an 80 PSI reading was as good as something that's 10X that, like real running BMEP. High pressure can help seal rings until they stick. Leaky valves and cracks not. I'd rather take a common compression gauge and crank the engine a few times with the starter to build pressure but that's not how it's done.

    Gary

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    Steve Pierce's Avatar
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    Quote Originally Posted by cubdrvr View Post
    A long time mechanic I've worked with has told owners to find another mechanic when informed that Savy was involved with the inspection.
    Said it was a big waste of time........too many back and forths regarding decision making.
    A mechanic buddy of mine got wrapped up in that with a new customer. Finally told the guy he wasn't dealing with Savy, he could find another mechanic. They wanted to grant him permission to do this or that. He talked to Busch himself. He asked Busch if he was gonna sign the logbooks. Busch told him the only logbooks he has ever signed are his own.
    Steve Pierce

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    frequent_flyer's Avatar
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    Quote Originally Posted by ;281546
    For the past few years my compression has read in the low to mid 60's (over 80 psi when measured cold).
    Post deleted.
    Last edited by frequent_flyer; 08-11-2022 at 10:06 PM.

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    I thought he meant 60 “over 80”, like 60/80
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    That was 18 years ago. I wonder what is is now?
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    Quote Originally Posted by RaisedByWolves View Post
    Weird when all mechanics share the same opinion of him.


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    Thanks to all who’ve contributed to this thread- this is one of my favorites in a while. I’m about sick and tired of some guy 3500 miles away behind a computer telling my customers why my opinion and analysis, which is based on years of real-world experience on 100’s of different planes, is wrong. I take issue with the accusation that all mechanics are knuckle-dragging, unthinking con-artists who all have an instantaneous reaction to this one tiny number. No consideration that maybe I’ve already looked inside, at plugs, engine/component times, type of flying/hours per year, and taken all that into account when I’ve made a recommendation...? Sorry for the rant, but I’m tired of the insinuation that we’re all greedy predators trying to sell top overhauls. And don’t even get me started on how it’s impossible to safely replace a cylinder in the field...
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    Pet leave of mine, are mechanics actually doing a compression check or are you truly doing a leak down test. BIG DIFFERENCE IN THE 2. Using a 2 gauge setup and looking at your leak down at ? / 80 psi is not a compression test, it is a leak down test / check. Or a cylinder differential pressure test. Look at SB03-3 from tcm
    Last edited by eskflyer; 08-12-2022 at 03:36 AM.

  37. #37
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    Quote Originally Posted by eskflyer View Post
    Pet leave of mine, are mechanics actually doing a compression check or are you truly doing a leak down test. BIG DIFFERENCE IN THE 2. Using a 2 gauge setup and looking at your leak down at ? / 80 psi is not a compression test, it is a leak down test / check. Or a cylinder differential pressure test. Look at SB03-3 from tcm

    I found TCM SB03-3 here - https://www.aircraftspruce.com/catal...cebulletin.pdf (only first 9 pages)
    but I could not find it in the TCM listing here - http://continental.aero/support/service-bulletins.aspx or with a search of the entire TCM site.

    Was SB03-3 withdrawn or am I looking in the wrong place?

    This FAA legal interpretation may be of interest - https://tinyurl.com/57n9n8d9 in the general context of this thread.
    Last edited by frequent_flyer; 08-12-2022 at 07:18 AM. Reason: corrected link for SB03-3
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  38. #38
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    Do you really think whoever wrote this in at the FAA understands what a compression test is? Or what a differential cylinder test is? Obviously not. I truly hope the mechanics that peruse this forum do. Even the way lycoming words it in places I have read gets the wording wrong. Please read up and understand the differences between the 2 if you have never thought about it and or been taught.

  39. #39
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    Quote Originally Posted by eskflyer View Post
    Do you really think whoever wrote this in at the FAA understands what a compression test is? Or what a differential cylinder test is? Obviously not. I truly hope the mechanics that peruse this forum do. Even the way lycoming words it in places I have read gets the wording wrong. Please read up and understand the differences between the 2 if you have never thought about it and or been taught.
    No part of my reply disputed your assertion that a leak down test was not a compression test. The FAA legal interpretation doesn't comment on that either. It was linked in reference to the question of whether SI and SB were mandatory.
    Last edited by frequent_flyer; 08-12-2022 at 08:13 AM.
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  40. #40
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    Differential compression test is kind of an industry standard no matter if you call it that or a compression test.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers
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