PDA

View Full Version : cam wear



Big AK
02-04-2004, 06:43 PM
Check the oil screen!

Just joking.

Pull a cylinder and take a look up there at the lobes. You'll get an idea from looking at the lobes that are visible, but you can't know the whole truth until you've seen every lobe on the cam.

Why do you ask?

T.J.
02-04-2004, 10:21 PM
delete

StewartB
02-04-2004, 10:27 PM
If they do, why does Lycoming make different lengths of pushrods?
SB

nanook
02-04-2004, 11:34 PM
Hydraulic lifters have variable travel. The spring tension and how much oil pump-up the lifter plunger produces is a little different in each lifter. You would be measuring the lifter plunger travel instead of the cam lobe, even if the lifter is maxed (pumped up) with oil there is still some plunger spring travel.
The different length push rods allow you to adjust for that lifter travel variance and also to adjust for rocker arm, valve face and seat wear. Most importantly is fixes the proper clearance in the valve travel equation. Too much clearance and the valve is not opening far enough and or there is more that the hydraulic lifter travel can compensate for and the parts start tapping. On the other end, too little clearance and the valve may be open all the time and burn.
> You could remove the plunger from the lifter and measure lobe travel from the inside base of the tappet body but it's probably just as easy to remove a cylinder and look.

cubpilot2
02-05-2004, 11:12 AM
How about removing the lifters, bleeding them down,reinstall everything and then take readings from the rocker arms while forcing the lifter to the bottom. This would only take a little time and some push rod housing seals. It should give you an indication of wear but if corrosion is suspect you would only find out by removing one or more cylinders. I have seen cams with only one bad lobe, so to be sure you would want to pull both cylinders on the same side to see the entire cam. I don't like to pull cylinders unless you have too. You could easily disturb the ring "seal" and create more problems then you started with.

Big AK
02-05-2004, 02:33 PM
I figured it was an old engine or one with suspect cam condition. This IS :D a Lycoming, right? :bunny

Anyway, measuring the lift at the rocker arms would give you a place to start.

But, If you're truly worried or concerned about the condition of the cam, you need to see those lobes. Pulling cylinders is where I'd be going with this.

I too, have seen a cam with only one lobe worn round.

cubdrvr
02-05-2004, 02:54 PM
Big AK's thoughts are like mine. Take a looksee. I flew my present cub home from MT and it ran beautifully........when I tore down the engine there were two lobes that were rounded so bad I don't know how it functioned.

StewartB
02-05-2004, 02:54 PM
And I just threw one away that probably would have measured to specs, but had rust pits all over it. It wouldn't have measured to specs for very long.
SB

Big AK
02-05-2004, 03:18 PM
That's the deal, how long will they go after corrosion is present?

FlipFlop
02-05-2004, 03:29 PM
That's the deal, how long will they go after corrosion is present?

Hard to say, considering if it's corroded, you toss it... I've seen some that I knew had operated for some time after they had sat for awhile and the cam corrosion didn't bother me as much as the metal scraped off from the followers...

Big AK
02-05-2004, 05:21 PM
Cuby, I was being facetious.

I won't let the thing out of the shop without a rebuild once corrosion is apparent on the lobes.

But, it's funny that I've always found bad lobes when changing a cylinder or solving a base leak, etc. etc.

One engine builder that I know believes that Lycoming's valve guide clearance tolerances are much too tight. I agree.

He believes this is the cause of stuck valves. I agree.

...And that the stuck valves and bent pushrods that result may also have something to do with the odd "one lobe worn" scenario.