No silk thread method?

Had o-235 that was assembled with star washers under cylinder hold down nuts.

bob turner said:

I would love to know how the silk thread prevents fretting.

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It doesn't. It forms a sort of gasket between the case halves. Mechanical force and a precise fit prevent fretting.

Web

I used hylomar & silk thread recently. New tube looked like cottage cheese but was ok after I mixed it up and threw the first little bit away. My take is that the hylomar never dries and does not resist torque. It serves to hold the silk thread while you assemble the case halves. The silk thread is to prevent oil leaks, nothing else. Fretting seems relative to power. Lycoming progressively needed to dowel cases and bearings as power went up.... My read from the Lycoming manual and SB's on "no thread" was that it worked ok with approved sealants as long as final assembly and torque was done right away. With thread and proper non hardening sealant, you can assemble cases and then install cylinders later.

I found much seemingly good info on motorcycles and boat motors that would seem to offer good advice but decided that following factory dirrections was best. While my engines are experimental, my life isn't.

I've gone back and checked torque and have NEVER been able to move a nut at max torque. I believe that if I did it would mean I didn't get it right the first time. Of coarse I only have low compression O290 and O320 engines. With higher PSI units YMMV for sure.

BTW, I have seen several lycoming O235 and O290 G/D engines with star lock washers under the cylinder base nuts. One is my own O290G that was assembled by an IA and has several 100 hours. I couldn't move any nuts at spec. torque. I suspect that if properly lubed the star washers will not prevent "adequate" torque from being applied and might not hurt, at least not on those low compression engines. But why in the world use them?

OTOH I had a first run O235C1 with 1500TT that was moderately fretted when opened at an aviation school. I had the logs and it was never opened. Makes you wonder?

Bottom line for me anyway is that there is an overload of infomation out there including this forum (which I feel is excellent). I decided to do what Lycoming said to do. My choice was a approved non hardening sealant(hylomar) and silk thread or one of the two approved "no thread" sealants. I chose the former since I wasn't planing on applying final torque for several days.

I've also seem to many cylinders that had obviously too much paint on the cylinder base flanges. I do not want to believe a certified shop painted them. I also took apart a very old high time converted O290G with paint under the cylinder flanges that seem none the worse. In retrospect, I've owned several aircraft over the years mostly E-AB but a couple certified. My conclusion is that the only way to KNOW that your engine was assembled correctly is to have done it yourself...and then hope YOU didn't have a senior moment...or two...or more...

Oh well,

Jack

Loctite 515 is anaerobic sealant

CharlieN said:

Might be fine with an EX engine but nothing you want to use in a Certified build.
One thing Lycoming noted about non approved sealants is people select a sealant like you mention, but that sealant was never approved to be used between critical machined surfaces. Most of those sealants are designed for timing case covers and oil sumps essentially engineered as a gasket replacement.
In those applications, even after torquing the fasteners the sealant has a measurable thickness. It then is prone to a slow creep out of the joint.
Two issues here, the slight thickness imposed by the sealant reduces bearing shell crush right from assembly.
Second the chance of slow creeping my result in loss of torque on through fasteners. Nether is a good thing.

There are sealants that could be considered safe such as those used on case halves of motorcycle engines as well as outboard motors. But I would truly not go there on a Certified engine and make sure you truly understand the sealant's properties.

The recommended and approved Locktite 515 is derived from that family of sealants.

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Loctite 515 is anaerobic

CharlieN said:

the slight thickness imposed by the sealant reduces bearing shell crush right from assembly.

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Catching up on some old posts….

Slight thickness... HA!

Many moons ago under the shade of a tree, a humble A-65 was overhauled, hung on a Cub, fired-up, found to have low oil pressure, removed from said Cub and subsequently sold. A good friend of mine bought the pig-in-a-poke engine for a song. He assumed that the low oil pressure was probably due to a worn accessory case having been re-installed but upon disassembly, the oil pump area and gears were perfect. Hmmm…

Further tear-down found the culprit: a large strand of rib-stitch cord had been substituted for silk. After the offending cord was removed and the case halves properly joined, the A-65’s oil pressure ran silky smooth.

HT