Silk thread case seal - aircraft engines

[Dennis Kalma]

Hello all.

Just saw a "How It's Made" episode that went through the build of a Lycoming air cooled aircraft engine. It was very similar to a Porsche 911 engine, quite interesting, but one thing stood out.

When it came to putting the case halves together, they ran a silk thread around the edge of the case as a seal. The process was first application of a sealant (not sure which one) and then putting the thread on the case half and then the conventional process of putting case halves together etc.

There have been some discussions on this forum about the silk thread idea, seems to me that if it is good enough for an aircraft engine it ought to be pretty good for our little ground based engines....and it seems that it would offer a bit of a gasket effect without increasing the bearing clearances.

Anyone ever do this and what was the result? I know there have been a number of good threads about case sealing, just wondering about this one.

Dennis

[cgarr]

Aircraft engines are not machined as nice as Porsche engines. Thus the silk thread.

[Dennis Kalma]

Originally Posted by cgarr

Aircraft engines are not machined as nice as Porsche engines. Thus the silk thread.

Interesting point, I would have assumed that that was equivalent.....so would the silk thread be a bad idea or just unnecessary?

Dennis

[cgarr]

Unnecessary,

Here is the sealing surface on an old C-65 case:

[Shoepop]

I learned the silk thread method from my father 30+ years ago and still use it on 911 builds. Not that big of a deal and I don't have the 'Porsche weep', at least on case halves.

[kenikh]

Sounds like a darned good idea...

[lucittm]

Silk was the strongest thread available at the time. But who would want to put a kevlar thread in their engine, it just isn't as sexy.

Mark

[304065]

Silk thread is probably part of the type certificate for the airplane, and therefore is probably REQUIRED to be installed, because that was part of the design when that engine or its predecessor was certified in the 1930's!

Modern sealants today are a hell of a lot better-- the Harley V-Rod seals the case with robotically applied grey silicone, and our friends at Porsche provided the engineering support.

Case leaks are more often than not the result of other problems, e.g. buggered mating surface, incorrect or improperly applied sealant, improperly torqued or stretched studs or improper assembly technique. They don't leak if you do them right.

[cgarr]

Here it is right out of the Continental Overhaul Manual:

c. On the parting flange contact surfaces of the castings, and on the small surface immediately below the rear camshaft bearing (around the 3/8 inch stud) spread a thin, uniform film of Aviation Permatex. do not allow the liquid to run into the interior of the case. do not apply Permatex on the other camshaft bearing bosses or on any main bearing bosses. use it only on the surrounding flange and around the 3/8 inch stud.

d. When the Permatex has become tacky, place a strand of No. 50, Grade "A" silk thread along the upper, lower and front flanges of the No's 2, 4 side casting. Place the thread outside the upper and lower flange screw holes and inside the lower front flange hole. Place a loose loop of thread around the 3/8 inch stud on the surface below the rear camshaft bearing.

[304065]

Next: how to pour your own babbit metal bearings and replace your leather prop flange seal.

[cgarr]

Believe it or not I did rebuild and had to pour babbit bearings in our old windmill shaft on the farm once.

[304065]

[kenikh]

This kind of reminds me of the RSR rocker shaft seal debate: "Not necessary", but if it adds a little more leeway, why not?

[304065]

Too great of bearing clearances, making the engine seem worn out. Who knows, maybe lycoming factored the squish of the string when they designed the bearing, hence the high degree of specification of the string. "No. 50 Grade A" etc.

[kenikh]

Given the rough looking tolerances of the mating surfaces on the Lycoming, I doubt it.