Silk thread

[jonesb930]

I was watching a video short from Nichole Johnson of a 911 engine build. There is a short part showing them placing a silk thread between the case halves in the sealer liquid. Is this something new? It’s been a while since I put one of these together.



https://www.facebook.com/share/r/1CdK4mJx7T/?mibextid=wwXIfr


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[Magyar Kiwi]

Hi
It is an old idea, was/is used on some horizontally opposed aircraft engines.
I would not recommend this practice on 911 engines.
Cheers

[stownsen914]

Google AI gave the following:
In certain aircraft piston engines, specifically those from Lycoming and Continental, silk thread is used between the crankcase halves to aid in sealing. It's typically used in conjunction with a sealant, like Permatex No. 4, and is laid into the sealant to prevent it from being squeezed out entirely when the parts are bolted together. This technique helps ensure a leak-proof seal for the life of the engine.

Haven't heard of doing this on 911 engines. Maybe someone here will know differently.

[Turbo_pro]

I watched the video short and it's obvious that Nicole doesn't build these engines for a living.
I'm no expert but neither is she. Follow her guidance at your own risk.
nuff-said.

[chucksmith]

Olsen Motorsports also does this on their rebuilds. I have no idea if it does anything or not, but like most things with these engines, there are many successful ways to build a good leak free motor.

[85RedCarrera]

I have used the silk thread to seal the case halves on a Lycoming engine, and it works amazingly well.

[Speedy Squirrel]

Lycoming specifies allowable sealants. They are not allowed to experiment in the aviation industry.

We can be more freewheeling. At this point I don’t think anybody would us an RTV sealant on a Porsche case. Loctite 518 probably would not respond well to a thread in it. It is a pretty thin coating.

https://www.lycoming.com/sites/default/files/attachments/Alternator%2520Crankcase%2520Parting%2520Surface%2 520Sealants.pdf

[Turbo_pro]

There is a complete thread on proper sealing techniques that has survived the test of time. Ignore the experts at your own demise.

https://forums.pelicanparts.com/911-engine-rebuilding-forum/284737-complete-engine-sealant-thread-38.html

[stownsen914]

Good point about the choice of sealant. Looks like silk thread thickness is typically .3 - 1 mm. I wonder if the thickness would impact sealing if anaerobic sealants like Loctite 574, etc. are used.

Also, would the thickness interfere with bearing clearances? I guess the through bolts would have to squeeze a little harder to overcome the extra thickness at the case perimeter. Especially at the #1 and #8 bearings.

[gorskined]

That was the norm many years ago a little added extra insurance until the newer better sealants came along . i always did this on my 356 cases, but i put the thread when possible on the inside of the studs unlike the picture in the video . i just did it on my 930 case, never had a case leak yet .

[dannobee]

Stick with modern sealants and techniques. Literally billions and billions of engines have been assembled with modern sealants as outlined in the complete engine sealant thread.

When these cars were new they would leak oil on the showroom floor. I know, I was there. Once the manufacturers started using the threebond type of sealants, the engines kept the oil inside, without so much as even a hint of oil seepage or "sweating."

But hey, it's your engine, do as you wish. There is a LOT of labor in assembling one of these engines, so why not listen to the people who have figured out how to stop (and keep) them from leaking.

There were so many things wrong or misleading in that video that I don't even know where to start.

[john walker's workshop]

Like gobs of 574. EEK! The factory used a little roller to lay just a thin coat.

[safe]

Originally Posted by john walker's workshop

Like gobs of 574. EEK! The factory used a little roller to lay just a thin coat.

If a little is good more is better!!

[Henry Schmidt]

The goal in assembling the 911 case halves is sealing the joint not gluing the case halves together. The hardware creates the mechanical joint.
The challenge when using Loctite 574 to seal the engine case is the speed at which it hardens and the fact that it hardens at all. It is anaerobic which means it starts hardening in the absence of oxygen.
If the sealant hardens to produce it's sealing efficiency, it must be compressed quickly and with consistency. In the case of the video, there is too much sealant applied to the wrong half of the case.
574 was the go to for years but things have changed. Adhesive chemistry has changed.
What we discovered is that 574 can offer a shear strength so we use it on the main webs in performance engines when shuffle pins are not an option. This technique requires an expertise not exhibited in the video. Speed is of the essence.
Instead of gluing the case halves together (574) we have opted for a liquid gasket technique. The sealant (Dirko HT Grey) is applied to the loose case halve (the half not on the stand). The liquid gasket is applied in a very thin, smooth and consistent layer.
Within about 4 minute the product will start to skin over. This is of no concern. When the case halves are torqued together the sealant squeezes out leaving a very thin layer between and around the compressed components. The curing time is then about 4 hours. The two components are now attached together with a thin bead of sealant on both sides of the seem.
This newer technique eliminates the need for speed in assembly and is flexible enough to resist cracks and breakaway during heat cycles. This is especially important with magnesium cases that grow and shrink excessively during normal run cycles.
574 hardens to create a seal and if that product is moved after hardening it can crack.

As for the "silk tread". Fluff with no real benefit. Sometimes old school is based in misconception as to why something was done.
Case in point: boat tailing 911 cases....zero benefit at the cost of main web stability.

[jonesb930]

Thank you everyone for commenting, this has been an interesting thread I started. Like I said, I stumbled on the video and questioned some of the things most have also commented on. Its been a while since I assembled my engine but I recently acquired another motor that will be the basis of my next project (still to be determined as I am looking for a rolling chassis).