Using silk thread with 574 to seal case
 

Hi all,
I’m rebuilding a stock ‘79 3.0l sc motor. I found the thread “The complete engine sealing thread” too late and have already sealed my case with loctite 574 and a silk thread around the perimeter as “they” apparently do on airplane engines from the same era.
After reading “the complete engine sealing thread” thread, I have a question for the sealing thread experts…
ISSUE :I am rebuilt to the point of getting ready to install pistons and cylinders.

QUESTION: Should I keep rebuilding, or should I tear back down, clean, reclean, clean again, and restart the rebuild sealing the case with the 4 adhesive method?

QUALIFIERS: This is my first engine rebuild process other than Briggs 2 piston lawn mower engines. I have decent skills, but definitely not a master mechanic. I hate cleaning, but will do it if absolutely necessary. I’m leaning towards continuing the rebuild and take my chances.

Always error on the side on caution but I see no reason to split the case unless you are truly uncertain as to your method. That sounds wishy washy but you are ultimately responsible for your own work. 574 was the go to for years and adding a tiny silk thread shouldn't change that. The issues are, how clean, how much 574 and how thick was the thread?

Thank you Henry for your advice.

The thread was purchased from an aircraft engine rebuilding company. It is 5-6 strand and fairly thin like heavy fabric thread. With the bolts torqued to 28.5 ft/lbs the crank turns freely but with good resistance. So I don’t think the thread impacted bearing tolerances. I’m more concerned about leaking. Will the thread help/hurt when sealing with the 475?

These little things will eat you up overthinking.

Think about this, there is probably 10’s of 1000’s of pounds of clamping force from the perimeter case nuts plus the inner bolts.

Are you sure with 100% certainty that the 574 and silk thread were installed correctly and didn't slip? Would it be easier to redo the case sealing now, with the engine on the stand and the cylinders not yet installed, or would it be easier after a few hundred miles of driving and you notice an oil leak at the case halves while parked in your driveway or garage?

And Lycoming approved the use of GE RTV-102 for case halves almost two decades ago, so even the aircraft guys are forging ahead into the 20th century.

574 needs to be torqued as soon as the case halves meet. If you waste time and assemble the through bolts and sealing rings, which takes a bit of time, and torque them first and then torque the perimiter nuts, there's a good chance the 574 has begin to set already and won't squeeze out properly. Then you get oil leaking around the #8 nose bearing and have to tear it down. Personally, I hate 574 for this reason and bailed on it eons ago. I would split the case and use gray Dirko instead. Easier now than later as noted.

What John Walker says is correct. We are building 80 transmissions a day at my job and have used 574 to seal all casings on them for 20+ years. Its a great sealant but if you put the halfs together you have about 15 minutes max and it starts to cure and if not torqued quickly it will leak. The good thing is it either leaks or it does not, its not going to start leaking later. In my opinion its one of the best sealants out there but you need to be quick with it.

BlueTarga78- Honestly, I think you are going the wrong way with the case sealing.... if you read the Lycoming data https://www.lycoming.com/content/safety-tip-sealant-use which says:

"The latest revision of Lycoming Service Instruction No. 1125 specifies POB No. 4 Perfect Seal and silk thread as the generally used items for sealing crankcase finished parting surfaces that do not employ gaskets. Two other products, RTV-102 or LOCTITE-515 applied as a very thin film without silk thread, may be used as alternate materials for sealing crankcase parting surfaces."
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The silk thread is used ONLY with a flexible sealant (POB #4). The alternate method of sealing the case is to use Loctite 515 anerobic sealant (a HARD sealant).

You would NOT use Loctite 574 with silk thread- Only POB #4 Perfect Seal.

I've done it both ways on Lycoming cases, and although the silk thread method seems pretty hokey, it seems to work just as well as the alternate method. As far as leak-free longevity with the silk thread method, all I know is that when I open a case with silk thread, it is typically VERY old and is leaking.

My other concern with the silk thread and flexible sealant is that I have observed more fretting along the flanges of cases that were sealed with the silk thread and POB # 4 (which I call "snot"- great stuff). What I have observed may have been coincidental, and is not a scientific observation...

Dave

I'd pull it apart. Having purchased a "freshly rebuilt" early SC that quickly sprung a nose bearing leak, the only real fix was to split it and reseal per the engine sealing thread. 80k miles later and it still doesn't leak a drop...

Dang, I hate cleaning. thanks everyone for the guidance. I truly appreciate it. I wish I had found “the complete sealant thread” thread before I used thread and 574. Like I said, it’s my first rebuild and for sure I don’t know what I don’t know. Really appreciate the help! Thx.

I had to split a case I'd just sealed for a different reason. I hated the idea, but it wasn't that bad. Much easier now than later.

Dragging this post out a bit longer, you see videos of "professional" engine builders gooping 574 around the perimeter and then on the main webs!! They better be fast!!

"Gooping" case sealant is not one of those "more is better" things.
574 on the main webs can be beneficial (increased shear resistance) but as John eluded to, you better be experienced at assembling the bottom end.