Top End Rebuild Question

[Wood Doctor]

I ran into this the other day while rebuilding the top end of a rather vintage Husky 51 that cosmetically was in good shape. It had virtually no comprsesion, the ring was fused to the piston, and streaks were everywhere on the exhaust side. So, I ordered a new top end P/C kit and was installing it.

I removed the four mounting bolts, lifted out the jug, and noticed that not only was there no cylinder gasket, there was no sealer cement separating the jug from the case. In short, the previous mechanic who had installed it simply bolted the jug to the case, metal-to-metal. I've rebuilt at least 40 top ends and this was the first time I had ever seen this on a burned out engine.

So, I have to ask:
(1) Is this a common practice?
(2) Is it a recommended practice?
(3) What are the most likely consequences of doing what the previous mechanic did?

I have always used either a gasket or a gasket cement sealer (Permatex, etc.). To me, metal-to-metal is asking for an air leak and lean burning. Anyway, Forum, please discuss and advise. T.I.A.

 

[Backtroller]

I think the previous “mechanic” should not be considered a mechanic.

Was he inept? Lazy? First build?

 

[Dub11]

Beer, I would blame beer.

 

[huskyboy]

Those saws have huge squish stock. No base gasket and 3 bond 1184/yamabond/hondabond/motoseal/dirko or anerobic 518 will be fine. Id install a new intake boot as well, it’s important it seals well because it’s a press fit. Rubber gets old... it’s cheap insurance.

 

[Wood Doctor]

For the good news, I rebuilt the top end using Permatex gasket sealer. It had good compression (150 psi cold). I also installed a new intake boot as HuskyBoy recommended. Still, the engine also needed a new carb. After that was replaced, it started, and it runs fine today.

Assuming that saw actually ran without the gasket or any sealer, would it likely run lean and would that have contributed to the burn out? Seems so, but I'm surprised it would have even had sufficient compression to start and run.

 

[RIDE-RED 350r]

You are correct, deleting the base gasket is fine so long as there is adequate squish clearance. But failure to use a good sealant like Threebond or something similar is practically guaranteeing an air leak and most likely at least a contributing factor in burning that top end down.

 

[Hedgerow]

For the good news, I rebuilt the top end using Permatex gasket sealer. It had good compression (150 psi cold). I also installed a new intake boot as HuskyBoy recommended. Still, the engine also needed a new carb. After that was replaced, it started, and it runs fine today.

Assuming that saw actually ran without the gasket or any sealer, would it likely run lean and would that have contributed to the burn out? Seems so, but I'm surprised it would have even had sufficient compression to start and run.

I got a $20 bill that says the carb was to lean and they ran it anyway.
Metal to metal is a bad policy, but whoever ran it did not know how how to tune it to compensate for an air leak.

I've seen saws come in still running and had massive air leaks from bad pro side seals and clapped out case halves.

 

[Al Smith]

It might come as a surprise but many of the real old saw engines with a removable cylinder head had no gaskets . Fact many race saws have no head gaskets .

 

[Wood Doctor]

I got a $20 bill that says the carb was to lean and they ran it anyway. Metal to metal is a bad policy, but whoever ran it did not know how to tune it to compensate for an air leak.

I've seen saws come in still running and had massive air leaks from bad pro side seals and clapped out case halves.

Also, Hedge, the old carb had plastic limiters on it and I hate those. Several times I have removed them and on a few occasions the additional adjustment was exactly what was needed for the engine to start and run. The new carb does not have limiters, and it required practically no changes as supplied.

 

[Backtroller]

It might come as a surprise but many of the real old saw engines with a removable cylinder head had no gaskets . Fact many race saws have no head gaskets .

Tolerances must be better

 

[Wood Doctor]

Tolerances must be better.

It seems to me that heat transfer and/or vibration as the engine runs could also set up very small gaps between the jug and the case. The gasket or sealer would compensate for that and maintain the air-tight connection. There has to be a reason why the OEM includes that gasket.

 

[smokey7]

Did it possibly look like someone had pulled the jug before? Then asssembled it real quick without the gasket?

 

[Wood Doctor]

Did it possibly look like someone had pulled the jug before? Then assembled it real quick without the gasket?

Yes, Smokey7, that is another possibility that I also thought of. Thanks for reaffirming my intuition. It would have been rather easy to drop that old dead jug and piston back on after discovering that a new one would not work because the carb and maybe a few other parts were were also toast. The old cylinder gasket may have been broken.

In fact, the muffler exhaust gasket was also broken in half. Even the bar oil tank cap was broken in half. Pull cord was snapped. Seems like a pattern, doesn't it?

 

[smokey7]

Good you thought about previous disassembly was a possibility. First thing that popped in my mind. Any remnants of old gasket on the base or case?

 

[Al Smith]

I'm not advocating not using a base gasket or sealant .However regarding say Power Products AH 58 engines with no head gaskets stop and think about it compared to base gaskets .With a base gasket you are dealing with the parting lines of the case halves .That fact makes it very different than a cylinder head where the cuts could be made with a much tighter fit .Plus the cylinder head will heat up making the fit much tighter as the metal expands .With a cylinder base that does not happen .

 

[drf256]

Beer, I would blame beer.

Blasphemer!

 

[drf256]

It’s not good practice, but if the machining is OTM, a saw will live a long time with metal to metal contact only.

Like Matt said above, probably tuned lean, or not tuned fatter to compensate for the air leak.

Build enough saws and you’ll wind up doing it once, then have an HS moment.

 

[XP_Slinger]

For the good news, I rebuilt the top end using Permatex gasket sealer. It had good compression (150 psi cold). I also installed a new intake boot as HuskyBoy recommended. Still, the engine also needed a new carb. After that was replaced, it started, and it runs fine today.

Assuming that saw actually ran without the gasket or any sealer, would it likely run lean and would that have contributed to the burn out? Seems so, but I'm surprised it would have even had sufficient compression to start and run.

What type of permatex did you use?

 

[Al Smith]

FWIW the early 10 series McCullochs were held together with Indian head gasket shellac .The later used silicon .I've used Permatex number 2 with good results on the clam shells .You're not talking about high pressure on bases or case halves .However I've never used just sealer as a base gasket replacement ,always a gasket .

 

[Leafy]

You can build it to not need a base gasket. But you gotta lap the assembled case till you can't tell it's 2 pieces and lap the bottom of the jug. Those race saws running no head gasket are just lapped on both surfaces until they have an ra under 8 uin and are flat within a few tenths.