Stihl - Cylinder Base Gasket Mystery?

[SteveSr]

Hello,

I have been contemplating on whether or not to try to rebuild a Stihl 4137 series (FS-75,80,85) trimmer with a bad piston and cylinder. I was looking on Ebay to see if anyone had a cylinder rebuild kit for a reasonable price. While doing this I found a seller that was selling used saw and trimmer parts. In one of his listings I saw a complete short block in what looked to be *very* clean condition for $55. Having a soft spot for old 2-strokes I decided to take a chance the piston and cylinder were still usable and hit the "Buy" button.

The short block arrived and it was indeed very clean... like someone had recently worked on it. From what I could see through the ports the engine had been run but only for a short time. The piston still had the horizontal factory machine marks visible on both intake and exhaust sides. The cylinder wall looked like a mirror. The rings were sort of a dull gray color like they hadn't even been worn in yet.

Now the big mystery. There were not one but *two* cylinder base gaskets between the block and cylinder. So it appears that someone had this engine apart cleaned it up and replaced the the piston and cylinder using two cylinder base gaskets. The engine was run for a short period of time and then the trimmer ended up in the junk pile and was disassembled for parts.

I suspect it was turned into parts because it wasn't running very well. I know that the extra base gasket definitely lowered the compression ratio but was this enough to make it run bad enough to be scrapped? My only thought is that some tech thought he was trying to fix a leak somewhere. Can you think of anything else? Unless someone gives me a very good reason not to I am planning on removing one of the cylinder base gaskets.

Speaking of things that can possibly leak. Is it advisable to change out the crankshaft seals at this stage in the rebuild process? Can the old ones be removed without having access to Stihl's special seal puller? If so can you recommend a method or procedure?

Thanks,

Steve

 

[tolman_paul]

It's possible whoever replaced the p/c checked the squish clearance and found it was too tight, so he put a second base gasket on the the cylinder to get the correct squish clearance. But if that was the case, then the compression ratio should be ok, and a tech savy enough to check squish clearance would likely be able to get the engine to fun correctly.

More likely whoever replaced the parts either got a p/c with two base gaskets in the package and figured if there were two in the package he might as well install both, or he was too lazy to remove the old one and doubled them up. But even with that being the case, I don't see a second base gasket as sufficiently dropping the compression and changing the timing to the point the engine won't run.

As to the seals, if they leak replace em, if not, don't mess with them. I run quite a few 20-30 y/o two struck engines with their original crank seals and they work just fine.

 

[SteveSr]

Paul,

Thanks for your insight. The mystery remains...

As for the crank seals I have no way to check them out definitively without putting the engine back together completely. This is why I was considering replacing them while the engine was completely disassembled. But you are right in that seals are a low probability of failure.

Steve

 

[056kid]

They come from the factory with 2 cylinder der gaskets..

 

[Vintage Engine Repairs]

They come from the factory with 2 cylinder der gaskets..

Only 11 years late

 

[056kid]

Only 11 years late

& it's still 100% relevant, today.

 

[Vintage Engine Repairs]

& it's still 100% relevant, today.

Very true!

 

[056kid]

Very true!

I've run across 2 threads where people posed the question, made assumptions, & then a conclusion. That being that 2 gaskets was not factory & it was definitely 1 from the factory.
It's nice to get the right info when in search.

 

[Gord404]

Another thing I didn't know is that the cases on short blocks were machined as a matched set and that a half off another probably won't fit on another properly, a friend came by the other day and mentioned this and took a case half I had and sure enough it didn't match up properly with his half- I have a complete short block saw that came in a while back and it has a compression release and orginal plastics to match- evidently only made for one year- It runs but needs a little tlc- also someone swapped the crank out for the larger nylon fin one, going to be a nice saw when I get at it.