Woodslasher's 056/066 hybrid build

[Woodslasher]

Another link to 056/660 piston sizes:
https://opeforum.com/threads/stihl-066-piston-measurements.26082/

 

[thompsoncustom]

Thanks for the picture. Looks like the 066 piston skirt is 63% of the bore and the 056 skirt is %68 percent of the bore so you gain 5% width relative to the bore.

Also the 056 piston looks thinner in a couple areas than the 066 piston so I'm guessing the weight is pretty close.

 

[Powerstroke Cowboy]

Just keep in mind that the 056 piston uses the 13mm wrist pin, so an alternate wrist pin bearing is needed if you don't have the 056 bearing.

My current wrist pin is 12mm the bearing pocket in the rod is 16mm. I have found a 13mmx16mm wrist pin bearing. So, I should be good to go.

 

[Woodslasher]

I've slowly been making progress on the saw, so far 99% of what I've done is just the cylinder mods to make it bolt on. But, I've encountered a hiccup. The piston doesn't clear the crank weights. I can work around it, but I'll need to trim the skirts ~1/8". Could that cause issues? What sort of timing #'s would I need to know to tell if it's an issue? I don't think it would matter on the exhaust side, but it might on the intake side. Currently the intake starts closing at 70 degrees and trimming the skirt would move that down to 76 degrees or so. Also, I need to allow for the longer stroke so currently I have a thick base gasket. If need be I could get the band cut if that would help any timing issues.

 

[thompsoncustom]

As long as your not free porting on the exhaust side after the 1/8in is removed I don't see a issue there. On the intake side are you saying the skirt breaks the top of the intake port at 70 degrees or is closed (skirt at bottom of port)? U can always fill in the intake tho not ideal but as long at the top of the piston doesn't break the top of the port it's probably workable.

I wonder if there would be any issue with removing the crank and taking the 1/8 from there. Also I assume removing 1/8 all the way around the weight is going to make the saw rev quicker from less rotating mass.

 

[Canadian farm boy]

If you bevel the inside edges of the piston skirts you might be able to take less then an 1/8”off the skirts.

 

[Woodslasher]

As long as your not free porting on the exhaust side after the 1/8in is removed I don't see a issue there. On the intake side are you saying the skirt breaks the top of the intake port at 70 degrees or is closed (skirt at bottom of port)? U can always fill in the intake tho not ideal but as long at the top of the piston doesn't break the top of the port it's probably workable.

I wonder if there would be any issue with removing the crank and taking the 1/8 from there. Also I assume removing 1/8 all the way around the weight is going to make the saw rev quicker from less rotating mass.

The skirt starts covering the intake at 70 degrees.

If you bevel the inside edges of the piston skirts you might be able to take less then an 1/8”off the skirts.

How sharp of a bevel can I put on them?

 

[Canadian farm boy]

How sharp of a bevel can I put on them?

I’d do as little as possible and only in the areas that need it.
Maybe you can take 1/16 off the skirt length and bevel another 1/16???

 

[Woodslasher]

I revisited this thing today and realized a few things. #1 was the case was the main clearance issue with the piston and #2 was that the other clearance issue didn't involve the skirts. There was an ear below the pin on one side that was hitting the crank weights. So, today I clearanced the case, ground the ear off the piston, trimmed a few fins on the cylinder to match the work I did on the 1st page, did some port work on the muffler, and cleaned up the bench. I've also got a case gasket and a few 066 big bore pistons on the way for the next time I work on this. I also have a public service announcement, don't forget to drain the bar oil out of the case before you split it.