Interesting 024 find

[John Lyngdal]

The wet(ter) season has arrived in Oregon, so I'm spending time getting caught up on some of my saw projects.
Grabbed an 024 off the shelf that just needed a new cylinder cover and a couple flywheel screws to be installed.
I decided to give the started cord a pull and "Wow" I thought, that's not as easy to pull over as I thought it should be. I pulled the spark plug and gave it another pull, and it turned over light and easy. OK, time to grab the compression gauge and I watched it soar with every pull. Looks to top out at 225 psi ( and yes, the gauge reads accurately), so I measured the squish at 0.013". That's a little tighter than I like. I was thinking that it should be OK for a small displacement, short stroke engine; or should I look for a thicker base gasket or just run it the way it is?

 

[pioneerguy600]

I run .015 on many of my 026`s. .013 is really tight.

 

[John Lyngdal]

I think I'll pull the piston and put it on the lathe and make into a semi-popup design to increase the squish at the expense of a few psi of compression that I can spare.

 

[John Lyngdal]

Pulled the jug and slug today and found this:


Clearly things were a little tight on the top end based on the peen marks on the top of the piston and the bright spots on the top of the cylinder.
It looks likes the piston has a semi-domed profile with a top band cut, but the Stihl mark can be made out on the top center of the piston.
It has a standard cylinder gasket present, so that didn't cause the squish issue. Looking at the profile of the piston , the combustion side forward of the ring looks narrower that normal.
I noticed that the underside of the piston has also been modified for clearance. So I'm wondering if this was really a 024S crankshaft case fitted with a 024 jug and the piston modified for clearance, or perhaps a modified 028 piston was used.

Going to proceed with putting the piston on the lathe to add some clearance and bolt it back together.

 

[pioneerguy600]

Looks like an 028 piston, the mods needed to make one fit follows what you found. Also looks like it was detonating at the perimeter, squish was too tight and likely ran on old fuel mix.

 

[John Lyngdal]

I machined the top of the piston and cleaned up the underside of the piston(which was a mess) as best I could.

I ended up removing more material from the top of the piston than I had planned. I targeted 200 psi when I started, but ended up at 180. The squish is now a healthy .026", so at least the top of piston and cylinder head are safe from future damage. I still need to finish re-assembling the saw, adjust the carburetor, and make some chips fly to see how it works in a cut.

 

[Justin Taylor]

I machined the top of the piston and cleaned up the underside of the piston(which was a mess) as best I could.
View attachment 791685 View attachment 791686
I ended up removing more material from the top of the piston than I had planned. I targeted 200 psi when I started, but ended up at 180. The squish is now a healthy .026", so at least the top of piston and cylinder head are safe from future damage. I still need to finish re-assembling the saw, adjust the carburetor, and make some chips fly to see how it works in a cut.

I think she will cut for sure

 

[pioneerguy600]

They actually don`t run any better with a 028 piston in them but maybe the flat machined squish will make a difference, they run much better with a totally flat top piston.