Proper Ring Gap

[GBertolet]

I am rehabbing a free Craftsman chainsaw. It is made by Poulan. Let me set the stage. The model id tag is missing. It is a 40cc 2.4 cu, with a 18" bar. I think it is a 358360130 saw. I am not sure what the Poulan model cross reference equivalent is. It has low compression of about 60 lbs. It is erratic in running. Runs great at times, then stalls out when hot. Sometimes won't start at all when cold. Anyhow adding a squirt of oil in the cylinder raised the compression to 90 lbs. Usually starts first pull after adding the oil. Bad ring was my amateur diagnosis. I tore it down, placed the old ring in the cylinder to measure, and had a .055 ring gap. I think this might be too much. What is a expert opinion on this?

I did purchase a new ring # 530038729, while I had it apart, and when it came, I noticed an end was chipped off, so it is junk. Anyhow the new ring measures .065 thick, while the existing ring is .059 thickness, so that's 12 thou added to the diameter. Should improve compression when I get it's replacement, I think. Am I on the right track here?

 

[Stump Shot]

Yes that is too much end gap. You should be roughly .004" per inch of piston diameter. Example: 2-1/4" diameter piston would be .009".

 

[GBertolet]

My piston diameter is 1.610, so .006 or .007 would be about correct then. My bore is 1.617. Is that too worn?

 

[Stump Shot]

My piston diameter is 1.610, so .006 or .007 would be about correct then. My bore is 1.617. Is that too worn?

Yes to the first and no to the second. There needs to be a little room for the expansion of the piston.

 

[GBertolet]

I got the new ring and installed it today. I placed the bare ring in the cylinder to check the end gap, and the ends almost touched. It was golden. I assembled the engine, and it started on the 2nd pull. Had to fiddle a bit with the mixtures, to get it running right. It has some pop now in the compression. Before I could place the saw on the ground and lift up on the starter cord, and it would almost all unwind, before the saw lifted. A successful conclusion to this adventure.