Chainsaw Repair

In an earlier post in this thread someone said

For an OEM 7900 jug, I came up with a 272 piston which should require about a .174" spacer under the cylinder. If a 13mm wrist pin could be used, a 281 could be used with only a .095" spacer.

For the 7900BB, I only found the 288 piston, which has a 13mm wrist pin and would require a .095" spacer.

Since I had an older red top 7900 on my bench for a rebuild, I thought I would test that, I used a stock 7900 jug a 272 & 038 magnum pistons. I cut out a spacer out of 0.305 material, thinking 0.305 should get me the required space for an 038 Magnum piston since the CH of the 038 is 22.5mm and the 272 is 24mm or about 0.059055 taller than the 038 Magnum. So if you take the spacer 0.305 add to it 0.059 you come up with 0.364 as how much room you would have. Subtract the 0.174 from the 0.364 leaves you with 0.019 no base gasket needed......COOL, thought I!


Wrongo! The 038 Magnum piston runs smack into the head with 0 squish, so that blows that number all to hell. Be back in a bit with the 272 numbers.

OK, with the degree wheel on the saw the 272 piston hits the top of the cylinder about 2.5 degrees before the 038 hit which stands to reason because the 272 piston is 1.5mm taller than the 038 Magnum e.g. 0.018-0.019 = 1°.

So my feeling is that the quoted set of numbers must be for a pop-up.