Need some advice on a k960 cut off saw

[ladrhog]

I just picked up a couple of cutoff saws from a landscape/construction company. The first is a k750 that just needed the air filter cleaned. Did a vac and pressure test and looked at the cylinder. The cylinder looks great. It has 140 psi compression cold. It did fail the vac test. I did a pressure test and it passed. I then tore it down to look at both seals. They looked good. I put some gas mix on them and put the vacuum to them and neither leaked. Looked everywhere else I could think of for a leak. Found nothing. While under 20 psi of pressure there is no leaks. I am stumped, either way it runs great. I am just going to set the carb a little rich and run it.

The second is a K960. It had low compression and upon looking in the spark plug hole I could tell the piston was bad. So I pulled it down and here are the results. Looks like a ring let loose and the the piston failed. After that a whole lot of things bouncing around. My question is can this cylinder be salvaged? I know there is a gouge in the exhaust port but other than that it seems ok. There is no aluminum transfer anywhere. I am thinking of cleaning it up and throwing a new piston in it. I have never seen pieces of ring lodged in the piston. Neither saw will be sold. all for personal use. What do you think?

 

[Thommo]

I would tidy up the exhaust port so that there is no burr sticking out into the bore and also try to smooth out the squish band on the cylinder. Make sure that there is no bits of ring stuck in the top of the cylinder that could dent your new piston and close the ring grooves.

 

[alexcagle]

I agree. Use It. The Stihl TS700/800 cylinders actually have a notch not unlike your saw's damage designed Into the top of the exhaust port. Take a dremel with a #502 paddle wheel and take down the raised part of that ring gouge so it doesn't hang on the new rings. Those K960's have 1.2mm thick rings, rather than 1.5mm rings. The rings on concrete saws if given enough time, will wear past the ring locator pins, allowing the ring to rotate around and hang on a port. Pretty common on saws with age or poor filter maintenance.
Also common on that saw is the clutch sides' seal to leak under vacuum, and the cylinder gasket transfer port end to leak even with a new gasket. The metal gasket type seals from the K970 work well, but can't be reused. The decompression valve hole needs to be completely clear of carbon too or it won't have enough velocity to slam the valve shut.
On your K750/760 intakeone bolt flange) are notorious leak points too. It can be sealed by block sanding about .005" off the face, (with the three rubber boots off), of course. It will give it more squeeze.
Also on the transfer port end of the cylinder gasket; It can benefit from a very small film of red silicone on both sides will help seal it up.
Lastly; shake out all the broken ring bits out of the muffler. They could theoretically find their way back inside the exhaust port, given the fact that most saws get thrown around in the back of a truck.

 

[ladrhog]

thanks for the info.