ATC 70 cylinder & head questions

[Baba Yaga]

Hi, 1985 ATC 70. Was burning oil and became difficult to start and run - fouled plugs within minutes. Compression was low, around 100 IIRC.

Pulled the head, and both valves were stuck open - like .010/.020 or so.
Tons of carbon on the exhaust valve, in the seat, etc. I'm surprised it ran at all. Full disassembly, cleaned valves, light lap, etc. I put the head back together, but am questioning if I should have replaced the valve stem seals. They look fine. For oil to get into the combustion chamber, it would have to either get past those seals, or bypass the piston, correct? Thoughts on re-using old seals?

Also pulled cylinder & piston. Everything looks good, wear on cylinder is under SM limit. Machining marks still visible on piston, so can't be too much wear. All ring end gaps conform to SM specs when placed in cylinder.

I have on hand, one new set of Honda OEM rings, stock size. I also have on hand a Chinese cylinder & piston kit.

I can either hone old cylinder & install old OR new rings, and use all original parts.

OR

I can install the Chinese kit. Chinese piston is a bit shorter, but everything pretty much looks the same.

Thoughts?

[Baba Yaga]

Talked to a local guy here who says the Chinese pistons are NFG, and won't make power. Interesting....

[atc500x]

Shorter piston will not make a difference,the be sure the distance between the pin and top is the same,sometime the difference is only the skirt.

I would check the cylinder and make sure it's not out of round,ask to and shop to check for with cylinder gauge.New ring in a bad cylinder may be worst than your old rings

[Baba Yaga]

The piston that came in the kit, has a shorter skirt, and a shorter distance to the top of the piston. So I'm using a different new Chinese piston, which looks to be a copy of the original Honda piston, with the new Chinese cylinder. Hopefully it all works ok.