anything we can do to repair/restore for exhaust port which cut too much?
can we use high temp epoxy?

Not much you can do to fix that... Only thing is possibly shave the base of cylinder ( or simply use thinner gasket) to drop it a bit but that will change intake and transfer port heights and will need to be readjusted... That is assuming the piston isn't bottoming in cylinder dome. It really all depends what has already been done. Most often it's simply better to start over unfortunately.

If too much is cut, the exhaust port will get weak and possibly bust off of the cylinder. I believe when you get around 188 deg. duration the roof of the tunnel gets quite thin. I have never opened the exhaust that much, but have seen pics of broken off exhaust ports because of too much duration.

However, there is a cheap way to fix yours (G290 PUM?) if you went up to about 4 to 6 degrees too much, but you have to modify the piston. For the 4-bolt 36mm PUM cylinder, you will not have to modify the head area if your using a stock stroke crank because it has about .040" stock squish clearance.

This only works for the G290 cylinders, unless the RCMK 34mm piston also have longer skirts than Zen. But I think the squish is tighter for the 34mm bore PUM. If it is, you will also have to modify the squish band, or cut it off the top of the piston, at a 9 deg angle, to get your .010" minimum squish clearance or more.

By taking .020" off the base of the cylinder (like Mark said), you will lower the exhaust port duration and increase the intake duration by 4 degrees duration (.005" per one degree of duration). The transfers will loose duration also, but we can fix that at the piston.

You can get a RCMK piston and use the Zenoah ring in it. The RCMK piston has .020" longer skirts than the Zen piston, so that will get your intake duration back to where it was. The height of the piston tends to be taller than Zen also (center of wrist pin to crown), and that may also remove 1 to 2 degrees more duration from the exhaust port time, and get your squish real close to .010" (I consider minimum squish clearance). For getting the transfer port duration back to normal, you can do what is called "Pie cutting" (can do with a small file) on the top of the piston at the transfer port openings only. You would cut about .025" to .030" deep at the piston edge (.020" off cylinder base plus the extra height of the piston if it is), and match your pie cut angles to the wall angles of your transfer ports (pointing to the rear, intake side, of the cylinder).


If your using the G290 PUM cylinder (the 36mm bore 4-bolt cylinder)----
Here is the RCMK piston to use. Your Zen ring and Zen top rod end parts will fit the piston.

www.davesmotors.com/ca822rcmk36mmpiston

Doug @ ESP

thank you for all your suggestion