Re: 3cyl 65-70-75 OMC Rebuild

[r-c]

One thing we have done on exhaust bridges for racing is to take your Dremal drill with a sanding disc and relieve it a few thousands in an arc from one end to the other. Finish smooth by hand with some wet and dry. A ball hone does a nice job to finish off all the edges smooth and leaves a nice finish to the entire bore. A 240 grit is fine.
So when the bridge expands from heat, because there is no way to cool it, it doesn’t expand into the bore. If it is expanding into the bore you will see rub marks on the piston, especially near the ring area. Bad enough it will stick the rings right there.
Also, take a round stone and radius all the edges of the port, especially the top, so the rings don’t catch an edge. The radius helps to move the rings back in the groove without catching an edge.
If I remember right these engines have an angled bridge. So you can’t put oiling holes in the piston to help lube the bridge. If it is a straight bridge, you can drill two oiling holes on the piston in line to the middle of the bridge. About 1/4" below the bottom ring and the other 1/2" below the first. A number 1 center drill works nice as it leaves a counter sink bevel and the hole is about 0.070" diameter. Of course don’t drill so deep that the bevel is wider than the bridge width. Take a round cutter tool and radius the inside holes of the piston.
As far as cutting the head, you can. But then you are a slave to higher octane fuel. Also if you are stuck with ethanol gas you may need to go richer on the high speed jets. The gain is minimal compared to more future problems. You can do more working props than modifying the powerhead and asking for problems there.