I've got an engine that I want to play with to see if I can make changes and make more power. This particular engine has some gaping holes machined inside the case that I suspect are big enough to hamper bottom-end pumping. The engine is, according to the manfacturer, only suitable when used with their "magic muffler/tuned pipe". When you run it open-face, it is a very nice engine that just doesn't really perform at all well.

I want to fill in some spots inside the case and inside the crank. What material is best suited for doing that? Some sort of filled epoxy I suspect, but I would like to hear if any have had any actual experience I could draw upon.

Bill

I have used JB Weld in Fox 35 bypasses instead of the bass wood stick for eliminating the burp, and haven't had any problems with it. I clean everthing well with laquer thinner and rough it up a little with 320 wet or dry sand paper before applying the JB.

Yes, J-B Weld.  A few years ago when my daughter was racing a 4 cycle kart, I filled the intake port with J-B Weld and then reshaped the intake port for a better flow pattern.  Our fuel was 40% methanol and 60% pure nitromethane.  She used the engine to win two season championships with no effect on the J-B Weld.

Another vote for JB weld.  While you're at it, fill in the inside of the crankshaft forward of the shaft port and fair it to the back of the port.  Makes the shaft stronger too.

You probably know this already, but after it's hardened for 24 hrs or so and is cured hard bake it in the oven at about 225degF as a post cure for a couple of hours.

Mark Smith or maybe Steve Yampolsky also recommended wire brushing epxoy into aluminum as a prep for gluing or filling.  The wire brush breaks up the aluminum oxide coating that forms on AL and lets the epoxy bond directly to the metal of the case.