I'm converting a K&B 3.5 marine outboard engine to aircraft use. I have all the parts I need except a backplate. The original is fitted with rear bearings, which i've removed. My main question concerns the volume of the crankcase. I've heard that either icreasing or decreasing can affect performance. Is there a rule of thumb concerning this? I have the ability to change/adjust the volume. Is it worth fooling with? Any ideas, suggestions or input would be appreciated.

K&B 3.5 ducted fan is basically the same motor so the backplate may bolt righ on, otherwise keep it as close to the back of the crank as you can get it without touching.

Thanks Gord. That's about what I figured. How many head shims (if any) are used in a DF engine? Do you need to keep the compression down for high R's?

Ewstap:
Sorry, don't know whether it has a head shim as I've never had it apart. Bought it back in the 70s. It's made to turn high revs in a ducted fan. I sits in a Hustler delta built back in the 60s. Has a rear tuned pipe and won't run on anything less than 25% nitro. I wuoldn't worry about the compression.