Hi Vauxhall,

I found a site with bore and stroke info:

http://archive.dstc.edu.au/BDU/staff/ron/engrev/

Lots of good info.

According to the info there, the Cox .09 has a bore of .497 and a stroke of .471 while the Cox .074 has a bore of .463 and stroke of .439.

I do know the heads will interchange between the Cox .09 and .074 and suspect (haven't tried it) the muffler will also fit either. If the cylinder to case thread is the same that just leaves some machining operations to modify for an .09 crank. This is all theoretical at this point.

If a rear rotary drum could be fashioned it would make a sweet-looking engine. It would be similar in style to the old Cox .15 Olympic or Space hopper .049, which are reed valve.