Hello Tim ; I have a YS AC 91 that I got from one of the guys at the field that had been in a crash & broke the cylinder @ the exhaust. I had the machine shop at the factory where I work drill deeper into the head & re-thread with a bigger tap. seems to work fine but I can't get a muffler to fit it now. My question is do you have an extra cylinder? It sounded like in your post that you had both old & new style. Mine is the old one with the clear gasket. If you have one & want to part with it please let me know. Also if you would like I could have our machine shop do the same thing as mine so it wouldn't be a total loss. Thanks Rob