Hi ShempHoward,

sadly the stock OS .91 DF only comes with the ABN liner. It is very confusing that OS states a true ABC assembly at their homepage which is NOT true.

http://www.osengines.com/engines/ductedfan.html

The only direct replacement ABC or AAC piston/sleeve assembly available for the OS .91 DF is offered by Dave Shadel at Performance Specialties.

http://www.pspec.com/html/pistons.html

BTW, the actual OS .46 VX-DF engine indeed has a true ABC construction