The current Ryobi engines in the Bolen weedeaters are the best of them all, dual rings, no slots, the new style exhaust port, they even made a little larger relief in the crankcase so the transfer ports match the cylinder better. Lowes had them for $64.95 the last time I looked. You'll have to machine the "shroud" off the front of the crankcase though, it doesn't bolt on like the older style motors.