Originally posted by BART
On my RB Concepts X12 the top of the sleeve is almost flush with the top of the block so I could not use it on there but it just fits onto my Mugen MT12.
The MT12 is a long rod engine which is different from the X12. The sleeve sits differently. That explains why you could fit it on the X12 because the O Donnel head was specifically made for long rod engines which are now defunct and almost non existant.