The weather was fairly decent today so I decided to talkie the piston Lump to head interference issue. Last week I had made a tracing of a pristine 57 235 head using pressure sensitive rubbing paper and made a cardboard template to duplicate the combustion chamber of the 235 head. Put it on the 261 motor and confirmed that the pop ups were hitting the exhaust valve combustion chamber. So did you ever have a duh-lightbulb moment? Week I did and realized that a 261 head at 95 cc Was going to be physically bigger and that notwithstanding my specifically ordering the pistons for a 261 block with a 235 head, it was possible the pop ups were cut for a 261 head. So I checked a 56 261 head and sure enough,I had clearance on the 261 head where there is none on the 235 head. I checked the casting number of the 261 head that I had with the cc numbers that Tom Langdon had gotten for me before he retired from gm and I had a somewhat nice surprise in that rather than the 95.5 cc Head, I have a 86.2 cc head, so not as bad as I feared. About 7.1 cc larger than the 848 head. Hopefully between the pop ups and a small cut on the head and can keep a little compression in the motor. So if you are going to order pop up pistons, make sure you specify which head you are using and make sure that specification appears on you order form. now off to build a new new head.