FMonster

iTrader: (2)

I'm betting on rings. Probably weren't gapped correctly and bound up when the motor heated up. This causes the ring lands to break. You may not see anything in the oil because the rings are holding the pieces in place. The oil consumption fits this and the broken lands will cause compression to blow by and give the high crankcase pressure. I had this happen to a motor when I bought pre-gapped rings. Same symptoms as you have, seems like it had no top end power too.

4doortypels

iTrader: (39)

Too much stroke. Nelson explains tall deck blocks are needed for 4.00 inch stroke cranks. 4.125 is a ton more than factory. Pistons are likely pulling rings past the max on the sleeve.

KCS

iTrader: (20)

Nelson is wrong, or at least he wasn't aware of recent designs when the video was made. There are of enty of 4.125-4.250" stroke cranks running around in standard deck height blocks without issues.

When he talks about the "gauge point" in the piston, he is correct that you can pull it down below the bottom of the cylinder at BDC, but that is with a incorrectly designed piston. He says the gauge point is about .500" below the oil rails, but in modern designs, it's more like .375". With those type of skirts, the gauge point stays in the bore with the longer strokes.