As Vic said, the NPR pistons are 30* and the early, pre 1964, heads are 22*. In order to make these a "bolt on" many people simply shim the cylinders so that the pistons do not make contact with the head at the small diameter portion of the squish. Doing so, shimming, gaurantees an engine with piss poor performance. This is because it will have a lower compression ratio and, more importantly, it will have very poor combustion. Without a tight and well controlled squish, you will lose a lot of power. You also lose fuel mileage and run a high risk of detonation because quite a bit of mixture is trapped in a void that is not "visible" to the spark plug. I bought a S90 with a newly rebuilt engine with this flaw. The difference after I corrected the mistake is shown in the graph below.
9-22-2008 8;56;06 AM.jpg [ 295.16 KiB | Viewed 532 times ]