The "p" bearings are a softer and a little more forgiving if you get small slivers going through. They can embed small particles into the softer material, thus not scratching up the crank as quickly. Mostly used in oem replacement and common in mild to medium builds. They do lack some strenghth on high hp builds as they are softer than the 'H" bearings. I like the "H" as they can take abuse and handle crank flex better on higher rpm and hp builds. The downside is if you get any particles through you will be more prone to cause damage to the crank instead the particle pressing into the a softer bearing. If your motor is seasoned and you are not doing a lot of internal machining, ie, oil galleries, crank work, etc...and have an engine builder that is very picky and cleans the block about four extra times you can easily go the "H" route. A softer bearing is usually used on most rebuilds as they are more forgiving on break in and when left over shavings, etc find they way through the motor. Engine builders like to use them so they can get away with having problems easier. Sorry about the long explanation.A lot of people doing their own motors just throw in "H" bearings "because they'er better" without understanding their use.
