Hello everyone,

I recently purchased a set of Mopar Performance P5007412 Pistons and it seems like there is very little information online about them. They are listed in the Building Big Inch Mopar Small Blocks book as Pistons for a Stroker engine but that is about all the information that I have found other than they are discontinued. I am curious if anyone knows who made these pistons for Mopar Performance. They are forged aluminum and have valve reliefs cut into the flat top design. They are the standard bore 4.00 inch pistons and I would like to put them in a low mile 5.9 magnum block and possibly put a 4 inch stroker crankshaft in as well without doing any machine work if I can get away with it.

My question is, what is my Compression Ratio going to be? They have a 1.418 Compression Height. I think the valve relief is 5cc's? I would like to have 10-1 or less and plan to use this motor as a street motor just for fun. I have a set of New Cast Iron Mopar Performance heads with 2.02 intake valves that have been sitting in my garage for years so I would like to use those if I can.

I'm not sure on the magnum block, but I built a 408 in an la with a .030 version. with the 1.41 comp. height and had to deck the block approx. .050 to get near 0 deck. Which if your looking for 9/1 compression you won't need to deck it. I want to say the rings were 5/64's also. I put them in at .0045 piston to wall.

Check the width between the pin bosses & see if they fit your rods.

I just measured them and they measured 1.345 between the piston bosses. I don't have a rod to measure. Engine is still assembled and isn't tore down yet. In the How to build Big Inch Mopar Small Block book, it says that the Magnum Rods are narrower than the LA Rods at the small end and the LA rods are 1.200. The Magnum Rods should fit?

It depends on what heads you use. I just did a 4"crank magnum with Mahle pistons, 1.460 compression height, 28cc dish, and 58cc magnum heads, piston .005 in the hole, .027 cometic and it is 9.89. Magnum blocks are about .020 shorter in deck height. If we assume your block us the same then you are looking at 11.3 with 58cchead and piston .046 in the hole, 10.70 with 63cc heads, and if you want a zero deck quench motor it will be about 11.9 with zero deck,.040 gasket and a 63cc head. Building quench at 10-1 with a 4" crank is near impossible with those pistons.

Well that doesn't sound good considering that I wanted to use magnum heads for the build and they would be 58cc. What if I just used these pistons with a stock 3.58 stroke crankshaft at some point down the road, what would the compression be then? Just wondering if they are even worth keeping? I really don't see myself running nothing but race gas in a motor build.

you'll end up around .039 in the hole in a magnum based of a 9.58 height.. Also I've only messed with 1 set of magnum heads and they were 64 cc when I actually measured them. Here is the way i see it:
swept vol. 823
deck vol 8 (using -.020 for the magnum deck mentioned above 9.58)
valve relief 4
head gasket 9
head 64 (I'm using this based on my measurement of an actual head)
908/85 10.68 to 1

Just so I'm clear, you believe that with a 4.0 inch stroke and the 4.0 in diameter pistons that I have, my compression will be 10.68 to 1 with the magnum heads that I have since they actually measure 64cc's? I guess iron heads would be out of the question then for pump gas?

If my math is right and I just used the stock crank 3.58 my compression ratio would be 9.67 to 1?

A 3.79 stroker crankshaft would bump the compression up to 10.17 to 1?

You would be smarter to just build a stock stroke engine with iron heads. My 671 iron magnum heads were 60cc unmilled, the edelbrock magnums are 58cc unmilled. I would not make any plans until you measure the heads.

Right, there is some assumptions in there (Head cc, deck height) but close enough.

The other combos would need different comp. height pistons
for example if you used them on a 3.58 crank you'd be .249 in the hole
swept vol. 737
deck vol. 51
valve pocket vol. 4
gasket vol. 9
head vol. 64
865/128 6.75 to 1

So I am assuming you mean the correct corresponding pistons.

Most MP catalogued B/RB/LA forged pistons during that time were either from Wiseco or TRW.

Much later, Diamond came on board with some very short lived Gen 3 products/post-2005.

Ok another quick question. In the How to Build Big Inch Mopar Small Blocks book it says that the LA motor and the Magnum Motor have the same deck height of 9.585 on page 16. Is this information not true?

I can say for certain that that might be the blueprint spec but they are all over place. I've never seen one much shorter than 9.59 . I saw one one time that was .030 different from one side to the other also. Also seen them vary wildly from end to end also. Most of my experience is with LA stuff.