Does anyone know if there is a difference in the pistons in a Gen IV vs. a Gen VI 502? I am in the market for some replacement std bore pistons for my Gen VI 502s and have found a set of Gen IV pistons which were never run and the look alike but are there sublte differences?
Thanks.

Shouldnt matter as long as the Bore size is correct, and the pin height is what you want. Putting a piston in that was designed for a 6.385 rod, on a 6.135 rod is no good, and vice versa.

You need to know the pin height, bore size, and pin style of what you are buying. If you are unsure of all of this, I would leave it up to the person building your engines to choose the pistons. Get a part Number off the pistons and maybe we can help.

I have a few sets of Gen VI pullouts if you need some. I do not know if they are the same. Since there weren't many Gen IV 502's built, we don't get the chance to see them.
Eddie

Both engines do use different piston part numbers from mercruiser, and the difference with the 502 looks to be between having fuel injection and carbureted and the GEN VI happens to be in the fuel injection years.

Merc part number 737-850477T is only found in the,
502 EFI (GEN. V) GM 502 V-8 1993-1995
502 MAG. MPI (GEN VI) GM 502 V-8 1996-1998 +

Merc part number 737-827664 is found in the,
502 MAG. BRAVO (GEN. V) GM 502 V-8 1992-1993
502 MAG. BRAVO (Gen IV) GM 502 V-8 1990-1992

http://www.mercruiserparts.com/selec....asp?type_id=8

Hope this helps, Good luck.

i would guess there would be a diff in chamber volume between older and new gen pistons. do some math homework so you know what CR you're gonna end up with.

if you do end up using the late gen6 stuff be advised that they are a metric ring pak with fairly low oil ring pressures and can have oil use problems over time. total seal makes a ring pak for these that fixes that.

Forget about the stock pistons, as mentioned the gen 6 crap uses a low tension metric ring pac and the rings alone are x2 cost over normal ones, J/E srp makes a nice drop in forgeds piston that is .002 larger and uses standard rings and real close to the same weight so if you choose not to rebalance you will be as close as you were to start with. With the .002 larger dia you can have motor tq plate honed too without having to re-bore blocks, I used them with great luck and with a little shopping you should be able to find them for about 450$ a set, Smitty
Link shows specs on J/E website:
http://www.jepistons.com/Products/281916.aspx
this link is where I have found J/E pistons many times for HUNDREDS less than most jobbers:
http://www.flatlanderracing.com/srp-bbcflatinvert.html

What everyone else said plus, some of the real early ones had a very oddball ring pack. We freshened one up a few years back and actually had to order two sets of rings to get the right combination because nobody actually made the correct set any longer I would stick with the correct ones even though it might cost a little more to begin with. The other issue is that the other pistons may be a different weight than yours and would require rebalancing the crankshaft to match.

Bill Koustenis
Advanced Automotive Machine
Waldorf Md

much better idea

I just tore down a Gen V 502 EFI & it had JE700 pistons in it. Two of the pistons said JE703, not sure what the difference is?