502 Piston History
 

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.:daz:

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?

Wow, 2 cylinders .030 over & the rest stock. WTF, who does that!

probably sounds like a Harley at idle.

OK, i have been studying pistons and bore clearance. It appears that the big difference between the Mark IV and Gen VI 502 pistons is the ring size. The Gen VI use the metric thickness rings and the Mark IV use the SAE sized ones. The question is why don't piston manufacturers give the actual diameter of the pistons? They all seem to advertise the bore size the pistons fit but not the actual piston size. Looking at the SRP Replacement pistons they say they fit a 4.470" bore but are only .002" larger than stock. If the stock bore is 4.466" then there appears to be an extra .002" clearance using these pistons. I assume that you are expected to hone the bore to get the desired clearance but it would be nice to know the actual piston size before ordering them. So my question now is, what size is the stock piston for the 502?

Thanks.

the adverage piston size on a gen vl piston is 4.464 measured .500 up from the bottom of the skirt. i have usd the J E 4.470 bore piston will come with a 4.466 size . i usually finish my bores .001 biggger for .005 pw , if i coat the pistons I still finish the bore to 4.471 .

they don't tell you the piston size because pistons aren't round. they are oval and tapered. which dimension would you like ?

beyond that, pistons change dimension and shape significantly with temperature. so when you buy a piston, if you are smart, you take the time to understand what its made of, how its made what the operating temperature of your system is going to be along with the general type of service it will see over what sort of duty cycle and use those factors to define the ring pack, final bore size and final bore finish.

the answers to your questions and confusion will come from the enginners at JE or weisco or cosworth who, if you call them might take the time to tell you in 25 words or less what they have learned in 75 years of fundemental engineering understanding. the same with the guys that make the rings. outside of the general folklore and voodoo , its a topic that is a science in its own right.