1994 Mercruiser 502
 

ok i have a quick question, i have a 1994 mercruiser 502 EFI and i was told that this motor has forged internals such as pistons, connecting rods and cranks. Can anybody verify this for me? it would help alot. thanks in advance

cq,
All of GM's big block Chevy rods are forged. Always have been. I am pretty sure that the only 502 crank that GM made was also forged, I have never seen a cast one. As far as pistons go, I dont think I have seen a Mercruiser 502 that did not have forged pistons. The older ones were Wiseco's and then I think they switched to JE. I have not seen any real, real new ones, they might have gone to a hypereutectic piston.

So answer is, yours probably is all forged internals.

Bill Koustenis
Advanced AUtomotive Machine
Waldorf Md

have had a couple of non mag 454 mercs with all cast stuff...

ezstriper, the non Mag 454's did have cast internals. They had a cast crank, 3/8 rods and cast pistons. The Mag 454's have the same parts as the 502, just with a smaller piston. The non Mags also use a two bolt main, whereas the Mags and the 502's have a four bolt main.

Conquest, your 502 does have a forged crank, 7/16 rods and forged JE pistons. As Bill said, I don't recall seeing one any other way.
Eddie

thanks for all the responses, but i did a compression test and it didnt look so good. It came back at 140, 125, 110, 110, 110, 115, 100, 100 so i have to do a leakdown test. The boat only has 275 hours so im thinking it is coming from the valves and once i put the new heads on i should be good. Anybody have any comment good or bad? u think it might be the valves?

shoot some oil in the cyl and re-check, if it goes up rings, stays the same, most likley valves...Rob

Has it been sitting for an extended period of time? Some of the leakage may be do to some rust on the valve seats if it has been sitting. I have seen that engine with 400 hours that looked like new and have seen one with 200 hours that was just worn out. And that was in the same boat. Can't explain it.
Eddie

i have a 99 502 mag mpi apart now. it had a bit less than 200 hours on it. sometimes run at max speed mostly at fast cruise. no overheating. no over rich, good maintainence.

the valve seats all look like garbage, and the cylinders are all glazed and there is oil in all the combustion chambers.

after doing some research from multiple authoratative sources, it seems that mercs attempt at gaining power form light ring tensions is essentially a ticket to early rebuilds and loss of performance at low hours and the internals of this motor confirm it. i will be putting std tension rings back in it.

i would suggest that when you have the heads off you are going to find the same situation and i would further suggest that you take the time to hone the bores and put a good set of rings in it. total seal makes a set specifically for this motor that fixes the problem. a simple 240 brush hone will clean and prep the bores without changing the dimensions.

for the minimal extra work, i believe you will get vastly superior results rather than just doing the heads... also... a compression test as a indicator of general wear is not particularly meaningful. if you want to continue doing a real analysis, warm the motor up and do a proper leak down. if the numbers are low, just listen for where the air is getting out.

and when you are all done killing yourself doing that, take it out anyway and freshen it. if you are doing it yourself, the parts to do new rings , NEW valves and a head set will be about 500 bucks...

be smart. do it right the first time.

Forged pistons, rods,& crank. I concur.

Steve,
I think that you will find that the reason for this is the Mercruiser engines are basically GMPP crate engines with minor modifications. GM made the decision to go with the low tension rings and they did have a lot of problems with them :(