Marine 502 into 1984 GMC 2500 454

x75x75x75 · Tuesday at 5:34 PM

I was pulling a trailer from Dallas to Austin when my radiator boiled over in the middle of nowhere. I let it cool down, splashed a little water into it, and slowly drove to the first gas station. Put in water and continued, but noticed steam-smoke along the way, got home and realized I had a cracked head.

Replaced the heads with some old 1968 closed chamber heads, and the engine ran fine, even stronger due to higher compression. Put maybe 200 miles hot-rodding around town, then one day lost oil pressure, from 50 down to 8 psi, and picked up a rod-knock. Sad ending to the bearings after 125,000 trouble-free miles. Still runs but sounds sick.

So a neighbor has a fresh Marine 502 short block that he rebuilt for a Corvette project, but then lost interest. Looks good inside, flat top pistons, forged crank/rods, windage tray, 4-bolt mains, fresh bearings/rings, etc.

So, I have a complete but blown 454 sitting in my truck and I'd like to install the 502 short block. I know the block height is different, but what from my old 454 will go in the 502?

I know the oil pan is different, but I think I can re-use my heads and I have a fresh valve train set-up from my 454.

what else fits?

Harmonic balancer
Flywheel (manual 4-spd)
oil pump
timing chain
intake manifold
starter
water pump
pullies, mounts, A/C set up and so on

any advice is appreciated

Bextreme04 · Tuesday at 6:43 PM

x75x75x75 said:
I was pulling a trailer from Dallas to Austin when my radiator boiled over in the middle of nowhere. I let it cool down, splashed a little water into it, and slowly drove to the first gas station. Put in water and continued, but noticed steam-smoke along the way, got home and realized I had a cracked head.

Replaced the heads with some old 1968 closed chamber heads, and the engine ran fine, even stronger due to higher compression. Put maybe 200 miles hot-rodding around town, then one day lost oil pressure, from 50 down to 8 psi, and picked up a rod-knock. Sad ending to the bearings after 125,000 trouble-free miles. Still runs but sounds sick.

So a neighbor has a fresh Marine 502 short block that he rebuilt for a Corvette project, but then lost interest. Looks good inside, flat top pistons, forged crank/rods, windage tray, 4-bolt mains, fresh bearings/rings, etc.

So, I have a complete but blown 454 sitting in my truck and I'd like to install the 502 short block. I know the block height is different, but what from my old 454 will go in the 502?

I know the oil pan is different, but I think I can re-use my heads and I have a fresh valve train set-up from my 454.

what else fits?

Harmonic balancer
Flywheel (manual 4-spd)
oil pump
timing chain
intake manifold
starter
water pump
pullies, mounts, A/C set up and so on

any advice is appreciated

Pics? Depends on what the "Marine 502" actually is. What the block casting number? Where did he get it from? Is it one piece or two piece main? Do you KNOW it is a tall deck engine? Most 502's would have been short deck, but many marine applications are tall deck. You can just measure from crank centerline to deck to know for sure. If it IS a tall deck, you would just need spacers to be able to use your intake manifold and you'll need a different distributor.

You won't know what will fit unless your neighbor knows what the rotating assembly was balanced for and whether it is a one piece or two piece crank seal engine. You should also pay attention to whether it has a fuel pump boss or not... many later street 502's and marine applications did not have that machined.

x75x75x75 · Tuesday at 8:11 PM

thank you so much for educating me, I did not know 502 blocks came in two deck heights. I will talk with my neighbor tomorrow and get the information to help figure out what we have. But we're pretty sure it's marine, the donor boat is sitting next to his garage. I owe you a beer

goldpack · Wednesday at 9:59 AM

if this one does not work out,...and looking for more big blocks.
...big truck engines are tall deck.
so like a 427 for a 1974 topkick c6500 is tall
vs. a 427 found in a corvette.

both possible to use...but in a pickup you might need a hood with bulge as the tall ones need 2-3 more inches of room to get it closed. (which you might in this case with the 502)

and welcome. ...how long did you have this truck that you have not found this great forum. ?

Jrgunn5150 · Wednesday at 10:36 AM

I don't know of any tall deck 502's. Everyone I've ever seen was 9.8" and used all standard Mark IV stuff. The caveats were they didn't have clutch ball provisions.

Regardless, tall deck 8.1 and 8.8's fit fine anyway.

Sad Sack · Wednesday at 10:36 AM

Bextreme04 · Wednesday at 11:30 AM

Jrgunn5150 said:
I don't know of any tall deck 502's. Everyone I've ever seen was 9.8" and used all standard Mark IV stuff. The caveats were they didn't have clutch ball provisions.

Regardless, tall deck 8.1 and 8.8's fit fine anyway.

The Marine 502's from the 90's-early 2000's would have almost all been based on Gen VI block architecture. That means one piece rear main seal, less bolts on the timing cover, roller cam provisions, no mechanical fuel pump boss, standard deck height. If it was a large boat that had a heavy duty setup it might have been the tall deck variant. This is the most likely one to find in the majority of boats of that era though:
https://www.perfprotech.com/new-199...tmDjLVkuAiuuYDvX1SVxvgNbhK4nBtYkC-meYgew8T0Z8

Honestly, if he still has all the boat parts... I would put the whole thing in just like it is and use an 0411 LS computer to run the factory marine fuel injection. The marine MPFI that they used on those is pretty legit. Pics always help as well.

x75x75x75 · Thursday at 12:08 PM

checked the engine short block, looks to be a 2007 model, standard height, no pump boss and no clutch setup. I will try to use as much of my 1984 454 as I can, clutch pivot might be challenging?

Also may run a standard hydraulic cam I already have, no roller cam due to expense.

Thanks to all for input.

Jrgunn5150 · Thursday at 12:48 PM

https://www.jegs.com/i/JEGS/555/601026/10002/-1

nvrenuf · Thursday at 1:05 PM

FYI, I’m pretty sure the Mark VI blocks require conversion parts to run a flat tappet cam.

85+ square bodies used hydraulic clutches which might make things easy (but not cheap).

x75x75x75 · Thursday at 1:52 PM

Thanks Jrgunn - I was hoping something like that existed, I fabricated a similar mount out of angle-iron to install a Windsor block in my 58 Ford 5-speed.

Ricko1966 · Thursday at 2:20 PM

It's actually not that hard to convert your manual linkage pedals to hydraulic then you also have options as to what style clutch slave to use.

Bextreme04 · Thursday at 4:35 PM

nvrenuf said:
FYI, I’m pretty sure the Mark VI blocks require conversion parts to run a flat tappet cam.

85+ square bodies used hydraulic clutches which might make things easy (but not cheap).

You shouldn't need anything to run a flat tappet. The Gen VI just has the machined boss on the front to run the cam retaining plate and it has the tops of the lifter bores flattened for the dog bones. There should also be the three threaded bosses for the spider retainer in the lifter valley. None of that will interfere with just running a standard flat tappet cam and related hardware. I would be more concerned with how much it is going to cost when that flat tappet cam wipes itself out though and takes the rest of the motor with it. I'm curious how this motor is only a short block though. No heads? No cam?

Might be more cost effective to just get a donor L29 locally if you can find one. Everything will be a direct bolt in. Bonus points if it came out of a manual trans GMT-400, as then you would have the correct flywheel and slave setup also.

x75x75x75 said:
checked the engine short block, looks to be a 2007 model, standard height, no pump boss and no clutch setup. I will try to use as much of my 1984 454 as I can, clutch pivot might be challenging?

Also may run a standard hydraulic cam I already have, no roller cam due to expense.

Thanks to all for input.

Keep in mind that a Gen VI has a completely different crank pattern and has it's own unique required externally balanced harmonic balancer and flywheel. The weight is similar, but slightly different, in placement and amount to the Gen V BBC and completely different than a Mark IV counterweight. You will need Gen VI specific flywheel and harmonic balancer at the very least.

nvrenuf · Thursday at 5:34 PM

My understanding is the Mark VI cam has a step on the front for the retainer plate and the timing gear is different because of this, to run a Mark IV type flat tappet cam you’d need the correct cam gear and a button since the retainer is no longer used.

Bextreme04 · 2026-06-19T11:48:57-0400

nvrenuf said:
My understanding is the Mark VI cam has a step on the front for the retainer plate and the timing gear is different because of this, to run a Mark IV type flat tappet cam you’d need the correct cam gear and a button since the retainer is no longer used.

No. All of your stuff just needs to match. If he had a factory style hydraulic roller cam installed already, he could just keep that. If he has no cam installed... it is just like you were putting any cam into any engine. The only difference between the Gen VI and Mark IV is that the Gen VI has the mounting provisions. You would need a button for a retrofit hydraulic ROLLER cam... not for a flat tappet cam. A flat tappet cam does not need a button or retainer, since the lifters themselves drive the cam back into the block. Cam retainer plate and factory roller specific timing setup is required if using a FACTORY hydraulic roller setup(which is actually way cheaper than trying to buy an aftermarket setup). For my Gen VI, I just used the factory roller cam and sent it off to Delta Camshaft to be reground with more lift and a better towing profile. It was $140 and two week lead to have it reground to my specs and shipped back to me.

Marine 502 into 1984 GMC 2500 454

x75x75x75

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