As seen in Speedboat Magazine’s
Jan/Feb 2025 Issue.

Mercury Racing 500EFI Rebuild Upgrade

Dear Bob:
I own a Formula 382 FasTech with a pair of Mercury Racing 500EFI engines that have about 500 hours on them. The engines were refreshed by a reputable machine shop in my area after they were removed and replaced by the marine shop that normally services my boat.

After about 35 hours of use since the rebuild, one of the engines failed because an exhaust valve seat fell out and broke into pieces causing quite a bit of damage. At this point, I do not trust the other engine either because it is making a ticking noise that sounds like a lifter. I want to keep the engines because it is a good combination for my boating, but I would like to be sure they can still be reliable. What suggestions do you have to enhance the reliability of the 500 EFI engine packages?

Chris Rankin
Michigan City, IN

The Mercury Racing 500EFI engine was the next generation of the HP500 carbureted offering. The engines are similar except for the camshaft, valve springs and induction system. The original engine is based off the same engine as a 502 Magnum with some upgrades to the valve train, including Manley Severe Duty intake valves and Extreme Duty Inconel exhaust valves. Both engines used a Crane 8620 core hydraulic roller cam with Crane lifters, Manley push-rods, and Crane rocker arms. The 500EFI engines were equipped with Comp Cams 929 valve springs.

The HP500 carbureted engines used the GM connecting rods and complete rotating assembly. The later versions of the 500EFI’s had after-market connecting rods and H series main and rod bearings. Both versions of the engine still utilized pressed fit wrist pins instead of the more desirable full floating wrist pins with bushings in the connecting rods.

Some engine building machine shops that specialize in automotive engines are not used to what it takes to make marine performance engines reliable. Engine bearing clearances are usually greater than standard automotive clearances. Valve seats should be wider to facilitate valve cooling. Valve spring on-the-seat pressures should be higher because of the constant high RPM use. Valve material is critical and should also include Inconel or better exhaust valves with the highest quality intake valves.

1: Mercury 500EFI going together
2: Manley 14060R Rods
3: Close-up of Manley 14060R Rods
4: Melling Oil Pump with Welded Pick-Up
5: 61559TCM Edelbrock Heads

Your 500EFI is based off the Gen VI GM 502 engine that is provided with rectangular port GM iron heads. The heads have Stellite exhaust seats, which have the tendency to fall out—especially in instances when the head has gotten hot at some point. The reliability is also hampered by the increased combustion temperatures that occur as a result of current fuel blends. For some unexplained reason, it seems like these failures occur more often after heads have been rebuilt. If you want to continue using the iron heads, you need to make sure they do not have cracks. It is common to have cracks develop between the intake and exhaust seats which in turn could also contribute to the exhaust seat falling out. If the heads check out ok, it would be important to replace the exhaust seats with a seat near 50 Rockwell C (RC) with a tighter interference fit to make sure they don’t fall out. We use Tucker Valve seats on all our exhaust seats. We currently avoid using iron heads on performance engines because the aluminum heads dissipate heat more effectively, and there is a greater number of options available in aluminum.

We have developed an aluminum head with Edelbrock that is designed to replace the iron heads on all the Mercury Racing 454 and 502 cubic inch based engines, which would also include any generation of Big Block Chevy engines. The base part number is 61559TCM.

The head castings are hard anodized for corrosion protection and come with wide 45- degree valve seats for increased valve cooling. The finished head assemblies are provided with Ferrea Competition Plus intake valves and Super Alloy exhaust valves. Isky Tool Room Valve springs are used with tool steel retainers. Complete assemblies include Comp Cams guide plates and ARP rocker arm studs. The exhaust ports are not raised so using them will not require modifications to your exhaust system and tail-pipes. The heads have the OEM accessory mounting holes that replicate the original iron heads.

The complete rebuild and upgrade of the 500EFI includes a multitude of steps. First, the engine and all components are inspect-ed during disassembly. The original bore is 4.467″ and in some cases can be honed to 4.470” to accommodate a replacement piston available from JE Pistons, which is part number 281916. The replacement piston is made with 4032 material, which is similar to the stock GM piston. The piston uses standard rings (1/16″, 1/16″, 3/16″) and can accommodate the use of double spiral locks for full floating wrist pins. If the engine block needs to be bored, JE Pistons part number 281919 is the same piston for a 4.500” bore. We also have a higher quality CP Piston in the same configuration that is made with 2618 material for heavy duty applications and has a more desirable ring land spacing for strength.

We don’t usually reuse the connecting rods because it is cheaper to replace them with new rods that are bushed for full floating wrist pins, compared to the cost of modifying the existing rods. Manley 14060R connecting rod is a good choice that comes with ARP 2000 rod bolts. Trend H-13 wrist pins are a good match with a .180″ wall thickness. Pistons and rods should be pin fitted to .001″.

Your main bearing clearances should be about .003″ and slightly more on the rear main bearing. The rod bearing clearances should be about .0028″ to .003″. Clevite H series bearings are available in various sizes to achieve the correct clearance (standard, -.001, HX +.001). It is commonly required to mix bearing sizes to obtain the correct clearance. Once you have the bottom end done, then the whole rotating assembly needs to be re-balanced which also requires including the damper and flywheel because the engine is externally balanced.

6 Melling Oil Pump with ARP Drive Rod
7 Cometic Head Gaskets and ARP Head Bolts
8 Clevite Rod Bearings
9 Johnson Drop-In Lifters 2510R-T

Replace the oil pump with the higher level Melling 10778 pump. This is a high volume pump with chromemoly shafts that are supported in the cap. You will be required to carefully remove the oil pickup from the old pump and weld it into the new pump. If you cannot get the old pickup out, Melling sells a replacement (part number 241S). Set the pump up on the main cap and position the pick-up so that it is level and proper clearance from the bottom of the pan is ensured. We recommend using the ARP #RPD6-520-1 oil pump drive shaft for maximum reliability.

Replace the cam bearings with Durabond CHP12 bearings. If the cam needs to be replaced, part number 169621-TCM is the 8620 core replacement cam. The GM timing chain kit number 12371053 is the stock replacement and the Rollmaster CS2095 is a double roller upgrade timing chain kit with an IWIS chain.

The generation of the head gaskets must match the generation of the cylinder block. In
this case, even though the Edelbrock heads are suitable for Mark IV and Gen VI engines, the head gasket needs to match the block generation for proper coolant flow. Cometic C5083 series head gaskets in the proper bore are the best choice. Usually it is best to have a gasket that has a bore diameter .010” larger than the bore. The standard head gasket is .040” thick, but a thicker head gasket might be desired if the was significant surfacing required on the deck. Usually, if the block is surfaced to about 9.795” and a .050” head gasket is used with the 60559TCM Edelbrock head, the compression will be about 8.9:1 on and 4.500” bore size.

We only use Johnson Hydraulic Roller Lifters. Part number 2510 is the “drop-in” style that uses the “dog bone” retainers. Trend .080” wall push rods are available in .025” length increments. If tie-bar style lifters are desired, the Johnson Part Number 2116BBR-T is the best choice, but you will have to provide clearance to the valley windage tray to prevent the tie bar from contacting it.

The combination of the Melling 10778 oil pump, the Johnson Lifters, Trend push rods, Isky Tool Room valve springs with on-the-seat pressure of about 175 pounds, using ARP RRS-4IS rocker studs and Comp Cams 1820 rocker arms is a recipe for reliability and endurance. The oil pump high pressure provides support for the short travel lifters that are opening valves with greater spring pres-sure which prevents valve float and dancing on the seats.

10 Project cart for Mercury 500EFI rebuild
11 Comp Cams 1820 Rocker Arms

In final assembly, do not omit the wind-age tray in the lifter valley because the PCV valve is located in the floor of the intake manifold and will suck oil out of the motor without the tray in place.

Rebuild your water pump and make sure the fuel pump is not leaking fuel into the oil chamber on the water pump. The oil in the water pump housing is the same as your drive oil and it should be filled to the sight hole in the side of the housing.

It is absolutely necessary to clean and flow you fuel injectors. And make sure that you replace the in-line high pressure fuel filter, which if dirty, could restrict fuel flow.

Make sure you pressure check your CMI E-Top headers to ensure they do not have internal leaks. Replacement may be necessary if there is a leak that cannot be fixed. Your leaking headers can destroy your new engines.

Replace the distributor cap and rotor and ignition wires. The spark plugs for the new heads will be different. NGK R5671A-6 or A-7 would be a good choice.

High quality synthetic/blend or full synthetic oil should be used. A couple good choices are Mercury Racing 25W50 or AMSOIL Dominator RD50, which is a 15W50 full synthetic oil.

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Check out the Teague Tech Library for diagrams and schematics of how to correctly install and perform various repair and upgrade projects. At Teague Custom Marine, we don’t ‘just sell parts’, we know how to install everything we sell. When it’s time to tackle a project, count on Teague Custom Marine to have the part and the answer.