Title: 4G63 forged build clearance advice – 94 mm crank / 153 mm rod / Manley Extreme Duty pistons / E85 / 40 psi setup

Hi everyone,

I’m about to build a forged Evo 4G63 and I’d like some advice on clearances and head gasket choice for my combination.

My setup

• Mitsubishi Evo 4G63
• Crankshaft: 94 mm stroke
• Connecting rods: 153 mm rods
• Pistons: Manley Extreme Duty 617005CE-4
• Fuel: E85
• Target boost: around 40 psi

1) Piston-to-wall clearance

What piston-to-wall clearance would you recommend for a 4G63 with Manley Extreme Duty pistons, 94 mm crank, 153 mm rods, running on E85 with a 40 psi boost target?

I know the final number depends on piston alloy, intended use, fuel, and operating temperature, but I’d like to know what clearance range you would normally run for a high-boost Evo build like this.

2) Main bearing clearance

What main bearing clearance would you recommend for this combination on a forged 4G63 that will see high cylinder pressure and around 40 psi boost?

3) Rod bearing clearance

What rod bearing clearance would you recommend for this setup with the 94 mm crank and 153 mm rods?

4) Head gasket recommendation with 1/2 inch head studs

I will also be using oversize 1/2 inch head studs.

What type of head gasket would you recommend for this setup? Specifically:

• MLS vs copper vs other options
• recommended gasket bore size
• recommended gasket thickness
• anything special to consider when using 1/2 inch head studs on a high-boost 4G63

Thanks in advance.

I Haven't personally used that piston kit so I would recommend you reference Manley's instructions. For a high boost application like your one however, I'd expect to end up in the 0.0035"-0.004" range for piston to wall clearance. For the bearing clearance I always ran 0.0025" on the mains and 0.002" on the big ends. I'd also recommend the Athena cut ring head gasket for this application. You haven't specifically stated what the engine will be used for but if it's something you want to keep streetable, I personally would stay away from copper gaskets. It's very difficult to prevent them leaking oil and water. The other option which I've used for this sort of setup is an HKS stopper-type MLS gasket paired with a stainless o-ring in the block with very light protrusion - Say 0.005-0.006"

Thanks very much for your detailed reply—I really appreciate your advice.

The engine will mainly be used for street driving, but I also plan to take it to the drag strip occasionally. My goal is to build a reliable engine that can handle around 40 psi on E85 while remaining dependable for street use.

Your recommendations on the clearances and head gasket options are very helpful. I'll also compare the piston-to-wall clearance with Manley's specifications before making a final decision.

Thanks again for taking the time to help.