Cometic 86mm K20 MLS Head Gasket C4300

Cometic 86mm K20 MLS Head Gasket - Part #C4300-040 / C4300-045

This is the head gasket for a K20 at its stock 86mm bore: an RSX, an EP3, or a K20A2/A3/Z1 swap that hasn't been bored or sleeved. Cometic's 3-layer MLS in .040 or .045 compressed, which is what you reach for when the block has been decked and you need to put the compression and valve clearance back where they belong.

What It Is and Why It Works

Cometic's MLS gasket is three layers of stainless steel: two embossed outer layers coated in a Viton fluoroelastomer that seals against the block and head, and a center shim that sets the compressed thickness. The Viton handles 250 C, the stainless springs back after every heat cycle instead of taking a set, and the layered design tolerates the slight movement between an aluminum head and block that kills a composite gasket. It's the same construction Honda uses from the factory, done with more clamp margin.

Thickness is how you set compression and piston-to-valve clearance after a deck cut or a piston change. Thicker gasket, lower static compression, more clearance; thinner, the reverse. Cometic sells the same gasket in several thicknesses for that reason, and the bore has to clear your actual cylinder bore, not the one the engine left the factory with.

Specs

Fitment

86mm is the stock K20 bore. The K24 is 87mm from the factory and does not take this gasket; use the 87mm C4311. The K20Z3 (06-11 Civic Si) uses a different coolant passage layout and takes Cometic C4561 instead.

What to Know Before You Buy

Surface finish matters more than anything else. Cometic wants the block and head decked to 50 RA or smoother; an MLS gasket on a rough or corroded deck will weep coolant no matter how it's torqued. Have the head checked for flatness and skimmed if it's been overheated.

Pick thickness by what you're correcting. Stock K20 gaskets are roughly .030 compressed; .040 drops static compression about a quarter point on a stock K20A2 and buys clearance after a light deck cut, .045 a bit more. If you haven't cut anything and aren't chasing clearance, the .030 C4300-030 is closer to stock.

Measure the bore. A block that's been honed a few thou is fine; one that's been bored to 87 needs the C4311.