502 Head Gasket Help
 

Hello all, I just joined the forum and am unsure if this is the right place to post this.

In August 2024, I bought a 1994 PowerQuest 270 Laser w/ a gen. V 502. Come September 2024 I had detonation and put a hole through my #4 piston. Root cause was me running 87 as I was told the motor was stock, come to find out the motor was far from stock and needed 93. I pulled the motor and saw that it had been bored .030 over, had a cam, roller rockers, etc, etc.

I was able to track down the shop that initially did the work on the block as I wanted to use them again to clean the block and heads up. After five months at the shop, I just got my block and heads back yesterday. They cleaned and honed the block as well as did a valve job and resurfaced the heads (0.008" removed).

When I tore the motor down it had a Fel-Pro 17048 head gasket (0.039" compressed height). When I talked to the shop owner he said that 0.008" was not enough to worry about a thicker head gasket, but my fear is that I don't know the history of the motor and I don't know how much the heads have been resurfaced or the block has been decked in the past.

Am I overthinking this and should I just use the 0.039" head gasket? Go up to something thicker? If so, what? Do I need to be concerned about my pushrod lengths?

I have more knowledgeable help putting the engine back together, I just want to make sure that all the parts I am buying are proper.

Thanks in advance for the help.

You need to determine the piston to deck clearance at TDC.

0.008" off is not a lot, but if you're running your quench tighter than at least about 0.040" with forged marine piston to wall clearance, you're likely to contact the head as the piston rocks over TDC when cold.

I like tight quench, but perf marine engjnes need a moderate approach, without going too big.

You may need to mock up the short block to check the piston to deck.

It is unlikely that it will make a difference and may even help. I've had heads decked .020 with stock magnum pistons and ran the .039 Felpros with no issue. To be 100% certain though, you need to measure the piston height vs the deck height. What pistons are in it??
Old thread The squish, or quench thread.. - Page 2 - Offshoreonly.com

FelPro makes a .053 MLS gasket

MPD55,

Cometic makes them in what appears to be about .006-.010" increments, from.027" to over .100" compressed thickness and several bore sizes.

Cometic MLS Head Gaskets CHEVROLET 8.2L/502 Chevy big block Gen VI - Free Shipping on Orders Over $109 at Summit Racing

Not going against what anybody else has to offer, but it would make sense to me to reestablish what Merc had already worked out. Really just replying to follow....

Thanks. Brad.

Thanks for the replies guys, sounds like I need to get the short block together and do some measuring. I just want to make sure I’m safe and don’t run my valves into my pistons.

Pistons are JE 281919, stock piston style, just .030” over.