Head Gasket
 

Does anyone make a .021 composite head gasket? I thought someone did but I can't locate it. Can I use a copper shim gasket. I've found one that is a 4.25 bore and a .021 thickness. This would help increase the compression on 454 to 8.9 vs the stock 8.6. Another question why is the 454 mag listed as 8.6 compression where as the Merc 400, 420 and 425 are all 8.8:1. It was my understanding that all of these engines used the same heads and pistons. Any idea as to the different compression ratio?

if it makes any difference I only boat in freshwater.

http://www.cometic.com/ call them on monday

I think the thinnest cometic makes is a 0.027.

If you want a cometic, let me know. I can probably get it for you cheaper. My friend Dan is a dealer.

Hey Mike, how far in the hole are your pistons? Have you had your heads cc'd?

I'd almost rather go for a good quench distance, rather than using the gasket to play with compression. Granted, I opted to go with thicker head gaskets, because my heads had been milled a bunch. I was told by a few knowledgeable builders the added quench area isn't a huge deal with blowers.

In the merc parts book, it lists the same part # for the 365HP and 400HP pistons. I agree the heads should be the same. Even the head gaskets are the same part #. I dont see how they'd be different.

Joe-
I agree and have been told that quench isn't as big a deal as a lot of people claim. I need to measure how far my pistons are in the hole but to my knowledge the block or heads have never been milled so I'm guessing the pistons are .02 in the hole. I need to have the heads CC'd as the actual chamber size on the stock 188 heads can vary.

wjb21ndtown-
If you don't mind see what your buddy can get the .027 gasket for.

Just curious, how much do you expect to gain by raising the compression 3 tenths ?

Yes, you are correct. .027 Just installed on my Gen 6 blocks. While you are at it, price him out a set of exhaust gaskets too. Like the head gaskets, good stuff.

GPM-
The cam I'm running I should really be around 9.2-9.5 so I would rather be .5 off vs 1 full point.

Rotax- Good idea, I will need exhaust and intakes as well.

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