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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Sorry for taking so long. He just got back to me today. He has been out with the flu. His price for cometic head gaskets is $70 per side, but his warehouse only stocks the 0.040 gasket. It may be more to have one ordered, but I may be able to have it sent right to your house if I place an order. Let me know if you want me to look into it more.


FWIW - did you ever just consider milling your heads down 0.030-0.050? You wouldn't hurt them at all, you could dial in your cc size, and it's not all that expensive (my machine shop charges $80 for 0.020, and they'll do the next 0.020 for less). You should be able to do some light bowl blending yourself, better protect yourself from detonation issues, and have a more dialed in motor. Just my $0.02...

The price sounds good. When it gets time to install the heads I'll let you know. As to the machining, I think I am going to have them milled a bit.. I need to CC them and see where I'm at.

Thanks for looking into this for me.

A 454Mag has a comp ratio of 8.75:1 and so do the 400, 420, 425. They are all 454's and use the same pistons, head gaskets and heads.

I would just mill the heads .020....in fact that is exactly what I did do with the 454mag I had in my 24 OL.

if he mills the heads,should intake also be milled??

not for only .020''

For .020, the intake will fit fine and in most cases .030 is fine.

Sounds good, thanks guys!

I believe roughly for every .010 milled you reduce chamber size by 2c. If you milled them .020, you should gain about .25 compression.

Griff, what kind of performance gains did you get out of this?

I did a lot of other stuff at the same time also. I did a complete top end with cam, carb, roller rockers, and Imco exhaust. The engine was very similar to Merc 420 when I was done.
I picked up 400-500rpms and about 5-6mph at WOT. Probably gained 60-70hp or so. I figured the milling was maybe good for 10hp.

I gave my specs to somebody else and he dynoed his engine at 425hp at 5000rpms. The only difference was he also changed intakes. I was spinning mine at 5400rpms.

My buddy had a pair of 370's, that were rebuilt and upgraded at some point years ago.

They installed crane 134561 cams (236/246 .553/.555 114lsa), and roller rockers. however, they left the stock cast iron manifolds, stock mag intakes, and quadrajet carbs. One of the engines dropped a valve, so we are rebuilding it, but now adding stellings headers, holley 750DP, single plane intakes. I think the engines should wake up with the new exhaust, intakes, and carbs. hopefully. That was alot of cam for stock exhaust.