Oversize head gaskets

Oversize head gaskets

Joined: December 22nd, 2009, 6:12 pm

June 30th, 2012, 6:39 pm #1

I would like to know the ramifications of using a head gasket that is significantly larger than the bore size. For example the SCE Titan Self Sealing Copper gaskets are as large as 4.160 while my block is just forty over stock.
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Joined: January 10th, 2002, 5:53 pm

July 1st, 2012, 12:15 am #2

Standard Felpro Blue gaskets are 4.100 on the sealing ring. You pick is only another .06 larger.

1967 Falcon 4 door 351C-4V
1970 Mustang 351C-2V
http://raceabilene.com/kelly/hotrod
Owner built, owner abused.
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Joined: December 22nd, 2009, 6:12 pm

July 1st, 2012, 5:46 pm #3

I would like to know the ramifications of using a head gasket that is significantly larger than the bore size. For example the SCE Titan Self Sealing Copper gaskets are as large as 4.160 while my block is just forty over stock.
It is probably a stupid question but I just wondered about the "dead air" space if one used too large a gasket. I see a lot of posts where guys are buying thicker gaskets to lose compression. Something I had at one time considered since my -3 cc flat top pistons are not very far down about and my port size is 62cc. I have the Fel Pro gaskets #38347PT1 that are 4.100 and .047 in. thick vs the Z1013 that are .041 thick. Now I have decided just to live with the high compression and keep my quench where it is at. Still I have that extra .06 x .047 all around the cylinder that is under the head so I wondered if I should slightly bevel the cylinders to encourage more flow through that area or not. ICS has a gasket that is .093 thick and for a 4.10 bore. There would be a lot a area under the head for who ever uses that gasket even with a pop up piston.
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Joined: December 23rd, 2004, 3:26 pm

July 1st, 2012, 7:01 pm #4

Not quite sure what you mean by "slightly bevel the cylinders to encourage more flow through that area". That would be a tricky machining job would it not? You could drop the compression way big time, if I am understanding correctly, and then you'd be screwed. And how would this "increase flow"?
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Joined: December 22nd, 2009, 6:12 pm

July 1st, 2012, 9:42 pm #5

I certainly wouldnt do it myself. I would have my machine shop would just chamfer the lip of the cylinder slightly closer to the gaskets diameter, kind of like gasket matching. It would not have to be much. I was thinking that would help pull more of the cylinders volume of gases into the cylinder and help get a more complete combustion of the air/fuel mixture. Also I would tend to think the upper edge of the cylinder bore under the gasket may retain more heat if the gasses there dont circulate.
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Joined: December 23rd, 2004, 3:26 pm

July 1st, 2012, 11:08 pm #6

I just don't trust the machine shop. But this could open up a can of worms, i.e. what angle bevel? Multi-angle? curved, i.e. semi-hemi? here we go...
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Joined: January 10th, 2002, 5:53 pm

July 2nd, 2012, 1:57 pm #7

It is probably a stupid question but I just wondered about the "dead air" space if one used too large a gasket. I see a lot of posts where guys are buying thicker gaskets to lose compression. Something I had at one time considered since my -3 cc flat top pistons are not very far down about and my port size is 62cc. I have the Fel Pro gaskets #38347PT1 that are 4.100 and .047 in. thick vs the Z1013 that are .041 thick. Now I have decided just to live with the high compression and keep my quench where it is at. Still I have that extra .06 x .047 all around the cylinder that is under the head so I wondered if I should slightly bevel the cylinders to encourage more flow through that area or not. ICS has a gasket that is .093 thick and for a 4.10 bore. There would be a lot a area under the head for who ever uses that gasket even with a pop up piston.
The gasket is keeping the fire sealing ring back away from the edge of the cylinder. Modifying the top of the cylinder to reach out to the gasket could result in gasket failure. Do the proper deck prep, install the gaskets and head and run it, quit worrying about it. Using 4.03 bore and a 4.1 gasket, .039 compressed - The difference between .07 inch oversize gasket bore vs .01 oversize (4.04) comes to .02 compression points. How much you want to spend chasing that? By comparison, .06 change in gasket thickness changes the compression by .13.

1967 Falcon 4 door 351C-4V
1970 Mustang 351C-2V
http://raceabilene.com/kelly/hotrod
Owner built, owner abused.
Last edited by Falcon67 on July 2nd, 2012, 2:00 pm, edited 1 time in total.
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Joined: May 20th, 2005, 4:48 pm

July 2nd, 2012, 3:02 pm #8

Only time I worry about gasket bore is when it's too close to the cylinder. If the fire ring gets hot, then it could cause some issues.

Not sure why you're looking at the copper gasket, but a regular Felpro will work in 9 out of 10 motors. If you need something fancy for high compression or forced induction, I'd use a Cometic MLS gasket and buy it to the usual thickness and diameter specs.

Brent Lykins
B2 Motorsports, LLC






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