Head Gasket Hole Sizing?

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nm9stheham

nm9stheham

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I've always use stock Felpro head gaskets but this time we are heading for some .045" thick Cometics. Our bore is 4.060" and we can get either 4.080" holes or 4.120" holes in the Cometics. (Or even 4.180".....) Is there a rule of thumb for how far back from the bore the head gasket's edge should be? I am thinking 4.125" holes, so any bore misalignment can be accomodated, but have never had to make this particular choice.
 
I don't know if there's any rule of thumb but I recently used a set of Mr. Gasket MLS gaskets with a bore size of 4.060 on my engine that has a bore of 4.030 with no issues. When I ordered them I was a little leery since that's only .015" per side but figured I'd check things real close and the bores were dead on center (should be since I put a ton of machine work into the block). I wouldn't bother go any bigger than 4.120 If I were you
 
I'd want the gasket bore dia no more than a hair larger than the cyl bore dia so no excessive crevice volume mixture that doesn't get burned. not sure if the fire ring can overhang safely at all & if not & if you got the smaller one & if was off a bit you may or may not be able enlarge slightly the bolt holes to where you can shift it in one direction to where all cyls are dead on. if the gasket holes are dead on as is then I definitely would want the smaller gasket bore gasket but a person would have to make the purchase (1 of em at least) to find out. Cometic is good but pricey & for that kind of money I would expect em to be dead on! Do ya feel lucky? well do ya. Holler how it turns out
 
Rapid Robert said:
I'd want the gasket bore dia no more than a hair larger than the cyl bore dia so no excessive crevice volume mixture that doesn't get burned. not sure if the fire ring can overhang safely at all & if not & if you got the smaller one & if was off a bit you may or may not be able enlarge slightly the bolt holes to where you can shift it in one direction to where all cyls are dead on. if the gasket holes are dead on as is then I definitely would want the smaller gasket bore gasket but a person would have to make the purchase (1 of em at least) to find out. Cometic is good but pricey & for that kind of money I would expect em to be dead on! Do ya feel lucky? well do ya. Holler how it turns out
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agree w/ Robert, the smaller the better, just don`t get them into the cyls., piston collision will hurt you!
 
If the gasket bore is too big, then you will collect carbon deposits in there and they may get a hot spot and cause pre-ignition. Try to keep the gasket hole and the bore close together without hitting the piston...
 
Have always wondered about this myself. Have noted alot of people using or recommending the Felpro 1008 previously however it does have the huge gasket bore diameter of 4.180"
Why so big?

Cheers,
Teedog
 
Thanks for all the replies and the good points. As for the Felpro 1008's, I don't have any idea either why they are so large. As for the edge gap, I have taken apart my racing 4 bangers and not ever found carbon build up. But, it might become a problem for any engine with excess fuel or oiling or or carbon fouling.

I think we are going to put on a 1008 and measure how concentric the bores are and use that as our guide to see if we can use the 4.080" or the 4.125" Cometic. The .032" gap of the 4.125" does not seem all that large.

Edit to update: We put the Felpro 1008's on and found that not all the bores were perfectly centered on the gasket holes. Cylinders 2-4-6-8 were pretty good, but 1-3-5-7 were all off a bit vertically. So 4.125" it will be.
 
nm9stheham said:
Thanks for all the replies and the good points. As for the Felpro 1008's, I don't have any idea either why they are so large. As for the edge gap, I have taken apart my racing 4 bangers and not ever found carbon build up. But, it might become a problem for any engine with excess fuel or oiling or or carbon fouling.

I think we are going to put on a 1008 and measure how concentric the bores are and use that as our guide to see if we can use the 4.080" or the 4.125" Cometic. The .032" gap of the 4.125" does not seem all that large.

Edit to update: We put the Felpro 1008's on and found that not all the bores were perfectly centered on the gasket holes. Cylinders 2-4-6-8 were pretty good, but 1-3-5-7 were all off a bit vertically. So 4.125" is will be.
Click to expand...


I use the 1008 on most of my engines, but if you have a choice, try to get a gasket that matches the cylinder and head alignment best.

The carbon build up is usually not a common problem. I've torn down some engine that had it and some that didn't and to different degrees. It usually won't cause a problem, but worst case it can get a "hot spot" and cause preignition.
 
For those that say (in my case) are dealing with a 0.020" over 318, the 1008 gasket is just too massive. I'm not seeing a lot of alternatives for me in the 0.035" to 0.040" thick range other than the rather expensive cometics. Felpro 1008 is their supposed one size fits all I guess.
Sealing Alloy heads here.

Teedog..
 
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