391 heads and gasket

[Robert D.]

Im in the middle of a rebuild of my L-71 motor when i cc'd the heads one was 103.5 chambers and the other was 107cc which is stock. obviously ideally they should be equal. my options are to take 18 thousanths off the good head and increase the compression ratio to around 12.5 which id rather not do, or use a thicker head gasket on the other side. another answer would be to open the chamber on the smaller side. which option would you guys use any other ideas welcome and are the head gaskets judged? if not id rather use better gaskets also.
thanx
Bob

[Richard M.]

Re: 391 heads and gasket

Bob,

It's unfortunate to have to decide whether or not to mill the "stock" head 0.018" to match the other side. Is that head now fine? Are you certain that it does not require any decking at all?

Also, continued head milling can lead to intake fit problems and require the intake to be shaved. I've gone through this on a L71 recently and it was a bit frustrating. The main objective was to replace good existing heads, which were dated incorrectly, with a replacement set properly dated. The replacements needed to be completely rebuilt and decked, and I then had intake fit problems after all of this was done and had to shave the intake to fit. This is not uncommon with old engine parts, but still frustrating.

Standard Felpro Composition gaskets have a compressed thickness of apx 0.039". There are also other type gaskets of various thicknesses. I investigated using the copper "shims" for another build but decided against at that time. I would think if you are planning to be judged that too thick of a gasket/shim would be noticed, but how much may be minimal.

Personally, I would take the deduct for that tiny snapshot in time(judging), which may not even be noticed, and go with a approach that would continue to be a mechanically sound and reliable result for the future.

In other words, I would use a combination of shims and/or gaskets on the decked head to attain equal compression ratios and not even think about milling a stock head just to match a decked head. Bringing the decked head back to stock height would also eliminate any requirements to shave that rare and expensive 3x2 intake manifold.

One caveat is that a future owner would not have knowledge of all of this unless it was documented. But this happens often and is very difficult to pass on when these change hands, particularly through unconcerned sellers.

Rich

[Robert D.]

Re: 391 heads and gasket

Thanx Rich

Thats kind of the way Im swaying to. This is after the heads were decked the heads were very straight so no more will be needed. i think i will go with the 2 different size gaskets. you mentioned felpro composite gaskets, were the original gaskets composite or were they metal the ones i ordered from long island corvettes came as steel shims

thanx
Bob

[Richard M.]

Re: 391 heads and gasket

Bob,

Composites were not used originally. Originals were metal and very thin. I forget exactly but IIRC around 0.013" .

Summit and others have thick copper shims, I've seen up to 0.080" thick, but I've never attempted to use those. Must be for those very rare occasions when a rare block needs salvaging.

I'd also like to hear opinions from the very knowledgeable engine folks here. There may be other options for you.

Rich

[Duke W.]

Re: 391 heads and gasket

It's not clear to me. Are you saying one head is 103.5 cc on all chambers and the other is 107 on all chambers?

There is more to CR than head chamber volume. Deck clearance is a big factor. Did you measure it before you disassembled the engine?

Has one of the heads been field machined? OE heads will have broach marks similar to the block deck. Field machined heads will show typical circular tool marks.

You should shoot for a final, true. measured CR of no more than about 10.3, and it's best if you can keep the max/min to 0.1.

See the Fall 2009 Corvette Restorer for an article that explains how to manage your compression ratio. If you don't have that issue, contact me via email through the TDB, and I will send you a pdf of the article. It exceeds the allowable file size that can be uploaded to the TDB.

Duke

[Robert D.]

Re: 391 heads and gasket

Duke yes one head measures 103.5 cc on the chambers and the other 107cc we took several measurements and used diamond piston compression ratio calculator deck clearance was just one of many number we plugged in as well as dome volume hieght to the first ring gasket volume ect. the 103.5 chamber came to 12.3 compression and the 107c chambers came to 11.6 compression ratio. when i calculated a 040 head gasket on the 103.5cc side they equaled out to within an acceptable range closer to the 11.6.

Rich i have a set of metal shims and they measure .022 ucompressed and jegs has several metal shims at .040 which should ae up the .018 i need

thanx guys
bob

[Duke W.]

Re: 391 heads and gasket

I recall the NHRA minimum chamber volume is 106.5 cc and that allows for a small "cleanup cut", so I expect both heads have been milled. The volume for the OE pistons specified in the F-M catalog is 36.3 cc, but John McRae measured a smaller value, which he posted a few years ago. You can search or contact him.

Rather than using thick head gaskets, you should select pistons with a smaller dome or cut the dome on the OE replacement pistons.

If you plan on using pump gas, 11.6 is too high and will detonate. I recommend no more than 10.3, which usually can be achieved using OE replacement components and a slightly thicker than OE gasket as long as the block and heads haven't been cut.

Duke

[Joe L.]

Re: 391 heads and gasket

Bob,

Composites were not used originally. Originals were metal and very thin. I forget exactly but IIRC around 0.013" .

Summit and others have thick copper shims, I've seen up to 0.080" thick, but I've never attempted to use those. Must be for those very rare occasions when a rare block needs salvaging.

I'd also like to hear opinions from the very knowledgeable engine folks here. There may be other options for you.

Rich

Rich------

Yes, steel shim-type head gaskets were used on all big blocks with cast iron heads. However, big block applications using aluminum heads used composition head gaskets.

[Richard M.]

Re: 391 heads and gasket

Rich------

Yes, steel shim-type head gaskets were used on all big blocks with cast iron heads. However, big block applications using aluminum heads used composition head gaskets.

Thanks Joe. I forgot about the aluminum head engines. Do you know what the CT was on those composition gaskets? Just curios.

Rich

[Joe L.]

Re: 391 heads and gasket

Thanks Joe. I forgot about the aluminum head engines. Do you know what the CT was on those composition gaskets? Just curios.

Rich

Rich------

My recollection is they were about 0.040". I'm out-of-town now so I can't check.

[Robert D.]

Re: 391 heads and gasket

Thanx guys
i dont mind the extra compression because i usually use vp vintage fuel in the car anyway because i have hard start problems once the car is hot and the vp fuel seems to fix that amongst what some people believe it actually works in my car. the thicker gasket will bring me down to low 11's which the vp fuel should be fine with.
thanx again
bob