3830711 SB head gasket

[Duke W.]

3830711 is the replacement head gasket listed in my '77 vintage Corvette parts catalog, and according to gmdirectparts.com this shim-type gasket is still available, and they list compressed thickness as .026".

According to the Chevrolet Power Manual the OE SB gasket compressed thickness is .018", so maybe the replacement gasket is thicker to compensate for typical head and deck machining when engines are rebuilt, but .026" is very thick for a shim type gasket.

If anyone has one of these can you measure the thickess. If it's unused, just measure the thickness of the material where is it flat and not formed. Also, does it have any kind of coating?

I'm developing a comprehensive listing of head gasket thicknesses that will be an aid to managing compression ratio on rebuilds. About .010" difference in thickness changes the actual static CR by about 0.25 (and deck clearance variation the same effect).

Right now it looks like head gasket increments of about .005" are possible, which is good.

Duke

[Michael H.]

Re: 3830711 SB head gasket

Duke,

Pretty sure the GM numbers are incorrect and have been for decades. If I remember the numbers correctly, the standard steel gasket was always in the range of .024"-.026". Even the 236? gasket found in the HD pages that was supposed to be something like .016"-.019, always measured around .022".

I'm not sure what an original from the 60's would measure. Seems like I've done that years ago but I don't remember the numbers now. I'll see if I have any originals from that era.

[Duke W.]

Re: 3830711 SB head gasket

LOL! I guess I wouldn't be surprised if the gasket thickness has been incorrectly specified for "decades". If that turns out to be the case, just add it to the other decades old errors like the 385 ballast being listed as .03 ohms instead of 0.3.

I installed double 3916336 (Did you mean "336" instead of "236") stainless steel shim gaskets in my engine that are specified as .017" compressed, but I don't recall measuring them.

I think they are discontiued, but if you have one laying around can you measure it?

Duke

[John M.]

Re: 3830711 SB head gasket

Duke,
Do you know what the thickest stock gasket might be for a BB? As you know, my 69 427-435 has given me a lot of problems with detonation on pump gas. The engine has been decked enough to obliterate most of the stamp pad characters and I was thinking of taking the heads off this winter to have a look. Maybe if I could get a very thick gasket the problem could be gotten rid of or reduced.

[Tracy C.]

Re: 3830711 SB head gasket

Duke,

I have a pair of used "assumed to be original" metal head gaskets at home. These came off a 66 250 hp 327 (Impala). I'll slap a micrometer on one and post a picture later this evening.

tc

[Duke W.]

Re: 3830711 SB head gasket

No, I haven't researched BB gaskets, but the Felpro line offers compressed thicknesses from .015" to .051" for SBs, so I expect they have similar offerings for BBs.

Dukue

[Duke W.]

Re: 3830711 SB head gasket

Thanks. Give them a careful inspection to see if there are any kind of ID marks- symbols or numbers. If any, they might help verify the gasket origin.

Duke

[Michael H.]

Re: 3830711 SB head gasket

Duke,

Yes, you are correct. The number on the "thin" SS gasket is 336, not 236. Some periods of mild brain fade occur at times now.

It's possible that the 3830711 may have been aprox .024-.026" since it's introduction in the early/mid part of the 1963 model run. I know that cars in at least the early part of that MY were equipped with two head gaskets but, supposedly, at some point in production this ended and one gasket was used. The new 3830711 may have been part of why the the change occured. The original 1st design gaskets used through the 62 MY and into the 63 MY were most likely the .016" dim that's been listed ever since. (I don't have the previous design part number)

Could be that this .010" increase in gasket thickness was supposed to reduce the C/R to around 11-1 instead of the 11.25-1, which is just about what it would do. (actually, the .010" reduction is closer to .32 in C/R, not .25)

If you happen to have an engine on a stand, it would be interesting to find the actual volume of the combustion chamber with the head installed and the piston at TDC. If you have a CC burrett, and I'm sure you do, would it be possible to provide the volume with one, then two head gaskets installed? We could calculate the difference using the volume of .010" at a 4" dia, which is roughly .125 cubic inch.

Still looking for some original 3830711's in an old package.

[Clem Z.]

the SS ones were .017 compressed

the one in the catalog now i bet is a rubber coated shim gasket and that would account for the thickness

[Duke W.]

Re: 3830711 SB head gasket

Nope, engine's in the car. I'm not sure how you could cc the entire chamber as the fluid would leak past the ring gaps.

My mid-production '63 was double gasketed, but the originals are long gone.

I always figured the CR spec reduction to 11:1 in '64 was due to the slightly larger chamber because of the unshrouding cut for the larger valve set, but in that case the '67 L-79 should have been a little lower because of the additional chamber volume from eliminating the plug-side quench zone on the 462 head.

In 1967 300 HP CR was reduced to 10.00 from 10.5, which would be about 4 cc if everything else remained the same. GM's "specified" compression ratios appear to be about as accurate as SAE gross horsepower and their "specified" rocker arm ratio, and it appears that they rounded or truncated to the nearest quarter point.

Duke

[Michael H.]

Re: 3830711 SB head gasket

Well, ya got me interested now, so I went to the big pile and dug out some GM numbers. For a 65 350 HP 327, (40.8407 ci in per cyl) GM shows a chamber volume of 3.97 cubic inches. That's with the cyl head installed and the piston at TDC. With a 4.000" bore and 3.25 stroke, the C/R would be 11.287-1, or slightly above the advertised 11.0-1.

If I use all of these values but add .010" to the head gasket dimension, (.026" instead of .016") The C/R falls to just under 11-0, or 10.973-1. I don't know if these numbers are actual fluid measurements or calculations using the dim's of all the components.

Unless very thick fluid is used to CC the completed engine chamber, it would indeed run past the rings. We always filled the top land with grease and added a spec amount to the volume total in the calculation but I don't remember the amount. Wouldn't be difficult to figure out though.

I'll see if I have any info for 63 and 64.

I agree, the reduction in C/R for 64 was most likely due to the unshrouding of the larger valves.

Sixty three #'s 17573 and 19746 had never been apart when I had them. Both had single head gaskets.

[Tracy C.]

Re: 3830711 SB head gasket

Duke,

These are the only markings I found (Mc 6631). They measure .020 thk.

tc

[Verne Frantz]

McCord gasket. *NM*

[Duke W.]

Re: 3830711 SB head gasket

Yup, that has to be a McCord. My circa 1993 catalog lists a 6631M, but it's a composition type. Part number 6910 is listed as a shim type, which might have a coating; 6631 is apparently, according to your example, an uncoated shim type and may no longer be offered - probably replaced by the 6910.

So it ain't "original". The heads have been off that 327/250.

Are both gaskets the same? And what is the bore diameter?

McCord also offers a couple of "performance" gaskets, which are six digit numbers with a 94 prefix, 94-xxxx.

This just goes to show you that EVERYONE should save the old head gaskets until a forensic examination is completed, and measuring the gasket thickness and deck clearance prior to block disassembly will allow you to compute the previous CR, which can be a guide to selecting a new CR for the rebuilt engine.

Duke

[John M.]

Re: 3830711 SB head gasket

Duke, My original, never been torn dowm 67, 327-300 engine head gasket has a steel thickness of 0.014". The bore diameter of the gasket is 4.080. I bought the car from the original owner and am 99% sure it's never been touched.