NOS head gasket thickness? Part Num 3993890 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

davepl · 05-04-2012, 05:20 PM

For my '70 402 we were unable to find a currently offered steel shim head gasket, and so wound up with a 0.040 composition gasket. I'm worried about my quench area, as I have about 0.020 of deck height.

I've located a pair of the original gaskets, part number 3993890, but I cannot find any info on what the compressed height of this gasket is. The parts book doesn't tell me.

Anyone have a reference that tells them what the compressed height of 3993890 winds up being?

Thanks,
Dave

SS Tim · 05-04-2012, 07:09 PM

Dave,
For what it is worth I have an article by Jim McFarland from 1970. It lists HD Chevy part swapping secrets...they are showing a Victor composition head gasket (red) 3935458...
"The piston-to-top-of-block deck height should adverage .0-.005-inch above the deck with a .040 Victor composition head gasket. Deck height may be reduced accordingly for thinner head gaskets. Piston deck-to-cylinder head clearance should be maintained at .035-.040-inch for best results." The numbers don't add up, unless that is the compressed thickness!
But another article in the same issue gives a 0.25 compressed unstated gasket thickness with a ZL-1 head swap. My Motion "cookbook" is somewhere. Digging it out is another matter.

SS Tim · 05-04-2012, 07:21 PM

I see what you mean, here is a Camaro parts clip.

davepl · 05-04-2012, 08:19 PM

Yep, if the 3993890 is about 0.020 then I'll be around 0.040 total, which is where I want to be. With a thicker composition gasket I'll be out to 0.060, lose some compression, and increase the risk of detonation because of poor quench.

The heads are already on (with the fat gasket) because we could not find a steel shim gasket, but if this old NOS one I've found is actually thin, I'm set. Just can't find any confirmation of what thickness 3993890 actually is!

1972RedNeck · 05-04-2012, 10:42 PM

http://www.summitracing.com/parts/FEL-7733SH1/

SS Tim · 05-05-2012, 06:47 PM

Found a Mr Gasket 0.020" shim at Classic.
http://www.classicindustries.com/fir...rts/71131.html
Also a general ref. page from GM Parts Direct.
http://www.gmpartsdirect.com/perform...CATID=381.html

davepl · 05-05-2012, 07:40 PM

Thanks... I had found one similar, but the bore is very large (4.370) compared to the 402 (4.125). I don't know if that really matters, and probably is too thin to change the chamber volume, but I was a little worried about the enormous size of the bore in the gasket. Think it matters>

05-04-2012, 05:20 PM	#1
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	NOS head gasket thickness? Part Num 3993890 For my '70 402 we were unable to find a currently offered steel shim head gasket, and so wound up with a 0.040 composition gasket. I'm worried about my quench area, as I have about 0.020 of deck height. I've located a pair of the original gaskets, part number 3993890, but I cannot find any info on what the compressed height of this gasket is. The parts book doesn't tell me. Anyone have a reference that tells them what the compressed height of 3993890 winds up being? Thanks, Dave __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

05-04-2012, 07:09 PM	#2
SS Tim Registered User Join Date: Aug 2010 Location: Edwards, CA Posts: 7,503	Re: NOS head gasket thickness? Part Num 3993890 Dave, For what it is worth I have an article by Jim McFarland from 1970. It lists HD Chevy part swapping secrets...they are showing a Victor composition head gasket (red) 3935458... "The piston-to-top-of-block deck height should adverage .0-.005-inch above the deck with a .040 Victor composition head gasket. Deck height may be reduced accordingly for thinner head gaskets. Piston deck-to-cylinder head clearance should be maintained at .035-.040-inch for best results." The numbers don't add up, unless that is the compressed thickness! But another article in the same issue gives a 0.25 compressed unstated gasket thickness with a ZL-1 head swap. My Motion "cookbook" is somewhere. Digging it out is another matter. __________________ Thanks, Tim * VIN/Model Decoders * Power Team Charts (engine/transmission/gear ratio) * Post Your Pickup SPID Blazer SPID Suburban/Panel SPID * RTFM ... Read The Factory Manuals... download 'em here Highlanders Do you have a 1972 Plaid Pickup? Plaid Blazer ** Plaid Suburban

05-04-2012, 07:21 PM	#3
SS Tim Registered User Join Date: Aug 2010 Location: Edwards, CA Posts: 7,503	Re: NOS head gasket thickness? Part Num 3993890 I see what you mean, here is a Camaro parts clip. Attached Images __________________ Thanks, Tim * VIN/Model Decoders * Power Team Charts (engine/transmission/gear ratio) * Post Your Pickup SPID Blazer SPID Suburban/Panel SPID * RTFM ... Read The Factory Manuals... download 'em here Highlanders Do you have a 1972 Plaid Pickup? Plaid Blazer ** Plaid Suburban

05-04-2012, 08:19 PM	#4
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	Re: NOS head gasket thickness? Part Num 3993890 Yep, if the 3993890 is about 0.020 then I'll be around 0.040 total, which is where I want to be. With a thicker composition gasket I'll be out to 0.060, lose some compression, and increase the risk of detonation because of poor quench. The heads are already on (with the fat gasket) because we could not find a steel shim gasket, but if this old NOS one I've found is actually thin, I'm set. Just can't find any confirmation of what thickness 3993890 actually is! __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

05-05-2012, 06:47 PM	#6
SS Tim Registered User Join Date: Aug 2010 Location: Edwards, CA Posts: 7,503	Re: NOS head gasket thickness? Part Num 3993890 Found a Mr Gasket 0.020" shim at Classic. http://www.classicindustries.com/fir...rts/71131.html Also a general ref. page from GM Parts Direct. http://www.gmpartsdirect.com/perform...CATID=381.html __________________ Thanks, Tim * VIN/Model Decoders * Power Team Charts (engine/transmission/gear ratio) * Post Your Pickup SPID Blazer SPID Suburban/Panel SPID * RTFM ... Read The Factory Manuals... download 'em here Highlanders Do you have a 1972 Plaid Pickup? Plaid Blazer ** Plaid Suburban

05-05-2012, 07:40 PM	#7
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	Re: NOS head gasket thickness? Part Num 3993890 Thanks... I had found one similar, but the bore is very large (4.370) compared to the 402 (4.125). I don't know if that really matters, and probably is too thin to change the chamber volume, but I was a little worried about the enormous size of the bore in the gasket. Think it matters> __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

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