L-72...AMA specs NEEDED!!!!

[Chevy454]

As you all may know, we are trying to participate in the "Certified Stock" tech at the Pure Stock Drags this year, and I am trying to find some specs on our block, so I can rebuild it accordingly.

In the "COPO Connection" book it lists most of the specs, but I couldn't really find any for the block. Would anyone have any idea on these? I am ultimately after the deck height figure, but so far NHRA specs are all I can find.

[Rowdy Rat]

Rob,

My hat is off to you for choosing to use the AMA specs for your engine setup rather than the more agressive NHRA Stock Eliminator specs allowed by the promotors of the PSMCD. Your approach is more in keeping with the spirit of this event, not "How much can we stray from stock and not get called on it..."

I have the AMA specs for the 1969 L-71 engine; I would imagine that the block specs would be identical to the L-72. I can forward what you need if you are time limited and require the information right away. If you can afford to wait a couple weeks, the "restoration package" that GM sends out on request used to include the AMA specs for the model/year requested (the only worthwhile information in the whole package actually). You may have requested this package before, but if not, the number is (800) 222-1020. There is no charge, but they will ask for your VIN.

Good luck! If there is any other way that I can help you out, let me know.

Regards,

Stan Falenski

[Chevy454]

Would you mind posting those specs for the block? I guess the number I am really after is deck clearance. The NHRA lists it at .008, but from what I have seen, that would require some serious decking of the block. Anyone have any numbers that could help me out? Did Chevrolet have a range for deck height?

[JoeC]

The Chevy Power book just gives the deck clearance as .035 minimum. Meaning you must maintain a minimum of .035 inch clearance between the piston and the cyl. head in any area where they may make contact. A composition head gasket may have a compressed thickness of .040 inch which would allow the piston to be above the deck .005. A steel shim gasket may have a compressed thickness of .022 which would allow the piston to be below the deck .013. These are minimum numbers. I know one engine builder that won't go below .060 deck clearance. An engine builder may use different numbers depending on experience and what parts they use. Is the NHRA number of .008 above or below the deck? NHRA may mandate that number for safety purposes. If the piston is .008 above the deck and your gasket is .040 only leaves .032 for clearance. I'm not sure why NHRA uses that number.

[Rowdy Rat]

Rob,

I can understand your frustration with the AMA specifications for the cylinder block. I hadn't realized how little information was there until I looked at it just now.

I checked an old copy of the Chevy Power book and came up with what Joe did; a deck height of .000" to .005" with a .040" composition gasket. The only cars to get composition gaskets in 1969 were those equipped with aluminum heads, which happens to be the application for the .040" gasket (part #3969865 - listed for use with the L-88). Your L-72 would have used a steel shim gasket with a compressed thickness of .021" (part# 330860) which pretty much dictates that your deck height is going to be at least .014" below the deck surface.

Having the piston even with or above the deck surface with the composition gasket should reduce valve shrouding (and be worth a few horsepower), but then, that isn't exactly stock...

Good luck with your decision.

Regards,

Stan Falenski

[Chevy454]

OK, I think I have half-way figured out my deck height problem. As best as I can tell, the only engine that Chevrolet worried about the deck height on was the ZL-1, as they were blue-printed from the get-go. In the AMA specs, I found ".000-.010" for the ZL-1 piston-deck clearance, but have found nothing for my L-72. So, I took and figured the stack height for my L-72 (1/2 the stroke+piston compression height+rod length) and subtracted that from the block length of 9.80, and you get .030 clearance. So, that is probably going to be the number I go with.

But, now a couple of questions. Stan, is the gasket you mention is correct for the L-72 still available from GM? Or, would I have to get one from Fel-Pro? Would anyone happen to know any specs for the correct gasket (like bore size, volume, etc)? Also, would anyone know the correct gasket for an LS-6? Would it have also used the steel gasket?

[Rowdy Rat]

Rob,

I'm told that the GM #330860 head gasket is still available with a current list price of $12.44/each. This gasket is a steel shim-type gasket which is nearly identical to the original gasket, GM #3863199. I'd bet the farm that the LS-6 found in the 1970 Chevelle/El Camino would have used the same gasket. GM also lists a Fel-Pro gasket which is far superior to the original steel shim gasket in sealing ability (with a corresponding high performance price of @ $80/each - my guess is much cheaper from Fel-Pro). This gasket is a composition type and has a greater compressed thickness and as such, is unlike the original. I really doubt that this would cause any problems during tech though(a call to Dan or Bob would most likely confirm this if you go this route).

BRC Pistons has a handy compression ratio calculator on their web site that might prove helpful to you as well.

Regards,

Stan Falenski

[JoeC]

There was also a stainless steel marine gasket with a low compression thickness spec. I believe it was sold by Victor Gaskets and maybe others. I don't remember the numbers for it. You may want to look up some old Stock Eliminator articles in the drag racing magazines. Those guys used many small legal engine building details that would ad up to lower ETs.