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Old 05-01-2004, 09:50 AM   #1
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Torque specs using Aluminum heads??? Different?

350 TBI w/ Trick Flow Aluminum heads.

I was wondering what the Torque specs were for the bolts going in through the manifold and for the headers. I know they must be different seeing how I'm bolting them to aluminum as opposed to iron.

Does anybody know? I have a shop manual, but all the specs are in reference to stock heads.
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Old 05-01-2004, 10:55 AM   #2
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Hello Gunny,

There was a thread on the board several months ago from another member asking pretty much the same question. If I remember correctly, the torque values are the same for the aluminum and cast iron.
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Old 05-01-2004, 01:31 PM   #3
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Quote:
Originally posted by Trickster
Hello Gunny,

There was a thread on the board several months ago from another member asking pretty much the same question. If I remember correctly, the torque values are the same for the aluminum and cast iron.
Really? Well that's odd.
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Old 05-01-2004, 02:05 PM   #4
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Yup, i remember the thread. The gist of it is that the tq specs refer to the fastener, not the item getting fastened.
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Old 05-01-2004, 04:51 PM   #5
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Quote:
Originally posted by Stekman
Yup, i remember the thread. The gist of it is that the tq specs refer to the fastener, not the item getting fastened.
Yep, because you want a certain amount of strech in the bolt when its torqued down... it doesn't matter if its aluminum heads or not, as long as the bolt gets the proper strech. Thats what the torque spec is for - to get the proper amount of strech
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Old 05-02-2004, 09:12 AM   #6
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With my AFR's it's 65-70#'s for the head bolts.
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Old 05-02-2004, 12:19 PM   #7
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There seems to be a bit of misunderstanding over which fasteners are in question.

First, head bolt torque specifications for aluminum heads and iron heads should be the same because:
  • The substrate material is still the same;
  • There is no "crushing" or compression of the head material itself;
  • SBC (and BBC) head bolts are NOT designed to be torque-to-yield, so there is no intended "stretch" of the fastener, thus, SBC (and BBC) head bolts or studs can safely be reused, unlike some others;
  • The only thing being distorted by the fastening of the head is the gasket.

The intake and exhaust mainfold bolts (as Gunny asked about) may have a different torque limit due to the shear strength of the base material. However, the overall force required to properly seat and seal the gaskets is still the same. The typical SAE torque limit values for a 3/8-16 120,000 PSI steel fastener into cast iron range from 30 ft/lb to 47 ft/lb depending on base material, and requiring a minimum thread engagement depth of twice the major diameter of the fastener - or in this case, 3/4" engagement. Common torque limits for fasteners in aluminum substrates are 11% lower than into cast gray iron. However, since peak torque limits for the typical 3/8-16 bolt are well above the required torque for either an intake or exhaust manifold on an SBC engine, the same torque values can be used. According to my information, intake bolt torque is 28-30 ft/lb, and exhaust manifold bolt torque is 18-20 ft/lb. Since you are apparently installing headers, retorquing the exhaust bolts after a few heat/cool cycles is probably a good idea.

Of course, you MUST have clean, lubricated threads, and should use an anti seize compound in those applications due to the dissimilarity of the metals. In this case, the anti seize compound will be used as the lubricant.
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Old 10-11-2006, 09:10 PM   #8
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Once again Vader comes through with the bottom line!!

I actualy was looking for that same information
thanks for explaining it in such detail
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Old 10-11-2006, 09:10 PM
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