exhaust manifold torque

[68rsssrag]

The GM spec for BB exhaust manifold to head bolts is 25 ft lbs. seems low but is there a reason?

Arno

[rich69rs]

During my engine (327) rebuild, torqued my SB exh manifold bolts to 20-30 ft-lb per spec.  Bolts are 3/8"-16, probably grade 5 material.  Torque of 20-30 ft-lb will "preload" the bolt properly.  Enough preload to keep the joint tight while not stretching the bolt to a point of permanent deformation.  Periodic recheck of the torque isn't a bad idea either.

Also, the gasket must be taken into consideration as well.  Using a high strength bolt and torquing to a higher value could impact gasket integrity.  Obviously the original specs are for original gasket, bolts, etc and might be different for performance upgrade components.

[JohnZ]

Also, the gasket must be taken into consideration as well.  Using a high strength bolt and torquing to a higher value could impact gasket integrity.  Obviously the original specs are for original gasket, bolts, etc and might be different for performance upgrade components.

Quite true for most applications, but no gaskets were ever used on any Chevy V-8 exhaust manifold-to-head attachment.

[68rsssrag]

John

Per your earlier post I am not running gaskets.

Rich

Interestingly the Bolts are 3/8 and grade 8 so 25 footpounds isn't going to stretch these at all. I was thinking more that during operation the expansion of the manitold would possibly stress the manifold and crack the front and rear "ears" where the 1st and last bolt are located?? There must be a reason for such a low torque. L6's are 30.

Arno

[JohnZ]

L6's are 30.

Arno

On the L-6's, only the bolts that use clamps to span the ears on both manifolds are 30; the end bolts (which go through holes in the manifold ears) are 20 ft-lbs.

[Dan]

On the L-6's, only the bolts that use clamps to span the ears on both manifolds are 30; the end bolts (which go through holes in the manifold ears) are 20 ft-lbs.

Actually, the outside hole in the exhaust manifold ear is a locating pin (see link).  The outer most mounting bolts span both manifolds and are spec'd to 20 ft-lbs.  All of the center mountings, a combination of studs and bolts, are spec'd to 30 ft-lbs.

http://mysite.verizon.net/pacesetter.camaro/L6Manifold.jpg

Dan

[JohnZ]

Shows how much I know about L-6's - I just took the specs from the Chassis Service Manual, which has a spec for the "end bolts" on the L-6. 

[Dan]

I'm afraid we L6s are a just about extinct.   

Dan

[rich69rs]

Hang in there Dan -

Saw a very nice '67 last weekend, 250 badges on the fender.  Car was at a local car show.  Hood was up and there was the original, 6 cyl with air conditioning.  Very nice - I believe in the 15 yrs I've had my '69 327 RS coupe, I've actually seen less than 5 L6 cars.  Keep the fairth!