Troubleshooter
Anchorage, AK
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Joined: 05/02/2002
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Evidently there is some differences in the maintenance manuals.
Again, my 1991 manual say for the 7.4L engines the torque is 40 ft. lbs. for the bolts/studs.
Now, it also states that for the 5.0L and the 5.7L engines, the two center bolts should be torqued at 26 ft. lbs, and the outside bolts should be torqued at 20 ft. lbs.
Then in the next line it gives a different dedicated torque value for the 7.4L engine.
This info is from the 1991 Chevrolet Light Duty Truck Unit Maintenance Manual, Section 6A5-38 under Exhaust Manifold Installation.
Bill
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bacollins
Lawrenceburg, IN
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Oops! I have egg on my face. I guess I was having a senior moment when I posted earlier on the torque values and was looking at the small block specs. The manual actually says 40 ft/lbs for the 7.4 My apologies.
'92 Itasca Sunflyer P series Chevy
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jenks60
MS. coast
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Joined: 10/27/2001
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My manual and my Chevy chassis guide both say 40ft/lbs of torque on exh. manifold bolts.
I put a gasket on mine. My new manifold came with a gasket.
jenks, fulltiming with the DW.
92 Southwind P3(32) 33 ft.
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bill h
el segundo
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Chevy recommends a graphite paste on the faying surfaces. The graphite hardens to a slate-like film allowing the manifold to expand and contract while still conducting heat. The manifold expands as much as 1/8 inch. They also recommend drilling the bolt holes 1/32 oversize to allow for expansion without breaking the end studs. My observation is that it is usually the right rear stud that breaks.
Chevrolet says milling a used manifold is better than buying a new one, as the old one is seasoned and will be less likely to warp again if a gasket is not used to impair heat transfer.
My own experience with a number of 396s, 427s 454s and two 502s indicates Chevrolet's advice is good. I also spot-face the bolt holes and use hardened Unbrako washers and graphite paste under the washers to allow expansion. I don't remember the torque numbers, but I use more torque on the middle cylinders than the end ones.
Cut a slot between each cylinder with a hacksaw or cutting wheel. Then have a machine shop surface grind the manifold. . Retorque on a hot engine after a few hours.
After all that, it will warp again, but not too soon. Install a right side splash shield to improve right side cooling. Avoid heat soak by idling for a little while before shut down after a hot run. Or better yet, have a thermostat controlled electric puller fan that runs for 5 or ten minutes after shut down. The fan will also avoid heat soak damage to hoses, wiring and the starter solenoid, and your Qjet is less likely to boil out all its fuel. Much of the Chevy MH hot starting difficulties are the result of the effects of heat soak.
You might also consider using studs and holding the manifolds on with brass nuts. They are less likely to seize.
Bill and Susan
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rickwils51
SE Michigan
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I recently went through the same process on both sides of my '95 TBI 454. I agree with the other posts that you should take a hacksaw and cut through the manifold between the ports to relieve stresses (Chevy recommends) and also drill out the holes about 1/32 over. I went even more to facilitate installation. I used gaskets that I got from the dealer, new studs, anti-sieze, and torqued to 20 first, then 40 ft. lbs. Totally quiet since then maybe 10,000 miles now.
I had three studs broken when I bought the MH, and audible leaks both sides.
Good luck.
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mfox20
Fort Worth, TX
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Thanks again for all of the input. I will be finishing the install this weekend.
The new replacement manifold has the expansion cuts already built in. I am going back with new bolts and with the final torque at 40 ft/lbs.
I am also going to try the factory install with no gasket. I figure if it leaks I can back out the bolts and install a gasket.
I will be adding cooling ducts to both sides bringing air from the front grill to help cut down on the high heat around the manifold. Also adding new plugs, belts and hoses. I purchased the unit last year with 38k and have traveled about 3k on the old belts and hoses. Figure I need to get them changed before my luck runs out.
Thanks again to all that replied and let me know if any of you have any questions about the project.
Michael
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tyndall
Winnipeg, MB
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Not sure if it matters but according to a Nov/96 service bulletin, a gasket is used. It states a new manifold and stainless steel gasket was developed for 1982-95 7.4L engines to correct the warping. The GM gasket # is 12551449.
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bill h
el segundo
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I would not go gasketless without resurfacing.
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PaulJ2
Northwest Oregon
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Manifold needs to get rid of its heat through the head and into the water jacket. Composition gaskets will cause the manifold to overheat and warp even sooner. Plane smooth and no gasket. However a thin steel gasket may be ok.
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bill h
el segundo
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PaulJ2 wrote:Manifold needs to get rid of its heat through the head and into the water jacket. Composition gaskets will cause the manifold to overheat and warp even sooner. Plane smooth and no gasket. However a thin steel gasket may be ok.
A soft copper gasket sometimes will seal better than steel, and will conduct more heat.
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