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exhaust manifold gaskets

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g3chevy / Mr Pontiac
dhw3971
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Post by dhw3971 Tue Aug 31, 2021 4:20 pm

In under 1000 miles of driving this summer I've gone through one set of gaskets on the passenger side and it sounds like the leak has returned. I torqued the manifold bolts to 30 lb-ft and put the gaskets in dry.

So first, what causes an exhaust gasket to leak? And what might cause them to fail so quickly?
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Post by g3chevy / Mr Pontiac Tue Aug 31, 2021 5:30 pm

The only thing I would question is the exhaust manifold warped? Maybe you could have a machine shop resurface it if so. Or try some Red High Temp RTV?
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Post by Joe73 Tue Aug 31, 2021 6:25 pm

Id have to go with warped as well. You might want to try doubling up the gaskets.
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Post by dhw3971 Tue Aug 31, 2021 8:20 pm

Well, between this forum and my friends at home there are four votes towards getting the exhaust manifold machined. Sounds like a good winter project.
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Post by g3chevy / Mr Pontiac Tue Aug 31, 2021 10:11 pm

Sounds like a good plan. Let us know how it turns out.
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Post by impalamonte Tue Aug 31, 2021 11:05 pm

When I had my LT1 in the car. I ran graphite exhaust manifold gaskets as they compress approx. 50%. They seal up well.
No leaks with the Hedman mid lengths.

I have installed graphite units on my LS swap. Have not started it yet...working through issues and details.
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Post by Iggy Wed Sep 01, 2021 12:47 pm

Dave, my first thought is also warped manifolds. Does your car have exhaust manifolds like these?
exhaust manifold gaskets 1975-710
They are less likely to warp
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Post by bucket Wed Sep 01, 2021 5:57 pm

In most cases, factory exhaust manifolds don't need gaskets at all. GM didn't run them, so I never have either. They can't blow out and leak if they aren't there to begin with.

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Post by dhw3971 Wed Sep 01, 2021 6:11 pm

Iggy wrote:Dave, my first thought is also warped manifolds.  Does your car have exhaust manifolds like these?
exhaust manifold gaskets 1975-710
They are less likely to warp



Tom--yes, that is what I've got, except for the connector bit between each collector.

BUT I did not get leaks like this on the original engine. Maybe the Jasper engine and my manifold don't mate up the same as the 1975 engine.
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Post by bucket Thu Sep 02, 2021 9:19 am

dhw3971 wrote:
Iggy wrote:Dave, my first thought is also warped manifolds.  Does your car have exhaust manifolds like these?
exhaust manifold gaskets 1975-710
They are less likely to warp



Tom--yes, that is what I've got, except for the connector bit between each collector.

BUT I did not get leaks like this on the original engine. Maybe the Jasper engine and my manifold don't mate up the same as the 1975 engine.

Your original engine shouldn't have had manifold gaskets when it was taken apart.

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Post by Iggy Thu Sep 02, 2021 11:31 am

The connector between collectors keeps the manifolds from warping too much. I had them on my 1977 Monte Carlo and a 1976 K5 Blazer I owned years ago. My 1975 Laguna currently has them, if you are interested in them - I am going with a 6.0 LS when I start working on it - break lines started leaking right before I put it up last so I have a lot of work to do before I put it on the road, fist up will be removing the 350 that is in it and the manifolds will not be used in the future.
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Post by dhw3971 Thu Sep 02, 2021 1:01 pm

Hmmm...I can't recall if the 1975 engine had gaskets when it was removed a year ago.

I've also been told by my mechanic Wayne that he often went without gaskets, so if I get another blowout I can try removing all of them on the pass. side and then see what happens.
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Post by Iggy Thu Sep 02, 2021 2:19 pm

I figure that you do not want to put headers on the station wagon, but have you thought of something like this which would keep the stock look:
https://www.castheads.com/exhaust-manifolds/log-style-chevy-ii-exhaust-manifolds/
The 300 HP 350 has been a staple of hot rodders for 40 years - flat-top pistons, smog heads (9:1 compression) and the old Wolverine Blue Racer WG-1159 cam (214* intake and 224* exhaust @ .050). The only thing they do not tell you is the 300 HP is based on headers and dual exhaust - with stock manifolds you are closer to 270 HP with dual exhaust. The stock manifolds from the mid-1970's narrow down to 1-7/8" outlet and that becomes the choke point. Gen I SBC engines are noted for having a weak exhaust port (BBC's are even worse) and the exhaust manifolds did not help that. That is why headers will wake up even a dish piston smog head SBC.
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Post by dhw3971 Fri Sep 03, 2021 2:58 pm

hmm, interesting. So these are higher-flowing, stock-appearing cast iron manifolds?

Yesterday, I ran the idea of headers past the wizards at our automotive service program and they all balked at headers...e.g. sealing problems, frequent tightening of mounting bolts.

I watched the video and I assume I have the manifold with the "donut"...can't tell if that's good or bad.

Further, I don't know if these log-style manifolds will bolt right up to my exhaust pipes...it seems like they should because the mounting flange bolt pattern doesn't change...but I've bought plenty of stuff that doesn't fit right and has proven to be a mistake or require more $$ to make it work.
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Post by Iggy Mon Sep 06, 2021 11:20 am

I put a set of Edelbrock headers on a 1992 K2500 Suburban with a 454 years ago. Used the good gasket that came with them, and carried a 7/16 open end box end wrench with me for the first week. Every time I ran the truck I would tighten down the header bolts once the engine cooled. After about 5 or 6 cycles the bolts would no longer tighten further. Drove that truck for 100,000 mile until the gasket needed replacing - the bolts never loosened up! The trick is to find headers with a 3/8" flange and not the 5/16". I have run headers since the mid 1970's and the only set I ever had an issue with was cheap set of Cyclone headers I put on a Monte Carlo (first set I ever bought) and that was not an issue with bolts loosening up, they burned through in about two years. I paid less than $50 new when the good ones were about $100 so I got what I paid for and learned my lesson.

As for the log manifolds that have been hogged out - those are the replacement manifolds for our SBC cars and trucks through 1979. The only issue you might have is the outlet has been redone to match is gasketless hook ups. They may be able to work your manifolds to maintain you stock hook up
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