Exhaust manifold leaking

[mark10337]

Hi

The exhaust manifold on my front cylinders (4,5,6) is blowing out of the top right corner again between the head and the manifold. It has done this before. Taking it off and new gaskets fixed it last time, but I want to check I'm not making some mistake before I fix it again.

Is this something that is quite common? What torque should these brass nuts be at to properly tighten the exhaust manifold to the head?

[PeterCrespin]

Same issue as your head nuts Mark. Either insufficient thread to get the clamping force, despite ample torque, or distorted face on a chunky cast component, leading to leakage at the warped area.

So take the manifold off and check that there are at least a few threads 'inside' the manifold clearance holes, Then surface-dress the manifold face until it is dead flat on a piece of plate glass and doesn't rock on the head face.

People who rebuild engines professionally, even previously well-running ones, perform checks like this on all existing and newly-fitted components and fasteners, rather than assuming anything is right, as most amateurs do. In terms of final outcomes, a careful and skillful amateur can save a fortune and have a good engine, but with pros you're paying for their time doing these baseline checks and assuming little. Often one hears comments like "What took him so long?" when you see the hours for something as simple as fitting a manifold or cover. Now you know why...

[cactusman]

If it always blows from the same place then I would suspect either a bit of foreign matter (old gasket) is present on the mating surfaces or a deformed exhaust manifold. It is unlikely to be a distorted head...and if it is then I would imagine there would be other head issues.
Remove the manifold. Make sure there is nothing left of the old gasket on either the head or the manifold. Be careful scraping the head as it is aluminium so no gouging with a chisel Now as Peter suggests, dress the manifold removing any traces of old gasket and then use a steel rule edge to make sure the manifold is flat across the whole face. If it is then it should seal. Use a new gasket. The nuts are brass so they should not be over tightened. Not sure what the torque setting would be but over tightening will probably damage the nuts. Only other thing that occurs is a partial blockage in the pipe leading to excessive back pressure...very unlikely as the engine will probably run badly...

[Mark Gordon]

What torque should these brass nuts be at to properly tighten the exhaust manifold to the head?

I searched through my rebuild records where I kept that various stated torque settings. All of the critical ones (i.e. bearing cap nuts, flywheel, head cap nuts, etc.) are listed, but no mention is made for the intake or exhaust manifolds. As mentioned previously, they are brass so be a little careful, but over tightening will strip the easy to replace nut and not the stud. After doing what is suggested above and installing a new gasket, snug them up nice and tight and you should be good. I know that you drive yours daily and that's good. After a week or so, go back and retorque those nuts as they have a tendency to loosen during the first few heat/cool down cycles. Fasteners being what they are, I think that it's a good policy to put retorquing all accessible fasteners periodically as part of routine maintenance.

[Hugo]

There's no torque setting cos it doesn't matter! If it is blowing, it won't be beacuse the nuts are at the incorrect torque. Thos gaskets will compress after a few heat cycles, but even that shouldn't make them blow. My guess would be that you have a warped face on the manifold.
On a sort-of-related topic, has anybody done any research into the relative heat insulation properties of coated manifolds, non-coated manifolds and stainless steel manifolds? The temperature of the exhaust gas will affect its velocity, which in turn will affect cylinder scavenging. It's probably one of those things that is purely notional, but having heard tales of flat spots due to too-large diameter headers or front pipes, it's something that crossee my mind in an idle moment.

[MarekH]

Dear Hugo,

Your exhaust gas temperature will move by 300-400'c as you whip up the rev range, so I wouldn't spend too much time worrying about how much a bit of insulation is affecting whether you have a flat spot or not.

kind regards
Marek