Engine out and back: a blowing exhaust manifold fixed!

[enigmas]

If the mating faces are are flat and true then why are exhaust gaskets required Phil? Any material in between both faces that is subject to either compression or heat erosion will eventually fail. Perhaps hand made copper shim gaskets would prove more effective rather than whatever is currently being used if you really have to have a gasket?

[Phil Nottingham]

On the V8s there is no need for gaskets as they are clamped tight. The IOE6 is meant to slide as it expands the special clamps and gaskets allow this. The manifold is quite long and it needs this expansion but both manifolds need flat mating faces across its length

[break23m]

OK, good to know, better not use silicone sealant in the exhaust system. 

[break23m]

I took the advice of my Rover specialist and retorqued the manifolt bolts on a hot engine. Not much movement and no result but a burnt arm. I released the bolts on the down pipe and scraped away the hardened paste. Put it back together with a bit of coppergrease. A bit of a result: no more blowing on idling but still not sounding good on the move, especially when pulling. Next step will be a very light smear of Holts Firegum on the joint of the down pipe. Getting tired of this, the car is running very smooth but the sound is annoying.

[enigmas]

May 24, 2024 13:14:57 GMT break23m said:

OK, good to know, better not use silicone sealant in the exhaust system.

I've been using heat resistant "high temp" red silicone sealant designed for exhaust systems and manifolds for decades, (not the general purpose black silicone gasket sealant/adhesive product; there is a difference) on all of my classic car and motorbike systems, all of which are custom fabricated. None of these systems leak exhaust gases. If the components are fitted properly and don't move about independently of the engine/trans unit, then they can't create continuous, persistent and irritating exhaust leaks.

[break23m]

This is how the manifold is fitted. Still not sure: are the clamps fitted the right way?

[3litrekiwi]

I'm pretty sure these clamps are around the wrong way. Mine are around the opposite way and this matches the orientation in the parts book. It also makes engineering sense as the machined boss would be there to give the nut and washer a flat surface to seat on.

Something that might be worth considering is the condition of the rest of the exhaust system. I have come across seemingly sound mufflers that had collapsed internal baffles causing excessive back pressure. This also resulted in poor performance but it seems the sealing on our cars is a bit marginal.

I machined the surface of the manifold prior to fitting recently with dry new gaskets. I must have had the exhaust manifold off and on three or four times and fitting this way has sealed ok each time.

[break23m]

Clamps the wrong way around - this was done by a specialist . Do I need new gaskets when I take the clamps off and place them back the right way?

[3litrekiwi]

I've found that the length of the studs and the design of the manifold means that the manifold needs to come away from the block a bit to get the clamps off. Once the seal is broken, the chance of leakage is pretty good so I think that a set of gaskets for £4+VAT would be a good investment. If the joints have been leaking, you should be able to see where and perhaps identify the reason. 

You will also need to either undo the clamp that holds the exhaust to the bell housing or disconect the manifold flange. This will either allow the exhaust enough movement to pull the manifold away from the block or allow the manifold to come off both joints.  Similarly here, you will be able to see if the flange joint has been leaking. As has been discussed previously, this joint theoretically doesn't need any sealant but given the age of the parts I have always cleaned both surfaces as well as possible and used a smear of exhaust flange sealant. If the joint doesn't need it, all will be extruded out as the flange is tightened, if there are some surface imperfections as is likely, the sealant will fill them.

I would at least slacken off the bolts on the bell housing clamp if you disconnect the manifold flange. This will allow the manifold flange to mate properly with the spherical end on the exhaust down pipe. Once the three flange nuts are tight, tighten the three clamp bolts.

It is important to tighten the three flange nuts evenly to get a good seal. The steel clamping ring should be reasonably paralell with the mating flange on the manifold.

As everything has been apart recently, this is a pretty simple job that should take less than an hour unless you do find some surface problems that need dealing with. A wire brush and  flat scraper are useful to clean up the surfaces and as prevoiusly mentioned some nickel paste if you choose to use it for the gaskets. I prefer to fit them dry.

Once the manifold is off and clean, I would check it for flatness as has also been mentioned previously. Without a flat reference surface, if you trial fit it to the engine with no gasket and tighten the clamps very lightly but enough to snug the two surfaces together, a 5 thou feeler gauge should not be able to slide between the joint. A steel rule across the manifold face with the manifold on a bench or table should also show any distortion. 

[djm16]

I routinely smear both sides of the manifold gasket with nickel grease. This way, not only can I re-use gaskets, but I rarely have a leak at this point unless the clamp nuts have worked loose. Now the manifold-downpipe joint, that is another thing entirely.

[break23m]

I received a new set of gaskets from my specialist. Remove all clamps and disconnect the rain pipe. Cleaning the surfaces of both the engine and the manifold. All easy, no problems to be seen. On the other hand, the manifold was uneven at the connection to the standpipe. Sand and scrape to remove old exhaust paste. The downpipe itself received the same treatment. Then apply a thin layer of exhaust paste and a very small amount of copper grease to the gaskets. And screw everything back into place - with the clamps the right way around. And Yess, a nice rumbling six.

Did everything go well? Not really. I tried to jack up the car to loosen the exhaust connection under the car but the jack stand itself moved, not the car. A new problem to solve. But that's for another time.

[MK IA Norway Viking]

Hello Joost - delighted to hear (!) that you have identified the exhaust gas leak, and that the engine is now running as it should. Difficult problems usually have simple solutions :-)

When you put the discs on, holding the manifold to the engine block, did you fit them with the round face inwards or outwards ? You know, there has been debate whether hte workshop manual/parts catalogue show these discs as fitted the correct way. Some say the raised faces on these discs shall be facing inwards.

Enjoy your Sunday.

[break23m]

Thank you. I fitted the camps the way the parts catalog shows, as I am adviced here.