V12 Exhaust Manifold Replacement

I am completing a long-term maintenance program for my 71 Series III. I just replaced the left side exhaust manifolds. It appears the front manifold base is aproximately 3/8" longer than the rear. The new downpipe pipes appear to be equal. Unfortunately I did not take a picture of the “before”. The right side manifolds are even at the base (bolting plates). Any insight would be very helpful!

Joe, I think you may have installed at least one wrong manifold. On my 74 S3, the two are exactly the same length on both sides

Off course, there could be a difference between a 71 and a 74.

Cheers … Ole

Dear Joe,

This is probably a function of your only replacing one of the manifolds on that side. If you look at the original item, you’ll see that it is quite thin and spidery and the commonly found failure point is at the end “ears” where the studs attach it to the head. If you look at the replacement part, you’ll probably see that it is a much thicker casting, particularly where it meets the heads.

This will give the new item a different geometry to the old item and assuming it is done up correctly, then replacing items in pairs is the only sensible solution. Failure to do that may mean that they heat up and cool down at different rates and the thinner older casting may crack after a few heat cycles if bolted up up tight and under tension against the newer, thicker, stronger item.

kind regards

Thank you for the help. It appears that the matched set is the key. I used a NOS (purchased locally) and a current new stock manifold from one of the usuals. I will order a matching manifold today. Also, thanks for the pictures. I am amazed at the size difference between the two.

regards, Joe

Dear Joe,

Before you order more parts, take a look upwards into the manifolds. If the inner diameters flare out at roughly the same height as each other, then all is good. The downpipe does not sit flush on the outer casing, but sits via a big thick crush washer on the inside diameter of the housings. It is not obvious from this angle that there is any mismatch at all.

kind regards

Dear Marek,

Thank you for the follow-up. I did double check. The forward manifold is 5/16" longer that the rear. Checking inside the recess both are the same 7/16". I was hoping the longer unit had a deeper recess.

Thanks againg for the great suggestion!!


If it clamps up and forms a gas tight seal then it ought to be trivial. All that happens is that the downpipe will point upwards. Since it has a large olive on its far end, it can always be made to join up with the rest of the exhaust.

If it is at too much of an angle to allow the downpipe to connect properly, then it is a problem.

kind regards

Hi Guys
I fitted new manifold studs when I rebuilt my engine, and came across an unusual problem with the new manifolds from Sng, well they are about 10 years old.
As Marek alludes to the newer product is thicker steel than the older ones, to the extent that I had to mill a bit off to allow the locknuts to get enough purchase.

Has anyone else found this?
Also make sure you fit the upgraded exhaust manifold gaskets, as used on the XJS. These have a reinforced inner ring. The earlier ones have a tendency to burn through. I had 2 go, and on strip down found others in poor condition.
Also when fitting up the manifolds, don’t tighten them fully up until you have fitted the lower down pipes. I did, and found the lower pipes impossible to fit until I loosened the manifolds.