Fitting new exhaust manifold nuts

Bought a new set of exhaust manifolds for my 3.8S, I also purchased new nuts to attach the manifolds. After I got them fitted I ran into the issue of the nuts hitting the manifold making it difficult to turn. The high spot on the nuts are hitting the manifold. Even the ones I removed from the old manifolds felt the same. I checked with McMaster Carr and ordered some castle style nuts with a flange, they are smaller in diameter on the flange than across the points and I can turn them down on a lathe if the flange is too wide so I get some clearance. since these are 3/8 12 point heads I should have the clearance I need. Now the originals nuts are brass, these are steel, will I get into issues with expansion and contraction as these steel nuts will hinder expansion where the brass would not.

Hi Jagarra,
I think the main reason brass nuts are used is to stop the stud and nut rusting together. Using dissimilar metals is meant to prevent this.
Have you got normal steel studs, or stainless steel? I’m sure one of the engineers on the forum will correct me if needed, but maybe ensuring the studs are stainless steel would prevent this 'rusting together’effect when using steel nuts…

I’ve never had difficulties installing new brass 5/16-24 nuts to the exhaust manifolds, though on 3.4 and 4.2 litre engines. Perhaps the manifold configuration is that different in your case.

With steel nuts you run the risk of binding to the stud from high temperature oxidation. Stainless studs and mild steel nuts won’t mitigate it due to galvanic corrosion between dissimilar metals. There’s also a tendency of stainless steel to spall. Better to use brass nuts on mild steel studs applied with a coating of anti-seize. Note that brass exhaust nuts are meant to be single use.

As far as I can tell the studs are steel, the magnetism seems strong, but not as strong as when i check the generator housing. Maybe the coating in the manifolds is causing the clearance issue, but I can’t turn them tight as it hits the manifold, the top rear one near the firewall is almost impossible to turn, it detentes as the point contacts the manifold. I don’t want to chip the new surface. Steel nuts and anti-seize seems to be the only option.

Can I suggest that unless you think you are a better design Engineer than those who designed the XK Engine in the first place, then its best to keep your manifold studs, washers and nuts of their same original design regarding size and material used. The original design worked well, so why experiment with changes.

So all your studs are MILD STEEL, but keep dimensions exactly as original, noting overlength studs can bottom out and break into the head water jacket, or if they protrude to far, then you can get fit/clearance problems with the exhaust manifolds.

The Nuts used on both the exhaust studs and on the down-pipe studs are standard-size hexagon nuts - totally unnecessary to fit long nuts - but the critical detail missed by many, including specialist Jaguar suppliers, is that the nuts are made of BRONZE, and not simply everyday hardware/plumbing supplies BRASS. So look for a supplier who clearly states that the are offering BRONZE nuts, and these are readily available from various specialty fastener outlets, as well as the better specialist Jaguar suppliers.
And the washers used are single coil spring washers with a square section - these lock the nut in place when torqued down correctly, with only ham-fisted efforts at overtightening leading to any stripping of thread. Forget stainless steel studs or nuts, definitely don’t use steel nuts, and make sure you use BRONZE nuts not brass, with some definite modern technology benefit in sparing application of a high temperature anti galling/seizing product to the exposed stud threads before fitting nuts…

That’s useful information. Thanks Roger. I always thought they were brass since that’s what’s commonly mentioned when the subject comes up.

McMaster Carr has them.

The problem is not what the nuts are made of, but the diameter across the points on the nuts. I am measuring .573 across the points, which are hitting the sides of the manifold preventing me from turning them in some locations. If I use the long nuts, I could turn a portion down to give me a built in stand off to give me some clearance, and I have ordered lock washers for a cap bolt which have a smaller OD of under .500 (like the English lock washers) that may help kick the nuts out a bit so they clear. As far as fitting, the castle flange units still seem that they would give me the clearance cause that is what I need. Anyway, there are orders coming from Barrett and 2 from McMaster Carr coming over the next couple of days, we will see what works.

Sounds like you have a problem with your MANIFOLD, especially if it is a reproduction one and not an original - as is, or restored. [ When you say 3.8S is that an XK150 or an S-type saloon? - different manifolds of course ].
If you have correct studs, and they are fitted properly, then a standard hexagon nut will wind on to the stud OK without hitting an original manifold.

Can I suggest you locate another ORIGINAL manifold - they are not that hard to find - and see if that fits OK. If so, return your repros to vendor as ‘not fit for purpose’

What you are proposing machining down long nuts sounds like a band-aid fix, that will be a constant source of problems, especially if you make matters worse using steel nuts…

Agreeing totally with Roger as usual, your manifold was poorly made, and should normally have enough clearance for any 5/16" UNF hex nut.
Mine has bronze nuts, and though I could wish for more room to swing the wrench in there to get the bottom 6, they all fit properly.
Bronze looks a lot like brass, but is a bit darker shade. It has different thermal expansion properties.

If you mean you have all the nuts on top tightened down, and then find you can’t get the bottom nuts on, you need to start with the bottom nuts first.

Dump the steel.

You can thank me later.

It’s best to fit the inner, bottom nuts first the ones behind the manifold finger tight. Then fit all the others. I always tighten them from the centre outwards but probably me being a bit of an anorak! If they are repro manifolds sounds like poor shape of casting!

Well you guys talked me into it. I ended up modifying the ends of the long nuts to give me a shoulder 1/2" across to match the OD of the lock washers. Didn’t have to remove much material, flats ended up the same length as a standard nut that I took off, got the clearance I needed and all is buttoned up. Did find another spot where clearance was tight, had to pull manifold away from stud to start nut then bring them down together to tighten.