Helicoil nuts for exhaust down pipe

I’m trying to find replacement nuts for the 3/8 -24 Helicoil nuts that the Jaguar factory used on the downpipes. I saved 7 when I removed the exhaust and one was destroyed during removal.

I may use the original nuts. Does anyone know a source. It appears the usual suppliers just have brass nuts. There is ample chatter on the jaguar forums that the brass nuts tend to loosen themselves up.


3/8 24 is a standard unf thread I think …should be heaps available on line …try mc masters they are usually 11/2 times normal thickness ….they would be pretty standard on us vehicles as well


I’ve used stainless studs (ARP) with the thicker brass nuts on the exhaust of my last three Lotii, a Singer, two MGs, and two E types (one with stock downpipes, one with headers) and there has been no loosening at all. Those having issues are probably using the thinner hardware store brass nuts and not properly compressing the sealing ring (donut) on initial install.

I believe that the original nuts were bronze. Whether this makes them less likely to come loose, I don’t know. I’ve seen 3/8-24 deep bronze nuts for sale on eBay occasionally, though I can’t vouch for their quality. Some have had success with Nordlock washers under the nuts to prevent loosening. Others advocate using a jam nut to lock the primary nut, or use a 3/8-24" threaded coupling for the additional length. The rest of us just check and tighten the nuts once in a while and rejoice in the fact that we know they will come off when we need them to…

Also available at McMaster-Carr, and probably some other places.

My original nuts were steel Helicoil nuts.

I had to look up what a “Helicoil Nut” is, as I’ve never heard of them. A little googling resulted in references to Jaguar using Helicoil Locknuts in the exhaust systems of some later cars (to secure flanges) with part number C17914. I don’t know whether these are 3/8-24 nuts though. The first 2 sites I looked at showed them as either Not In Stock, or No Longer Available, so it may be moot. If you want to use that particular nut you’ll probably have to look for a distributor for what appears to be a single manufacturer (Bullhoff) item.

I have done an “experiment” on my 420G, (for about the last 5 years) it has had double locked brass nuts on one manifold with copperslip, and the other manifold has double locked SS with nickel anti-sieze, the studs are new steel, some had norlocks and others spring lock washers.

I now have the original style long bronze nuts which I got at " Hi Tensile ", where I get all my quality fasteners.

They supplied them as generic “exhaust nuts” not Jag parts

My donut blows some gas, so I guess something has worked loose.

I did check them and they seemed tight, but doing it properly is not easy

I will replace the donut and install the bronze nuts, hopefully before I misplace them, I think they are all in the trunk with many other spares

They are still made, ie by Böllhoff.
Not sure if new UNF nuts are available. Mine were also “helicoiled”, 69 XJ6

My education continues……


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Golly, I hadn’t thought of these things for decades!

Dad use them on his race cars, but then, for whatever reason, he kinda would just fell away from using them, so I plumb forgot about him.

I just learned long ago to replace them with the socket head bolts, and that made it so much easier to install and tighten them, and with Nordlocks, they don’t come loose.

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I would guess that their performance would be comparable to distorted thread nuts. Distorted nuts are more readily available.

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