stripped exhaust port threads - Page 2 - Triumph Forum: Triumph Rat Motorcycle Forums
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post #11 of 14 (permalink) Old 07-19-2012, 12:52 PM
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Also, it's worth noting that if you are doing a shrink fit type, heat the entire head uniformly. Best to do it in the oven, as previously stated.

If you heat cast up on one side it CAN crack.

Freezing the insert will shrink it so it goes in a lot easier, too. If you do this then I would personally put the whole lot back in the oven afterwards to let it cool down slowly and uniformly. This just reduces the likelihood of cracking, once again.

Probably don't need to do the above but I find it's better to be safe than sorry
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post #12 of 14 (permalink) Old 12-12-2012, 06:20 PM
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I'm replying to this thread at this time because I am now addressing the loose spigot I have on the left jug of my '66 T120R. Today I picked up a replacement spigot and it screws in nicely. But it does not bottom out in the head and I suspect this is why the spigot that was in there was loose. Fortunately, it did not destroy the threads in the head.

I did a search here because I remembered this topic and Mr. Pete always driving home the point that the spigot MUST bottom out in the head in several threads on the subject of exhausts.

When I thought I could make an aluminum shim from 1/16" aluminum plate. It's not an easy machining exercise and because the actual port is not centered in the threaded spigot port, the shim either has to be offset or very thin. I managed to make it, but when I looked at it, decided it was a poor fix. So I will be machining back the threads on the spigot so it bottoms out and if necessary relieving the threads at the front of the spigot as well.

You have to wonder why they don't make the replacement spigots a bit longer and relieve the threads at the internal portion right off the bat.

I just wanted to thank Mr. Pete for sharing this important tidbit of info here. Otherwise I think I would have just tightened up the new spigot and move on.

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post #13 of 14 (permalink) Old 12-12-2012, 08:20 PM
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After wrestling the pipes off my '70 - which took three days - had to cut the cross over tube, I found the spigots to be pretty badly rusted with large flakes of corrosion coming off. I thought that new spigots were in order and contacted TMS in Nottingham for a pair. When they came they had the last two threads machined off just like the originals and just how the Triumph sages recommend. The chamfer (sp?) on the exit side was a bit not concentric but, oh well, how much can that hurt.

My point is that the last two threads need to be removed and of course the spigots need to be long enough to bottom out. Still don't really get the last two threads bit but will follow along on blind faith.

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post #14 of 14 (permalink) Old 12-12-2012, 10:06 PM
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The last two threads or in some cases more have to be removed because it is not possible to thread the port all the way to the bottom with a complete thread. So by removing the first couple of threads it allows the spigot to be screwed in without the threads bottoming out where the internal thread stops and this lets the end bottom out at the base of the port.

The new spigot I bought today has maybe 1 thread machined off the end. After inspecting the threads in my port, it looks like I'll have to machine 3 threads from the spigot. I'll measure it up tomorrow to be sure before I chuck up the spigot in the lathe. Guess I should take some pictures for the resto thread so this is clear to everyone asking about this in the future.

By the way, I thought about the requirement to bottom out the spigot in the ports. I'd never given it much thought before and figured it was to prevent hot gasses from impinging on the threads and possibly erode the threads from behind. But when Mr. Pete said it would beat out the threads, it all made sense. If you tighten it until it binds just on the end of the threads. It is fully supported only by the thread alignment and really only held tight by the last thread. If the exhaust is subject to vibes, it can, and based on Mr. Pete's comments, will eventually move the spigot side to side and once that happens it will continue until the threads are obliterated. But, if you bottom the face of the spigot on the bottom of the port, you now have a firm square base against which the spigot is held tight by the entire length of threads. Plus, all the threads are loaded against the flank of their back side. This puts both opposing lateral forces and compressive force on the spigot by the port. So, the ability to loosen the spigot and cause the spigot to move side to side is greatly reduced and apparently totally eliminated. Since I have one bottomed spigot that is rock solid in the head and the other bottomed on the threads only and was found to be loose, I've got real life proof right in front of me.


Last edited by Snakeoil; 12-12-2012 at 10:22 PM.
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