Exhaust nut woes

Tornado said:

Sorry it was Precision Cycle, not machining...owner is a member here... @Dzldanz

Web addy:
Precision Cycle https://share.google/Yqt72kWrz85AdBUfv

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Thanks for getting back so quickly. Appreciate it.

There is probably not even one Japanese motorcycle in existence which uses the Norton way of attaching exhaust pipes to the cylinder head. The sensible way is to have stubs threaded into the head, and the pipes attached with slip joints and springs. My 2 into 1 system is attached that way. I have tabs under the screws on the rocker-box to hold the ends of the springs, and I use Yamaha insulating rings behind the stubs to create a seal. With a road bike, you would need to modify the pipes with sleeves to fit over the stubs.

krullbeast said:

Hello there,
Has anyone had any luck with gooping up your exhaust port threads with high-heat jb weld and tapping new threads with a 1 15/16 tap? I am at my wits end with my 74 850 exhaust nuts. I can't afford to take the head off and ship it to mass. for those brass inserts. Thank for any information.

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Can you get a machine shop to machine some straight threaded tube to 1 15/16 + .020 or + .030 ? Obviously give the machinist a new standard nut , but bore out your old nuts press in and tig weld ., get 3 made out of say high strength hollow bar , it's really nice to machine , one of them can be a little bit + and add some flutes at the right angle , you don't need HSS steel to tap alloy . Cheers

This issue with the exhaust port threads is unique to the Commandos and was known about during the production period. Of course there was occasional cracking of the pipe flange on other models but the thread worked well.
The difference with the Commando is the rubber mounted attachment to the frame that does not support dynamic silencer loads generated during normal operation of the bike. These loads are reacted by shear, rotation and bending moments at the threaded joint which eventually loosens the nut and wears the thread.
The solution does not involve re-engineering the head to pipe interface.
It is necessary to relieve the loads by adding non-rubber supports between the exhaust pipe and both the front and rear engine plates. Using just one of these extra attachments is probably fine and I successfully only use a front one, but both will guarantee it.
The rear attachment is tricky but not impossible. I include it here because some on this forum have reported doing it.

I realise that this is of no help to anyone who has just stripped the exhaust port thread.

After a few years of running my new 850 Commando the headers needed tightening quite a bit and started to wear the threads but once I converted the motor to the Featherbed frame back in the early 80s and hard mounted motor I have never had any more problems with the rose or threads or exhaust coming loose, I use no locking rings or anything but I did have to run a thread file around the threads at first to clean the threads up on one side, my crank was balanced for the Featherbed frame, so a lot of problem with having a smooth bike with everything rubber mounted as a lot of shaking with Isolastics system, my headers were made for my bike and it just mounted by the rose and one muffler bracket bolt at the rear, I can pull my whole exhaust off in one piece and only takes a few minute to do so.
1982 my header pipes were made, they getting a bit old now and a few prick holes have been brazed up just on the first bend, not bad for 43+ years on the Norton and they only cost me $20 at the time to bend up and another $20 to get them chromed, a muffler shop who had a mandle bender and knew how to use it, 30 mins for each header.
As hybirdrider has said its the rubber mounting system that cause the problem with crack headers and thread problems, ever Commando motor I have rebuild for mates or worked on had the same problem of flogged out threats, some was an easy fix and a few had to be sent away for repairs, but that's the cost of a smooth running bike, but my 850 Featherbed runs pretty smooth for a hard mounted motor if balanced right.
One Commando motor I fixed up had the stud set up and bolted on flanges but who did it was a bit rough and the thread damage was just down the bottom of one thread and could have been fixed easy without doing a bodge job as the rose was still tighting up when I screwed in a spare rose I had.
Not sure what happened to the bike as I did what I was asked and the owner while still building it passed away from the big C, RIP Kim.

Grandad said:

Quite some decades ago when the roadster suffered worn threads ( and I was skint - again ) I used plumbers hemp and bosswhite, this lasted about two years until I sold the bike.
T’was a bit smelly for a while though !
Hemp goes for up to 200*C
I understand ptfe will do 350*C and comes off easily.
You’ll need to wind on the tape in the correct direction eg clockwise as per the nut.







Hemp and bosswhite worked back then, but nowadays PTFE is the go-to — it handles higher temps, comes off cleanly, and many plumbing services in Vallejo recommend PTFE or purpose-made sealants for reliability and easier future maintenance.

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Yeah, PTFE tape is definitely the better option these days, cleaner, easier to apply, and handles heat far better than hemp ever did.