ET: Engine (Twin) Exhaust Washers

[Peter Holmes]

I think the oversize thread is a very small increase on the existing thread, so welding and recutting in its entirety is not required, you would almost be using what is left of the old stripped thread as a lead and just recutting it a little deeper, and using an oversize exhaust nut to suit, if that were the case I guess it could all be done in situ using a hand tap.

 

[erik]

What I did on my Comet when the thread was not that good was widening the existing exhaust nut by pressing a suitible bolt through the nut.I had to do it two or three times with a wider bolt trying every time how the exhaust nut is screwing into the cylinderhead. Erik

 

[bmetcalf]

That is a very big bolt, is there another name for the item?

Also, how did you support the nut while pressing through it?

 

[erik]

The nut was resting on the fins . The bolt is about two or three tenth of a millimeter wider than the threaded part of the nut . I did it on a 40 ton press of a friend ,but I think a 10 ton press will do the job. And with the comb end of a thread file I scrached out the thread in the cylinderhead.This solution is good now for 15000 miles ! Erik

 

[Vincent Brake]

I would not weld on a Head, as its heat treated (percipitation hardening material) like the UK norm LM25. unless you do the same treatment again Vic???

 

[bmetcalf]

I never measured it, but I thought that the bore through the nut was tapered and the bolt (maybe called a mandrel?) might be slightly tapered and would make first contact at the end of the nut that bears on the exhaust pipe flange.

 

[erik]

The inner diameter of the exhaust nut is towards the fins bigger than towards the thread. What I call bolt is a piece of steel which has the right outer diameter to press it through the nut. On a lathe you turn the pice of steel down to the right diameter.( Rundmaterial) .Erik

 

[oexing]

When you got a lightly stripped ex thread - how so, what´s left then - I could imagine that sort of tap you get for a decent spark plug helicoiling, see link below. This tap got the first part with 14x1.25mm original plug thread for finding into the old windings. And then it will cut the new helicoil tread with correct alignment from the old windings. Otherwise I would not believe the job on the Vincent ex thread could find an acceptable outcome. Big snag is you don´t have a through hole for the stepped tap so you´d need another tap for bottoming the short thread - not a cheap operation. At least no CNC required and once a company was found for producing these special taps , instead of helicoiling (at that diameter I´d have no faith) the Spares Co may provide standardized oversize ex nuts ??
Well, welding on cylinder heads is standard repair - on BMW heads with their alu nuts on alu heads . Cylinder heads are meant to accept 200-250 degrees C or a bit more in the heat of 100 percent power like in owners manuals of aero engines, so as to prevent them from turning soft or cracking. I don´t know much about Vincent aluminium found in cylinder heads , so no clue about heat treatments after welding but believe a lot of welding was done in past decades on Vincents without much troubles from this, so I am positive . . . .

Vic
repair tap

 

[vibrac]

Well dear old Rofor did it on my trials Comet outfit two days before the trial. I drove over to his place he did it while it was still sitting on my trailer, he popped the tap in by hand went back in shed took one of his blank nuts slid it in a mandril on the lathe set up the cutting tool and made a few passes spun the new nut in to check fit
and off I went to fit on the pipe.

 

[oexing]

Nice story, but we miss a few details: What sort of damage was on the old thread - what sort of tap, oversize or just cleaning up ? Oversize at what amount - and if so, I´d rather not want to do the job, even less so on the rear head on a twin on the bike. Thinking about all these fu***d up threads just by dodgy mounting the ex pipes and nuts by less talented owners a tap is a bad idea to give to them. And having an alu tube welded there for a bigger ex pipe to go over it is not a lasting repair, thinking of post war Horexes with alu heads. They had exactly this over-stub design. In real life the alu gets quite soft with all that heat and with a steel pipe and clamp it just collapses in time for a wiggly ex system, unless you accept this and have a few more positions to fix the pipes and all thoroughly to the bike so vibrations plus heat growth in cycles will eat less into the alu stubs. Not the best idea for long lasting joy.

Vic