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Exhaust thread

ytzen

New Website User
VOC Member
The tread in the exhaust holes of my twin isn’t damaged but only worn-out.
My idea was to cut the tread with a small over size. To make a new exhaust nut with oversize tread is no problem.
The question is how to make a tap on my lathe with which I can make a cut.
 

Glenliman

Well Known and Active Website User
VOC Member
The tread in the exhaust holes of my twin isn’t damaged but only worn-out.
My idea was to cut the tread with a small over size. To make a new exhaust nut with oversize tread is no problem.
The question is how to make a tap on my lathe with which I can make a cut.

The threads were worn out on one of my exhaust ports too so I used a co**** thread tap of the correct number tpi to cut threads further into the aluminium. One of my National Co**** Taps (can't remember if it was 3/8" or 1/2) did the trick. I mounted a wooden handle on the tap and then very carefully scraped around the port using the remains of the old threads as a guide. Since it is only soft aluminium, it's rather easy to cut oversize threads this way, as long as you have a sharp tap.
After about twenty minutes of this I had nice new threads in the port. It worked so well that I did the same procedure to the other exhaust port, however it only required a few minutes of scraping to clean the threads up.
The main problem with the second port was carbon buildup in the threads, so the tap easily removed that.

Fortunately I was able to procure a set of Bronze Maughan Exhaust nuts which were a bit oversize. These were some which Tony Cording had purchased years ago but was unable to fit into his Prince exhaust ports.

After a bit more scraping to get the individual fits , the nuts threaded quite beautifully into the slightly oversized ports.
 
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pifinch

Well Known and Active Website User
VOC Member
being poor and broke in the 70s I tried the BMS method, degrease the port, smear isopon around the knackered thread, leave for ten mins then carefully screw the exhaust nut in. It lasted about 4 years, I only fixed it properly because I had the motor down, otherwise I might still be using it today!
 

stumpy lord

Well Known and Active Website User
VOC Member
exhaust pipe threads

Hi,
I have some where in the garage an oversize exhaust pipe nut[0.015 if I remember correctly.This was was made by Frank Willerton Of york,and if you are lucky he might still have the speacial tap that goes with it. If it is of use to you I am sure we can come to an aggreament.
stumpy lord
 

Tom Gaynor

Well Known and Active Website User
VOC Member
Exhaust threads and musings thereon

I've never had a Vincent nut slacken, while the Manx nut (which being a finer thread ought to be more secure) has to be wired in place, twice. The tightest wire will snap first, hopefully giving warning that the nut has begun to slacken and that burnt fingers beckon if it isn't caught before the second wire snaps. Or a long wait until everything cools.
Examining the MV three at MAD in Milan I noticed that every nut that was wired was wired in the same fashion, two sections of twisted wire between thread and anchor, with a loop about 1/8" diameter between the two twisted parts. Done on every wiring, it had to be deliberate. I reckon that by doing it that way one can tell instantly if a thread has started to slacken, because the loop closes up. However, alternative theories (Italian wire locking pliers with a short stroke?) are welcome.......
 

Glenliman

Well Known and Active Website User
VOC Member
I should point out that in order to use the method I described above you will need to use a bottoming type tap . If you do not have a bottoming tap in the correct TPI, you can do as I did by sacrificing a standard tap. I ground the starter section of the standard tap away . (shorten tap)
 

Roger Barton

Active Website User
VOC Member
Over size tap

Just been looking at this thread I may be able to help as I have an over size tap that my Father had made many years ago.
Just looked in KTB the orginal was 1.875" Dia.
The dia of the one I have is 1.90" so looks like its 25thou over size.

Regards Roger B
 

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