Author Topic: TS Thread repairs  (Read 401 times)

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Offline SunbeamS7

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TS Thread repairs
« on: 13 Nov 2021 at 19:40 »
Greetings fellow Douglas owners,
I have several TS cylinders in rather good shape-- except for the threads on the intake flanges, which as we know, are cast integral to cylinder. Just wondered if anyone has ideas/solutions/experience/ a service, on how to repair these? I had wondered about building the flanges up via brasing and then set up an alignment jig (?) and re-cut threads with 1 1/4 x 20 BSCY die (once i can find one!) to original size/thread. Solutions/Ideas appreciated, as i suspect i cant be the only one running into this.
Regards
bryan

Offline cardan

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    • Leon's Vintage Motorcycle Page
Re: TS Thread repairs
« Reply #1 on: 13 Nov 2021 at 23:32 »
Hi Bryan,
Inlet threads are usually a bit easier to work with than exhausts that have often had some serious heat treatment.
How bad are the threads? If not too bad, you can have a go with a thread file, or a thread chaser, or a combination of the two. It's slow but if you go carefully it should be possible to end up with a thread that is both workable and round. Measure as you go. Finally make nuts to suit.
When I restore stuff, I try to avoid drastic treatments like welding unless it's absolutely necessary. My experience is that bad things happen - not always, but mostly to me!
Cheers
Leon

Offline Doug

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Re: TS Thread repairs
« Reply #2 on: 13 Nov 2021 at 23:56 »
It is difficult to sync with the existing threads when they are brazed up. Also, threading in and out of braze or weld material and the base metal tends to not give a good finish unless you completely undercut the old thread and cut the new threads in homogeneous material.

One method is show here an an A31 head:

https://www.douglasmotorcycles.net/index.php?topic=3420.msg12526#msg12526

Turning a new stub with the thread saves having to swing the entire head in a lathe to chase a thread. The cutting off of the old stub and boring a hole for the new stub was done in a vertical miller where it was easier to setup for odd angles. 

-Doug

Offline eddie

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Re: TS Thread repairs
« Reply #3 on: 14 Nov 2021 at 10:04 »
I concur with Leon's comments. In the majority of cases, it should be possible to obtain a satisfactory thread without going too far undersize (the threads often look a lot worse than they actually are!) - a split die will often close down enough to clean up the thread. You then just need to remake the nuts to suit - either by screwcutting or obtaining a suitable undersized tap. When re-assembling, give the threads a coat of 'Coppaslip' or high temperature Molyslip grease to prevent further wear/damage. Unless you are extremely experienced, dont be tempted to go down the route that involves any heating - these barrels are now near on 100 years old, and therefore likely to suffer all sorts of cracking and distortion - it's best to keep the repairs relatively crude and simple!

   Regards,
                Eddie.

Offline SunbeamS7

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Re: TS Thread repairs
« Reply #4 on: 16 Nov 2021 at 20:47 »
Many thanks to all of you for your considered responses and advice, greatly appreciated. Unfortunately the threads are largely non-existent so there isnt anything left, to try and clean up.
I would also be remiss in not saying that although i have owned/ worked on multiple years/makes/models of motorcycles almost all my life, i am both grateful for, and extremely impressed with this forum, the wealth of information and the willingness of fellow Douglas enthusiasts to so kindly share their knowledge. Getting a belt drive Douglas (now 3 of them!) was a dream come true for me and this forum has been invaluable in realizing/restoring that dream.
Many thanks,
(if anyone knows a source for the required 1 1/4 BSCY die, for inlet flange,  i'd appreciate it).
bryan

Offline chris mac

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Re: TS Thread repairs
« Reply #5 on: 17 Nov 2021 at 03:56 »
Don't underestimate the ability of high temperature epoxies such as Lab Metal or JB,  generally good to 1000F
Bore out the old stub, thread a new stub on the lathe and epoxy in place