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General => General Douglas Discussion => Topic started by: Dave@NZ on 07 Mar 2013 at 06:01

Title: Repair method for worn manifold attach threads, 1915 cylinder
Post by: Dave@NZ on 07 Mar 2013 at 06:01

Hi Gents,
I am looking at reconditioning a 1915 cylinder barrel one of the issues it has is worn threads where the inlet attaches on the cylinder, the engine reconditioners who looked at it said as one fix may be machine remaining thread off then machine a new thread on it and have a new flange nut made to suit the new smaller thread.

Not quite so keen on this as I would rather keep it original size threads and flange nut.

 One idea I have been considering is removing all the remaining thread lathed off and a new thread piece (standard size) made which can be heated up and slipped on then tacked with some bronze weld not sure if this is possible.

Any ideas from the Douglas engineers would be appreciated, thanks in advance.
David.

(https://www.douglasmotorcycles.net/aa-files/images/dave@nz/2013/Douglas-manifold-thread-repair-750.JPG)

Title: Re: Repair method for worn manifold attach threads, 1915 cylinder
Post by: eddie on 07 Mar 2013 at 08:09

Hi Dave,
              Unfortunately, which ever way you go, this is not going to be an easy repair. Assuming the rest of the barrel is in good condition (not needing a rebore, etc.), I would opt for a 'cold' repair. A 'hot' repair may well cause distortion or cracking. As I see it, the major problem is going to be holding the barrel without the aid of jigs whilst a new thread is cut. This may well involve screwcutting it as there is not much room for a conventional  die and holder (if you can find one with the required thread!).
    I guess you have already thoroughly cleaned the old thread - they often look truncated if they are loaded with rust from a previously ill fitting nut. Even threads this worn are not beyond recovery if you have a slightly undersize nut and the patience to sit there with a triangular file and carefully recut the thread using the new nut as a gauge.

    Regards,
                   Eddie.

Title: Re: Repair method for worn manifold attach threads, 1915 cylinder
Post by: Dave@NZ on 08 Mar 2013 at 05:02

Hi Eddie,
Thanks for the advice, I haven't tried the file method, it does need boring out due scoring from the gudgon pin and the other repairs required are  valve guides, valve seats cutting and the odd broken fin repair.

I have been looking at it over the last week and it is a rather tricky area to carry out permeant a repair with out some other posable problem popping up.

Thanks
David 

Title: Re: Repair method for worn manifold attach threads, 1915 cylinder
Post by: Doug on 09 Mar 2013 at 00:27

David,

A similar repair for an A31 exhaust port thread can be seen in this post:

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

-Doug

Title: Re: Repair method for worn manifold attach threads, 1915 cylinder
Post by: Dave@NZ on 09 Mar 2013 at 07:31

Hi Doug,
Thanks for the information and photo link to the repair you have done it looks ideal and should be no problem for the reconditioners to do.
Regards,
David.