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General => Douglas Motorcycles - General Discussion => Topic started by: tck on 20 Dec 2020 at 16:07

Title: Head stem thread MK
Post by: tck on 20 Dec 2020 at 16:07
I am fitting a SA2760 cone on to a fork stem prior to fitting the forks the cone is extremely stiff on the stem to the point where I am afraid I may strip the thread. After Eddys help on the crank shaft thread I think this one is 1" X26  cycle thread the cap is off another set of MK forks but the thread looks the same and the cone goes on both easily for a few turns so I dont think the thread is different can that thread size be confirmed?
Title: Re: Head stem thread MK
Post by: cardan on 21 Dec 2020 at 04:28
Take care, because the 1" thread in the British Standard Cycle Thread series is 24 tpi, so this would be the thread most commonly found on head stems. But on Douglas head stems, I couldn't say!

Cheers

Leon
Title: Re: Head stem thread MK
Post by: tck on 21 Dec 2020 at 08:59
I checked the thread by using a BSB (british standard brass) 1" tap I have. BSB  is 26 tpi (but it has a 55 not 60 thread angle that cycle has) at that point  it did look the same pitch,however following the warning from Cardan I set my vernier to 1" and started counting, with a needle, three goes and I came up with 20 each time so its 20 TPI and I hope cycle many thanks
Title: Re: Head stem thread MK
Post by: eddie on 21 Dec 2020 at 09:40
Tim,
        The main problem is that the SA2760 cone is case hardened - the case hardening process usually causes the component to shrink by about 2 thou per inch of diameter - thus making the cone tight on the stem. In the past, I have resorted to lapping the cone in to the stem. A light smear of fine grinding paste and about 10 - 15 minutes of winding the cone on and off the stem, will have the cone fitting nicely. As is often the case, lapping usually removes more material from the hardened component than from the soft one, so the thread on the head stem will hardly be affected.

   Regards,
                  Eddie.
Title: Re: Head stem thread MK
Post by: tck on 21 Dec 2020 at 10:24
I have just discovered a compound called time saver paste I needed it to take the last few thou off my new 26057 front PB bush to give the necessary clearance it leaves no abrasive residue I will clean up the thread on the stem with a die and lap it in, thanks for the tip re hardness of the head nut.
I think this Douglas build has increased my engineering knowledge better than 10 years on other rebuilds :-)
Title: Re: Head stem thread MK
Post by: dalgrae on 21 Dec 2020 at 11:28
Hi as Eddie has said,this is exactly as he advised me,I added some light oil to my grinding paste,and gradually went back and forward many time,all the time advancing a bit further ,do not be tempted to go too far in one go ,as it will jam.  Slowly Sloooowly

Colin
Title: Re: Head stem thread MK
Post by: Vitesse on 21 Dec 2020 at 15:09
As is often the case, lapping usually removes more material from the hardened component than from the soft one . . .

Okay, I'm intrigued.  How does that work?
Title: Re: Head stem thread MK
Post by: eddie on 21 Dec 2020 at 15:36
The hard grit particles tend to get embedded in the softer of the 2 materials, then wear away the harder material as it is worked backward and forward. Proprietary laps for hardened shafts work on this principle and are usually made from light alloy so that it is the shaft that wears down.
Title: Re: Head stem thread MK
Post by: tck on 27 Dec 2020 at 08:57
The thread needed a little repairing too so I found an old 1"X20 die and since the space between the fork stanchions prevent a die holder, and the depth is deep, so I surrounded the die with a jubilee clip and tightened it up progressively winding it down with two all thread prongs and a top clamp. Its getting there.