Author Topic: Gearbox bolts  (Read 4137 times)

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Offline ste

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Gearbox bolts
« on: 17 Jan 2010 at 10:22 »
Three bolt size questions:

1. Does anyone know the correct size and type of bolt used to mount the gearbox in the frame for a 1924 TS?

2. Also the same question for the bolts and nuts holding the two halves of the gearbox together. I have the long bolts but no nuts.

3. Finally, what's the size of the bolts holding the motor in the frame? I have one!!

Is there a way of buying suitable replacements? They don't seem to be listed in the club parts list.

I've also seen: https://www.douglasmotorcycles.net/index.php?topic=1102.msg3721#msg3721
which is very comprehensive. But, how do I work out the length of each bolt?
« Last Edit: 17 Jan 2010 at 10:35 by ste »

Offline Chris

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Re: Gearbox bolts
« Reply #1 on: 17 Jan 2010 at 12:21 »
Hi Ste
     1. The gearbox is held by studs and nuts, not bolts. The studs are threaded from each end and are 3/8" x 28tpi. The length can be determined by measuring how deep into the gearbox the stud will screw plus the thickness of the frame lug plus the top hat washer thickness that goes on the stud below the frame to centralise the gearbox in the slot of the lug plus the thickness of the 3/8" x 28tpi nut and a washer. The rear one should also be longer still to allow for the chain guard arm.
     2. The studs for the gearbox halves are 3/16" diameter. The original threads are an unusual very course size and it is very much easier to make new studs from 3/16" rod and thread 2BA each end. The lengths again can be calculated quite easily by measuring across the width of the gearbox halves and then adding the thickness of two nuts and two washers. If you have the gearbox with kickstart mechanism then the top, bottom and rear studs need to be longer by the width of the lever extension plates each side.
     3. The nuts and bolts holding the engine to the frame are Douglas special threads 5/16"x 25 tpi. and the bolts are also a Douglas special with a domed head and a locating peg under the head to engage with mating slots in the engine mounting brackets.
     I regret that none of these parts are held in LDMCC spares but they are all easily made by any enthusiast with a small lathe. Chris.
« Last Edit: 18 Jan 2010 at 08:43 by Chris »

Offline ste

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Re: Gearbox bolts
« Reply #2 on: 17 Jan 2010 at 17:31 »
Thanks Chris, that's a big help. You have an amazing wealth of knowledge.
Stephen

Offline Chris

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Re: Gearbox bolts
« Reply #3 on: 18 Jan 2010 at 08:35 »
Hi Ste
    Further to my answer above reference the engine mounting bolts, I initially gave a response from memory. Thinking about it afterwards knowing that the special bolts with dome heads that hold the crankcase halves together are the Douglas special threads 17/64" x 25 tpi. I wondered why the engine holding bolts would be BSF. Having checked, I find as I suspected that Douglas used their special threads for these also and they are in fact 5/16" x 25 tpi which are the same as the cylinder holding down stud threads and the steering head clamp bolt threads. Clearly if you are making these items from scratch and do not have the special thread taps and dies available from the LDMCC Spares Scheme then BSF threads (22 tpi.) will do the job. I have corrected my previous posting.  Chris.
« Last Edit: 18 Jan 2010 at 08:42 by Chris »

Offline eddie

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Re: Gearbox bolts
« Reply #4 on: 18 Jan 2010 at 10:03 »
Hi Stephen,
                  As Chris said, the engine mounting bolts are drilled for locating pegs to prevent them turning. Most veteran/vintage machines had the bolts drilled just under the head at right angles to the bolt - resulting in a weakness at that point. Douglases obviously were aware of this problem because on the later machines - like the Dirt track - they moved the drilling to be in line with, and alongside the shank of the bolt - leaving the shank intact.
                               Regards,
                                           Eddie.