Author Topic: Dollshead Conversion  (Read 2206 times)

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Offline OCTypeCanada

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Dollshead Conversion
« on: 21 Jan 2018 at 17:07 »
Can anyone provice pictures showing a Norton dollshead transmission fit into a 1920ís Douglas?  I have a 1926 OCT type Douglas I am considering installing the dollshead into but would like to see how someone else may have done the conversion.

Thanks,  Gord

Offline Alan Cun

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Re: Dollshead Conversion
« Reply #1 on: 22 Jan 2018 at 08:09 »
Yes the Norton dolls head is a good choice for a Doug. The best way being the box set into a bracket that fits the original frame fitting. A plain flywheel is needed. As the clutch will be dry the best move is to fit Jawa speedway plate internals. I have used Norton boxes on 2 of my OHV bikes and used the clutches on other Doug gearbox models where I was short of the Doug parts. The Norton box is very robust  and you can easily do changes without the clutch. Just one point if you can find a dolls head with the upper plates dont overtighten the thru bolt as one side will break.
regards Alan

Offline eddie

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Re: Dollshead Conversion
« Reply #2 on: 22 Jan 2018 at 10:26 »
When we fit Dollshead boxes into our sprint machines, we get a 1/2" thick plate welded to the side of the box, to provide a rigid mounting plate, and then trim back the original mounts. That way, damaged cases as described by Alan, can be re-used. Of course, fitting the Dollshead box moves the final drive to the other side of the bike. This is fine if you have the dirt track hub, as a separate sprocket can be fitted to the other side of the hub without having to make modifications to the brakes. I run a plain front drum on the back of my sprinter. If you can find a Norton Atlas clutch, that will fit straight on and has bonded plates that we have found to be more than capable of meeting the demands of our sprinting activities. If you find a Dollshead box with a completely knackered case, you could do the same as me and machine a new one from a billet (see photo) - I made it in 2 parts - the main case and the plate to carry the camplate, and had them welded together - it's probably cheaper than finding another usable case!

  Regards,
                Eddie.

Offline cardan

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Re: Dollshead Conversion
« Reply #3 on: 22 Jan 2018 at 11:22 »
... you could do the same as me and machine a new one from a billet ...

Was there swarf Eddie?

Gord - The OC is a rare thing compared with the DT, built in large numbers in 1928-29, raced to death in the 20s, 30s, 40s ..., and the place where most gearbox conversions seem to happen. Have you considered keeping the Douglas box? I'm not sure how many OCs survive, but I'd guess not more than 10 or 20 worldwide.

Cheers

Leon

Offline eddie

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Re: Dollshead Conversion
« Reply #4 on: 22 Jan 2018 at 13:03 »
Was there swarf Eddie?

  Yes, just a little!!!!

  It was worth the effort, though! The 'box has been faultless - even when changing up under full throttle, with just a flick of the clutch lever. Even under that provocation, the Norton Atlas clutch hasn't shown any sign of slipping.

  Regards,
                Eddie.

Offline OCTypeCanada

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Re: Dollshead Conversion
« Reply #5 on: 22 Jan 2018 at 13:11 »
In answer to Leon's comment I should add that when I bought the bike, it came with a DT5 engine, no wheels and no gearbox.  I will place an ad in the proper section requesting an OC suitable gearbox asap along with some back wheel parts to suit the Douglas driveline.

Gord

Offline cardan

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Re: Dollshead Conversion
« Reply #6 on: 22 Jan 2018 at 22:57 »

Hi Gord,

I recall OCs used the TT gearbox. Doug will no doubt confirm, but that will make it tricky to find. The OB box has the shafts located differently in the housing, so I don't know whether an OB box will fit an OC.

In Australia, every 1920s OHV Douglas ended up on the dirt track, usually looking nothing like it looked when it left the factory. Was it the same in Canada? There's a good case for restoring vintage Douglas specials "as they come", particularly since so many of the original parts were blown apart in the day.

Cheers

Leon

Offline OCTypeCanada

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Re: Dollshead Conversion
« Reply #7 on: 23 Jan 2018 at 13:25 »
Leon, I am surprised I found a Douglas of that age in Canada. Our seven months of winter might have some influence on when bikes became popular.

Eddie, thanks for the pictures you provided, nice work on the gearbox! 

Gord

Offline Doug

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Re: Dollshead Conversion
« Reply #8 on: 23 Jan 2018 at 15:20 »
Gord,

The OC, TT, and DT/SW shared the same gearbox shell. It was different from the previous OB/OW gearbox in that the position of the main and lay shaft were swapped around. The main shaft was now at the forward end of the gearbox. Later on, the DT gearbox was reinforced and changed shape slightly, and there was a variation used on the F/G31 that had provision for a kick starter.

The only difference between a OC, TT, or DT/SW gearbox are the internal ratios, and what prefix code is stamped into the serial number. The OC, TT, and SW used a "UG" prefix box, which had conventional road ratios. The "YG" prefix is the most commonly seen, and use on the DT; it has ultra close ratios. You might think this would be desirable for your rebuild, but you would be wrong! The ratios are so close, that you cannot get a decent spread between low and top gear. Due to lack of choice, folk have used them in DTs converted into SW spec for road use but they are not satisfactory. You either set the bike up with sprockets to give a normal first gear and the engine is screaming its guts out at fifty mph, or you set it up for a decent cruising speed with a first gear good for 25 mph before the clutch is fully home! The "YG" gearset was really intended for Dirt Track racing where you did not shift gears during the race. You selected one gear that best suited the size of the track (1/8, 1/4, or 1/2 mile ovals) and you left it there for the duration.

There was also:
"TG" prefix, wide ratio used for the OC model.
'SG" prefix, close ratio used for the OC model.
"VG" extra-wide ration used for the TT model.

Gearbox shells are not impossible to find, it might take a while but they are about. Decent gears on the other hand are in short supply (hence more shells around than gears!) My SW5 had DT ratios inside the "UG" prefix 'box, so I made a few sets of the road ratio gears. The full spare sets sold long ago, only 2nd gear pairs remain.

If you go the Norton gearbox transplant route, it is important to build keys into the gearbox joint face to mate with the shallow grooves cut into the frame lug. The chain exerts quite a powerful pull on the gearbox, trying to wrench it free from its moorings. Early conversions that just relied on the studs failed as it just hammered the studs back and forth until they loosened up in their holes. 

One of my DTs came from Ontario, where it was used for hill climbs in the forties by Richard Gower.

-Doug



 

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