Author Topic: DT5 INFO  (Read 5377 times)

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Offline andy

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DT5 INFO
« on: 05 Oct 2006 at 13:14 »
Dear Doug,
         Many thanks for your help so far with my DT5 project,its really appreciated.
 I have got the gearbox in bits at the moment and as you probably know 1st and 3rd gears have meshing dogs but 2nd is just a "crash" type.Everyone ive talked to say you cant change gear on the move without wrecking the teeth.Do you know if this is the case with road going Douggies - ie SWs,F31s etc,and could i use these internals in the DT box? .
          Im also wondering what jet sizes to use , as i want to put the bike back on petrol and also what the ignition timing should roughly be ?.
Thanks again for your help Doug.                     PS had great fun at the Chalfont Hill climb last weekend on the Douglas - it went superbly.

                                                            Regards,
                                                                  Andy

Offline Doug

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Re: DT5 INFO
« Reply #1 on: 06 Oct 2006 at 02:51 »
Yes, second gear is of the 'crash' type. To facilitate engagement of the teeth, the ends are rounded over. But they do lead a hard life and can get rather battered. When the whole length of the tooth takes on a barrel shape, they become prone to jumping out of gear. They are available new here.  They were used up to the F/G32, but superceded by the four-speed box which was all constant mesh with dog clutches on the 1934-35 OHV machines. The few crash types I have ridden were not really any more difficult to shift through second than first of top. And do not forget all Indian Chiefs as well as many other Indian models used the same type crash second gear concept. The key is a slow and deliberate shift, not unlike nursing a ponderous Burman gearbox into the next gear.

As for jetting, I have not seen any literature for straight petrol. The data sheets the factory sent out were for a 50-50 petrol/benzol mix. There are also two series of carbs fitted, the Amac and the Amal. I have the Amals fitted to my SW, and I could check to see what they have fitted in the way of jets. It is basically as I got it, and I only ran it a little before I discovered it needed a complete overhaul. But it was run about a quite bit in the UK on the road during the early fifties in this configuration.

The ignition timing will depend on if you have the anti-oil fouling pockets still present in the cylinder heads. If so, you will find 40-35 degree advance is about right, though I have seen up to 45 quoted. This is because it takes so long to get the mixture lit! I ran my 1934 at 35 degrees and it still has the anti-fouling pockets. Some heads have had the sparkplug hole drilled right through into the combustion chamber and long reach plugs fitted. Then anywhere from 35 to 25 degrees advance is appropriate, depending on compression ratio, state of tune, etc. It is hard to give an exact figure as so many things have an influence. Since the opposed twin is inherently smooth you will readily notice the engine get rough if you have the ignition set too advanced.

On petrol/benzol the compression ratio was 7:1, and on R.D.I alcohol, 9:1.

-Doug

Offline andy

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Re: DT5 INFO
« Reply #2 on: 06 Oct 2006 at 14:14 »
Dear Doug,
             You continue to astound me with your knowledge and promptness in replying,thankyou very very much.Yep - my DT has got the anti-fouling pockets and the ignition is at 45 BTDC at the moment ( on dope ).It looks like it has compression plates fitted ( about 1/8" ) but not sure,im hoping to look at Henry Bodys bikes soon,so ill check then,and find out what comp ratio its on at the moment.
             Ive bought a Norton box off e bay,but might see how i get on with the original one ( its got a norton clutch - so might be ok ).
              I wouldn't dream of asking you to take your carb jets out Doug just for me,but if you do happen to have one apart one day,the jet size would be very handy,but ill experiment with different ones when i get nearer to completion.
               I think im going to use a BSA "B" type hub for the front until i can get a Douglas one made so i can MOT it and get it registered.
                Saw Jim Gregory,Terry Stone etc doing exhibition races at Oxford speedway last week on Douglases/Rudges - absolutely fantastic.
                                     Many thanks again Doug,
                                               ill be in touch,
                                                      andy

Offline Doug

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Re: DT5 INFO
« Reply #3 on: 07 Oct 2006 at 03:51 »
Andy,

I have found some info re- jets. For petrol-benzol, the Amal jet numbers are 110 to 130. If using the older Amac jets the equivalent range would be 38 to 40. If running the high compression with alcohol, the factory recommended increasing the jet size 40%. Not sure if that meant by diameter or area. E.W. Knott’s “Types of Motorcycle Carburetters” pretty much suggests that the same jet size was appropriate whether running petrol-benzol or straight petrol.

My SW has 130 jets fitted, which pin gage at 0.039”, or a #61 drill. The slides are 6/3, and the jet blocks are stamped 39 on the top, and 30 on the bottom. As it so happens they are apart just now so I can send the bodies out for re-plating while rebuilding the engine.

You have to be careful about barrel shims. It is not uncommon for folks to cut some off the bottom of the base flange to increase compression.  Shims may have been fitted to restore it back to where it was. The original barrel length for the long stroke DT/SW (62.25mm x 82mm) was 4.825”.  The early style barrels had thinner flanges to begin with, and it is not unknown for these to crack if shortened; especially if a thick gasket is fitted, the case halves are miss-matched, or the studs not pulled up uniformly. Later barrels have a base flange a uniform 5/16 thick and are a little more robust. Another possibility is aftermarket pistons were fitted and a little more head clearance was required (an 1/8" shims sounds like rather a lot.) So if the barrels measure the original length, you will want to check the piston-head-valve clearance with a bit of modeling clay. If that is o.k., then I suppose they just needed to lower the compression. The pistons in my SW were genuine Kingswood, and had a modest 5/16” dome, so I think these were the 7:1 ratio. But I never measured the combustion chamber volume so do not know for sure, and they were in no shape to reuse anyway. The old Hepolite replacements for the OHV roadster engines used flat-top pistons, so I suppose these ran a little lower compression. I built up the crowns of some Hepolite pistons of unknown origin, moved the valve pockets about, and cut the skirts to suit a +15 rebore.

-Doug

Offline andy

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Re: DT5 INFO
« Reply #4 on: 07 Oct 2006 at 09:44 »
Doug,
         Yet again,so grateful for the information again,Im at work in Brussells at the moment,and get back home next thursday,when ill delve into the engine and let you know what I find. Ive got a feeling your right about the bottom of my barrells being machined off a bit and someone has fitted compression plates to restore a more friendly ratio. Not sure what pistons are fitted yet.
                     
                                          thanks again Doug,

                                                regards,
   
                                                     andy