...can you fit an SW gearbox internals into a DT box...
Yes you could, if you could find SW internals! The YG, SG, UG, and TG prefix gear case housings are all machined identically, the prefix just denotes the gear ratios originally fitted.
The Dirt Track would have been fitted with the ultra close ratio YG box. Internally the reduction was 1.5:1, 1.25:1, and 1:1. Too close for road use, and as I understand you did not even shift it during a cinder track race. Rather it was supposed to provide some optimization for 1/8, 1/4, and 1/2 mile tracks; you stuffed it in one gear and left it there for the duration. The DT was not originally supplied with a clutch, and rolling starts were the norm. Later on most folks removed the gears and used the gear case as a counter-shaft; setting up the overall chain ratio to suit for their favorite tracks. What happens when using these cinder track ratios for road use is you either chose your sprockets to have a good getaway and peak out at 55mph with the engine screaming its lungs out, or you set the bike up for 70mph and have an incredibly tall first gear. My SW has DT ratios fitted and the former owner set it up for a good top speed. Consequently you practically burn out the clutch trying to get it rolling in first.
The UG prefix box was the one fitted to the I.o.M./TT machines, and I think possibly the SW too. There are far fewer of these sets about than those for the DT. The unavailability of the road ratio sets was so acute, I ended up making a few sets of new gears. The extras sets sold out so I never got around to advertising them, but two illustrations from the brochure are included for reference.
Larger view
Larger viewAnd a PDF of the brochure
here , 850kb download.
(Note: These are sold out)The TG prefix is a wide ratio box, also fitted to road machines. I have a damaged shell stamped TG, but I have never seen the gears fitted to one. The SG was a close ratio road box, listed as being optionally available for the OC model. I have seen the sad remains of individual gears for some of these alternate ratios. The specific ratios for any of these boxes other than the YG are not known. Douglas did not always publish the ratios for the OHV models, and when they did it was the over all ratio. Backing out the inter-box ratios does not always yield an exact match for what is possible given the limited tooth count pairings feasible. A PDF chart listing all the possible ratios is
here, most find it too cryptic.
All these different ratios were obtained by altering the tooth count of sleeve gear pair, and the first gear pair. So far as I know, all the boxes shared the same second gear pair, a 18-19 tooth combination. Top was, of course, direct.
Sleeve gears come in two flavors; short taper and long taper. The long taper is later, and the better one to have.
I know of only one person sprinting a DT with the original gearbox. You will have to accept the inevitable slow times using the three-speed, hand change box. That, and the lack of suitable road ratios, is why most folk do the Norton "dollshead" gearbox conversion.
Since these boxes use a second gear that slides the teeth in and out of mesh (first and top use dog clutches) the teeth on the second gear pair take a hell of a battering. Second gear pairs are still available. See advert
here.
-Doug
[remove text character glitches. 19Sep16 -Doug]
