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Help to identify Douglas air box - DT

Started by cardan, 06 Nov 2023 at 04:52

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cardan

Hi All,

Here's a high air box, presumably from a 1928-29-ish Dirt Track. Can anyone comment on its original use? It has various cast but unmachined features, for example for a drive to an oil pump in the sump (?), for a cross shaft for air slides, and for an oil pressure gauge on top. Were all DT high air boxes like this, or is this an early one made from a TT casting?

Not sure exactly when the small air box came in, but presumably around 1930.

Comments appreciated.

Thanks

Leon

Doug

Leon, 

You surmise correctly. the castings were 'left over' from the 1926-28 TT/I.o.M. models Those had a vertical shaft to an oil pump directly below the airbox and attached to the side of a sump which was slung below the engine/frame. The vertical shaft had a gear at the top and was was driven by a three-start worm on the tip of the idler gear of the timing chest. There was a large 15psi oil pressure guage in the top face of the airbox.

I have yet to have seen a tall airbox that did not have those vestigial (albeit un-machined) features, so I do not think they ever bothered to revise the pattern. It was superseded by the small airbox. My understanding, also, is that the small airbox first made its appearance in 1930 catalogs and magazine articles.

-Doug

cardan

Thanks Doug. Funny they didn't modify the pattern, given there were probably more DTs built than TTs!

It's interesting that the main casting has (unmachined) bosses to accommodate a second cross shaft to operate air slides in the carburettors (as used on the RA), but so far as I can see even the earliest TTs used the AMAC 15TT25 carbs which had the cable-operated air slides on the outside of the mixing chambers. Another relic of earlier design.

Cheers

Leon

Hutch

Leon,

I have a tall DT air box that would have been cast from the same pattern as the one you show in the pictures. I concur with what Doug says about its origins.

I note that your airbox has a few extra holes that are not on mine, so possibly later modifications to fit a oil sight / drip feed and maybe an oil pump off the camshaft? I circled these on the following marked up version of your pictures.

Interestingly your airbox also has a "problem" with where the quill oil feed to the crankshaft goes very similar to mine. It appears the casting did not quite have enough "meat" on it when they bored the hole!!. On mine the leather seal on the quill only just covers the gap in the circumference of the hole. Not a good thing for a racing machine (!) but I gather it must have worked well enough.... I marked that with an asterix on the picture air-box-2 marked up.jpg

Douglas were renowned for not letting anything go to waste and this airbox looks like another example of this! I wonder if the factory fire of 1927 would have been partial reason why they may have had a surplus of these parts? They would have been concentrating on supplying EW machines during the recovery at the expense of all other models - in particular manufacturing specialist machines such as the TT? ...then came along dirt track racing in Britain and they had a ready market for the left over bits?? Not sure.....

Cheers

Hutch


Doug

#4
Changing the casting would have required altering the pattern and several core boxes. That requires time and equates to money, so presumably they figure it was not cost justified. Particularly for a cinder track machine where aesthetics was secondary. Who knows how big a stockpile of left over castings were on hand. And even if not a massive stockpile, they could have had a situation where when they ran out, then figured just running off another quick batch of castings would see them through. At some point they would have realized this dirt track phenomenon might stick around for a while. The more likely reason is you could still buy a I.o.M./TT model in 1928, and while the 1929 catalog shows the I.o.M./TT model as using the Dixon twin cam engine, I have my doubts that model ever went into production. So a 1929 I.o.M. model (if they sold any) would have likely been the same as the 1928 model. So it would behoove them to keep the pattern around unaltered, in which state it would suit either model.

I had a look though my collection of images and could not find one where the second carburetor control shaft was installed.

The tall airbox for the DT was drawn up in October 1929. There must have been an earlier drawing for the I.o.M./TT model, but it has not survived. Had it done so, it probably would have dated to around November 1925, when the 1.o.M./TT airbox lid was drawn. The 1929 drawing still has the features for the mechanical oil pump drive (see above rational), though the boss for the oil pressure gauge is not shown. The small airbox was drawn up in November 1930. Drawings seem to have been created after initial production was underway, and September-November was a popular time for such drawing work judging by surviving drawings.

The hole in the bottom of the tall airbox for the throttle shaft return spring is clever in its simplicity. They dropped a cotter pin (or split pin) through the hole and bent the tails over. Then used the eye of the cotter pin to hook the spring to. Quick and cheap to manufacture.

-Doug 

Hutch

#5
Quote from: Doug on 07 Nov 2023 at 04:41

....

The hole in the bottom of the tall airbox for the throttle shaft return spring is clever in its simplicity. They dropped a cotter pin (or split pin) through the hole and bent the tails over. Then used the eye of the cotter pin to hook the spring to. Quick and cheap to manufacture.

-Doug

Thanks for the information Doug - very interesting and useful. On my airbox there is a fitting (re-purposed clevis pin?) for the return spring. Could not find any mention of it in the DT parts list - but then again I didn't find reference to the spring either! (I may have missed it?) I agree that a split pin would be a cheap and effective spring retainer tho'! I guess if Douglas did supply a fitting for the spring it would have been easily lost and a split pin would be an attractive replacement.

Of course the carburettors have their own internal slide return springs so why the extra spring?? In my experience of motorsport the scrutineers want to see a secondary backup throttle return spring so it may have just been something that was easily added to comply with local rules and may not have been originally supplied by Douglas?

I also note that on the airbox I have there is a flat machined section on the inside adjacent to where the magneto drive would be (on the timing chest side) - maybe another unused option provided by Douglas to mount a OC type oil pump driven by the magneto gear? On mine someone has scratched by hand where the magneto shaft would be on the timing chest side - but thankfully they didn't drill it through!

Cheers

Hutch