6 cont'd. Inlet Stubs

Inlet Stubs for 32mm Flange Fitting Amal 10TT9 Carburetors

I always knew it was going to be impossible to find a set of early racing inlet stubs, and do you know what? I wasent disappointed!
I did advertise once in the Vincent Owners Club magazine but I was not surprised when I did not get a response. I guess I had always figured that eventually I would have to make them myself, but I left the job for as long as possible, just in case miracles happen.

Actually, I would like to see a real pair of early Black Lightning inlet stubs, just to see what they look like and what bore they are. If you look at early photographs you can see they definitely made some for flange fitting TT carburetors, however I doubt these were ever bored to 32mm. I would like to see if they are production items, or easily identified as one-off's. If anyone has more information I would be interested to know. I do have photographs of the bike in the National Motorcycle museum, fitted with these stubs, and they do look quite well made (I shall have to go back there and examine in more detail).

Inlet Stubs on Gunga Din

This picture of Gunga Din shows angle of inlet stubs well

When I came to make my own, the first thing I needed to decide was if I should make a pattern and have them cast in aluminum or fabricate them from steel. A few years ago I had acquired a set of Series 'D' stubs, which are aluminium and are made to accept Amal Monobloc carbs. I was hoping I might be able to bore these out to 32mm but when I looked closely, it was clear that even if they did bore to this diameter (which I doubted), they would be perilously thin. I also think that the racing stubs, certainly for the front pot, were designed to give a more direct (downdraught) path for the fuel. As the flange on the Monobloc is the same as a 10TT carburetor, what this did tell me was that making my stubs out of aluminium would very probably make them too thin to hold the carburetors without the risk of fracture.
One good thing with this plan though, was that I would not have to make wooden patterns for having the stubs cast, which I really was not looking forward to.

1948 Drive Side

1948 Drive Side showing rear flange fittng inlet stub

Fabricating Steel Stubs
Not suprisingly, I decided the easiest way to make the stubs was to find some steel tubing with an internal diameter less than or equal to 32mm. Unfortunately, despite contacting numerous pipe suppliers, I was unable to find any that met this specification, so in the end had to resort to using solid bar and boring it on the lathe to a 32mm internal bore.
For the flanges, I had available a length of xxmm X xxmm steel bar, from which I cut 4 individual segments using my power hacksaw. (For anyone who has ever had to cut large pieces of steel using a hand hacksaw, they will appreciate what a godsend a power hacksaw is!). Once the blanks were cut I then mounted each flange on the lathe and bored them to just fractionally larger than the external bore of the tubes. By making these flanges a loose fit over the tubes it would allow me to position them enough to achieve the correct angles (see accompyning photograph).

Raw materials, with bar machined into tube and flange with hole bored to accept tube.


Once the tubes and the flange blanks were cut I then positioned them between engines and carburettors, using archive photographs of early Black Lightning's as reference to find the correct position. Once I felt I had achieved the correct position I marked the blanks with a scriber and used copious quantities of Blu-Tac to hold them in place while I removed them ready for brazing.
As you might imagine, this part of the job was the most time consuming and fiddly. Once I started to braze the flanges there would be no further opportunities to change the position, so it was very important that I got it right first time! To this end, I made up a short 32mm diameter alloy plug, which could be fitted into each cylinder head inlet port and then the tube of the stub fitted onto it. This would at least ensure that the internal bore was absolutely parallel (which after all was the most important requirement for a racing engine).


Finished Stub

First finished stub next to Series 'D'.

Notice difference in size of holes!

I then braze welded each flange to the tubes, taking care to make sure only enough braze was applied to build the fillet between the metals, so it looked similar to an aluminium casting. The nice thing about brazing, as opposed to welding, is that braze has very good gap filling qualities, so providing the parts were well fluxed, the braze would easily fill the gap between the pipe and flange.
Carrying out this job took the best part of a day all told, but it was well worth it. Once each stub had the blank flange's brazed to the tubes, I could then offer them up to the heads/carbs once again, and scribe the final position of the oval flanges on to them. It was then a simple task of applying elbow grease, to hacksaw and file the flanges to shape.
Final job was to drill the holes on the flanges for fixing to the heads and securing the carbs. The heads had studs in place, so for the this side it was simply a case of drilling clearance holes. For the other flanges, these would hold studs, so they were drilled and tapped to accept 5/16" studs.

The finished products look nice and I am very pleased with them. I have to admit I am not yet 100% sure that I have got the position correct, as I will only know this once I have attached swinging arm, suspension, petrol tank etc - lets hope so, as I do not want to have to go through that rigmarole again! The rear carburetor tract is almost horizontal, while the front is very downdraught, it using a remote float chamber to overcome this. I am expecting to make a larger cutout to the bottom of the petrol tank to fit, as per the originals I have seen.
I have held off nickel plating them yet (Dec 2002), until I have finished final assembly and can be sure the position is correct. Once this is confirmed I will also give the internal bore a final polish and ensure the inlet tract is absolutely seamless.
By the way, if anybody out there would like me to make a set of inlet stubs for them - forget it, I would rather look for the originals than go through that again!

Finished Stubs

Finished stubs now waiting to be dull nickeled.
Series 'D' stubs in background for comparison