There were a few outstanding jobs before the engine build was complete. I already had a rebuilt BTH TT magneto but this needed new studs to secure it to the mag platform - 3/8” BSW one end and 3/8” BSCY the other end.
New sprockets and chain were used and the chain tension was checked before the inner and outer timing cases were fitted.
The timing was set to 370 BTDC fully advanced (I intended 380 but having tightened the nuts on the sprockets it came out at 370).
Last, but by no means least, the carburettor needed a bit of attention. I was extremely lucky in already having a 1 3/32” RN carburettor – the exact size of the inlet tract in the cylinder head. I also have the remote float chamber which, with a downdraft-angled inlet, is necessary. The RN – Remote Needle – carb was an intermediate step between the TT and GP carburettors and I don’t think was produced for many years. There is more information here for anyone interested in reading more about these instruments.
The component parts of this carb and float chamber look like this.
Amazingly, some spares are available for these carbs – see here – which was fortunate because I needed all of the components for the air valve and a couple of other bits and pieces.
After reassembly and making new gaskets
the carb was fitted. The 2 drilled bottom screws will be security wired in due course.
The last job on the engine was to change the position of the engine sprocket – it needed moving outwards. You may recall that when I built the crankshaft – MOV flywheels and “K” mainshafts (see here) that the distance between the flywheels is less than the original “K” design. A consequence of this is that the mainshafts are further inboard and this will also position the crankshaft shock absorber (and hence the engine sprocket) further in.
The way I have solved this is to make a thicker K-145 friction disc (5mm thick) to move the sprocket outwards relative to the main body – the original is also shown in the picture below.
As ever, every step has a consequence and here, this also moves the K-93 left-hand threaded screwed collar outwards which was then impossible to engage with the main K-88 shock absorber body.
Luckily, I had a screwed collar with a longer section of thread in my collection of bits but I then found that the start of the internal thread on the shock absorber body was in poor condition and I could not start the thread. How to clean up an internal LH thread? I measured the pitch with a thread gauge – 24 TPI and, luckily, I had a 5/16” UNF LH set of taps and a die (also 24 TPI although the thread angle is 600 vs what is probably 550 for the shock absorber – but that really doesn’t matter too much for this job) and was able to use the plug tap, held in a tap wrench, as a thread chaser
to clean up the start of the thread in the lathe.
This worked perfectly and the shock absorber assembly is now installed and the primary chain is in perfectly alignment.
At the time of writing this blog the engine has actually been installed in the frame
….more next time.
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