The push-rod Manx
Penned by Paul Reed - HMCCQ Brisbane
The push-rod Manx at the Esk 2002 Rally
The project was to turn a
very mundane, overweight and underpowered, 1962 350 Model 50
Norton, into Norton's answer to the Thruxton and Gold Star.
(Well, not quite.)
The first step was to increase its capacity to 500cc, by using a BMW R80GS barrel and piston, giving it a bore and stroke 0f 85 x 88, instead of 71 x 88. This was not difficult to do, just a matter of plugging up the holes for the through bolts and re-drilling them, re-sizing the gudgeon bush to suit the BMW gudgeon, and making a spacer for the cylinder base to get the piston height correct. The alloy BMW barrel also saved many pounds in weight compared to the cast-iron original. The cylinder head was not altered, the original carburettor and inlet valve were retained, (giving a wide squish-band) as I was not looking for peak power at high revs, just good solid mid-range performance.
With this in mind, Ivan Tighe advised against re-grinding the cams, as the Model 50 cams are already a bit sharper than ES2 cams, to try to compensate for the excess weight, I suppose.
Next, I did away with the alternator and shortened the crankshaft to suit. As the Model 50 had coil ignition, I replaced the contact breakers with a flange-mounted Lucas magneto, ex Velocette, but fitted with manual advance. I had a fire sale, disposing of all original tin-ware (it weighed a ton), and the wheels, exhaust system, footrests, brake pedal, handlebars, headlamp, fork covers, tanks, everything went. I was now committed.
I bought a pair of Triumph conical hubs for $100 each, reconditioned them, and spoked them into flanged alloy rims. I ordered, from England, alloy petrol and oil tanks, the latter with a battery compartment, a set of Unity rear-sets, a Manx primary chain guard, and a Manx seat. Also a Scitsu electronic tachometer, and a pair of Tingate clip-ons.
It was then just a matter of bolting it all together. The tricky bits were getting the rear wheel and chain alignment right, and working out how to anchor the front brake. I made an engine sprocket from an old BSA A7 one, to give a 4.2 top gear for easy cruising.
In this form, it had potential, but was far from right. With the light 250 flywheels and big engine sprocket, it was hard to generate enough momentum to kick it over its 8.5 to 1 compression, with my puny 60 kg. The standard AMC gears were too wide, and while it accelerated well, and reved smoothly, it would not happily pull the 4.2 top gear. The dry weight was just over 300 pounds.
I was determined not to settle for a lower top gear, so the options were to increase the flywheel weight and the engine capacity, and try for closer internal ratios. A pair of ES2 flywheels (100mm stroke) increased both the capacity, to 565cc, and the flywheel weight, making it easier to start. This also necessitated using a shorter con-rod. I also fitted close ratio gears from a pre-war International, giving ratios of 1, 1.1, 1.33, and 2.33. It would now pull the 4.2 top gear with ease, and the top 3 ratios were a joy to use, but the low first gear was a pain, and the motor was nowhere as smooth as before. You fix one thing, but introduce another problem.
After experimenting with the balance factors, I finished up discarding the heavy BMW piston, with its low gudgeon, and machined up a BSA semi-finished piston which Dave Dettmar gave me. It required some accurate machining to replicate the cam-grind and taper of the BMW piston, but I felt that this was would be essential with the BMW plated bore.
After all this, it still vibrated, with a heavy B50 gudgeon pin. It was slightly better with a lighter B33 pin, so I knew I was on the right track and obtained a much lighter, tapered (internally) Gold Star gudgeon pin. This has finally given me an acceptably smooth motor, but don't ask me the balance factor, I started off with 65%. The final improvement was to import from Ken McIntosh, a pair of "Daytona" first gears, giving a 2.00 to 1 ratio, the highest you can use with a kick starter. There is still a bit of a jump from first to second, which can catch you out at times, but it is much better than the Inter first gear. I also found that the clip-ons gave me a sore back, so I shelved them in favour of a normal set of bars, with extenders. The long Manx tank looks good, but with the seat so far back, it is too much of a reach to the clip-ons.
It is NOT a Thruxton or Gold Star beater, but with better cams, and a bigger carby and inlet valve, I'm sure there wouldn't be much in it, with such a big weight advantage. As it is, it is very satisfying to ride, with excellent handling, (not as good as the Manx though), excellent brakes, (don't believe people who knock Triumph conicals), and effortless cruising at 65 to 70 mph. There is a lesson to be learned here though. If you want to build yourself a special, you have to be prepared to do the development work, the same as the factories do.
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