My NSD

1928 Triumph NSD, registered January 1929, my first Triumph.

It was all there, had compression and a spark but would not start. I felt it would be best to get it running and take it for a spin to assess its condition. This proved not too difficult. The timing was a bit out (and I’ll come back to that – the magneto mounting was bodged such that the chain didn’t run true and the sprocket tended to loosen and let the timing slip). The stopper, however, was blockage in the carburettor.

Riding NSD

Decompression Knob
On the timing cover, the NSD has a funny little mushroom shaped chrome plated button.

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I had not seen one of these and could not guess what it was.  I enquired of a number of people, including the National Motorcycle Museum but did not solve the riddle until I contacted the wonderful VMCC Triumph Marque Specialist for real triumphs (his contact details are available to members through the VMCC website).  He explained it to me.  He called it a ‘decompressor knob’ and quoted from his NSD Instruction Book which advises to pull the knob up for starting and press it down again as soon as the engine fires.  When I got round to opening up the timing case, I could see exactly how it works.  There is a sort of half-cam on the exhaust camshaft that blips the valve slightly  open when it would ordinarily be fully closed, breaking the compression and allowing the engine to turn over more freely

.Exhaust half-cam

On my bike, the spring that loads the camshaft had gone soft and allowed the cams to wander about and giving the knob an inconclusive feel.  I obtained a new spring [I’ll add details later] and this mechanism now works beautifully; it really does aid starting.

The Carburettor
The carburettor is an AMAC 25MDX, 7/8” bore. Consulting the fuzzy reprinted Replacement Parts Catalogue, 1929 Models, Ref No. Book 1159, this is correct for the bike (the engine persists in running rich and sooting the plug, I’ll come back to this).

The Petrol Tank
The petrol tank looked rather good in authentic saxe blue, but it transpired that it had been refinished by the previous owner with paint that instantly dissolved in petrol. Also, the inside had been treated with one of those patent tank coatings that form a gooey gunge in petrol (they say it’s the ethanol in regular grade petrol) and it was this that had, of course, found its way into the carb. After researching and finding all sorts of ‘good advice’ and marvel products, I decided to use my own instinct. The tank did not seem to have any leaks and therefore, in my estimation, did not need sealing so I acid washed the inside with weak phosphoric acid. This did a good job of easing off both rust and unwanted product, and provided a passivating finish to the raw steel. Phosphoric is used in proprietary rust killing paints. It combines with the steel to form iron phosphate and inhibit further corrosion. I used a 15% solution, swished around in the tank for 5 to 10 minutes, drained and repeated. I then washed thoroughly by running water from a hosepipe through for a couple of minutes, drained and repeated. Finally, dried the inside by opening all orifices and leaving a heat gun on low setting blowing through until it was bone dry – about 15 minutes.
I eased a 1/8 BSP tap through the petrol tap connections to ensure the threads were clean. I made a couple of plugs from 3/8” OD aluminium round bar I happen to have, turned to 11/322 and cut with a 1/8” BSP die (28tpi). The connector for the brass screwed-on cap (1 7/8″ dia 20 tpi) was a bit rough and would not have allowed a good seal so I filed it level (I served an engineering apprenticeship and learned to use a file effectively), made a liner for the cap out of heavy gasket material and refitted the sealing washer that was in good condition. I added the gasket material to compensate for the small amount of material I had filed off, such that the cap would be able to close on to the neck and make a good seal.
I had the tank repainted, lined and badged by a local car paint shop. The badge was a transfer purchased from VMCC Shop
The petrol taps are brass, tapered plug type with gauze filters. The filters were a bit of a mess so I purchased replacement ¼”filters from seagullrestorer on ebay, and soldered these in, more or less following the Soldering gauze filter U-tube clip.

The Gearbox
My NSD was prone to slipping out of second gear, especially climbing hills – just when you need the second gear.  I decided to strip the gearbox and investigate.  This is quite an easy job; the NSD is well designed and pleasant to work on.  It is very thoughtfully put together; I suppose it was designed just before the Wall Street Crash when pressure to reduce cost had not yet bitten hard.  The manuals I have do not explain disassembly and assembly so I have captured important details in some pictures in this Gearbox Overhaul note.

NSD gearbox ready

The Gearbox – ready for installation

The Clutch
Another nice piece of engineering.  All I needed to do was clean it, replace the NSD Clutch Locking Washer and reassemble.
I chose to clean all parts with petrol to remove the grease and grim, then clean with brake cleaner.
On the NSD, the primary chain has a guard and not a full case so the clutch runs dry.  I left it dry.
The manuals neglect to recommend lubricating the roller bearing but this is clearly necessary so I dropped some gear oil into it before reassembly.  I lubricated the clutch rod as recommended and left the gearbox oil to take care of the rest.

On reassembly, I realised that the clutch spring compression nuts had been replace by plain nuts and washers at some point in the bike’s history.  I felt I need to remedy this.  They are quite easy to make and it’s enjoyable to make such simple parts so this is how I made replacement nuts for my NSD Clutch Springs.

NSD clutch assembled

The Clutch – assembled

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