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.


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; this is an ongoing investigation.  I progressively dropped the needle till I got to the lowest setting but then starting became very difficult.  Raising the needle up one notch has fixed the starting problem.  Now, with everything carefully adjusted, I needed a run to see how rich it was running – still much too rich.  Main jet fitted was a number 30 (0.018″) which seems to agree with old information I can find.

Joe Pletersky ( refurbishes these old carbs and he very helpfully supplied me with a new needle. I found a number 24 jet (0.0165”) at the Banbury autojumble and tried that – too lean, plug is very white and engine pinks under load. I machined up a new jet and used a 0.7mm drill. This is probably not very accurate but gives something near an AMAC number 27 (0.0276”). The bike runs well on this with good plug colour. Needle is part no. 4124, jet is part no. 4122.

The Mag-Dyno – a little improvement – Vernier Sprocket

The mag-dynamo is a Lucas MDB1, original equipment on the deluxe NSD (with lights).

It feels right to keep a vintage bike as original as possible, but I don’t see anything wrong with an improvement that might have been done in the day by an enthusiastic owner. On most road going British bikes, adjusting the ignition timing is a crude business.  You pull the sprocket off the magneto spindle and push it back on to the taper hoping to get it where you need it to be.  And you keep doing this until it is near enough.  Some works racers, even quite early on, fitted a Vernier sprocket which allowed adjustment in discrete steps.  I made such an arrangement for my NSD.

A Vernier sprocket consists of a hub with flange that has a number of small holes to take a locating pin. The sprocket (the part with the teeth) has a different number of holes on the same pitch circle and is machined to fit on the hub. The position of the sprocket is located on the hub by a pin.

The magneto spindle has a taper to take the sprocket (the hub in this case). The taper in the hub has to be machined accurately if it is to sit snuggly. It took some care to measure the taper which I found to be 1 in 10.  This was confirmed when I looked in the excellent Radco book, Vintage Motorcyclists Workshop. I should have checked this first!

 I gave the hub 13 holes (27.7 degrees apart) and the sprocket 12 holes (30 degrees apart) so rotating the sprocket to move the pin to the next aligning hole is a step of 2.3 degrees. The stroke of the 550cc NSD is 99mm. The handbook recommends fully advanced ignition at 7mm before top dead centre.  At this point, 2.3 degree is approximately 1mm.  We should bear in mind that fuel available back in the day was not what we have today – so slightly different timing may be appropriate.

I used a steel pin 3/16” diameter.  Holes were drilled on my hobby milling machine with a rotary table.   The sprocket was drilled first with 5/32” holes, stepping 30-degree increments.  Using this as a guide, I then drilled through the sprocket and hub, stepping 27.7-degree increments.  This ensured the holes on each part would be on the same pitch circle.  I then drilled all holes out to 3/16” (drilling undersized holes first avoided accidentally enlarging when drilling through).  The pin was fettled to give a smooth fit and a 4BA thread was cut in one end to allow a simple tool to be made to facilitate removal.

I machined up a nut and washer and set clearances so the sprocket would be slightly clamped to the hub when assembled.

My Lucas MDB1 mag-dyno spindle has a woodruff key slot.  This is normally not used but I decided to fit a key to avoid the possibility of slipping – another advantage of this arrangement.

I hope the features described will be apparent in the pictures.

The Mag-DynoMaking a clip for Lucas MDB1b mag dyno

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