Restoring An Old British Classic Bike


Dec 9, 2008
I bought a 1952 BSA M33 in 2011, with a view to restoring it to original condition, when time allowed.

BSA M20 1 s.jpg

'Orrible colour, Chiang Rai mud brown. What on earth was the owner thinking...?
But, the bike was running and appeared to be mostly original with good tinware.
BSA M20 2 s.jpg

After I bought it, I rode it around for about 6 years, about half that time it worked, identifying and sorting out some issues.

During this period, when it broke down, I brought it home in a pickup about 4 times and pushed it home several times..

There was no speedo and the gearbox was missing it’s speedo drive shaft. I bought another gearbox in Mae Sai which had the speedo drive shaft and bush. As the thread was damaged, it needed modifying with extra thread cut and an extension piece then added.
2015-03-09 09.44.52.jpg

I stripped both gearboxes and rebuilt one box using the best parts. It needed new bushes, kickstart return spring, rubber stop and many more. Oddly, as original, the speedo cable passes through a tube welded into the oil tank. Actually, I rebuilt the gearbox 3 times before I got it right. I found that taking the box right out of the frame was the best way to get the gear cluster meshed in correctly.
2013-02-18 15.33.44.jpg

Epoxy glue used to try to prevent oil leaks.
2012-06-02 13.49.42.jpg

2013-02-18 15.34.23.jpg

The bike was fitted with a too wide final chain size; 525, 3/8” x 5/8” instead of correct 520, ¼” x 5/8”. The gearbox sprocket was the centre of the correct original, but with a wider outer part welded on. Possibly from a later A50 twin. The locking nut had also been welded on. I bought a new gearbox sprocket and milled the rear brake drum sprocket teeth down to the correct thickness, to fit the correct chain.
2014-07-07 13.22.51.jpg
2015-04-15 17.13.23.jpg

The cush drive engine sprocket was completely worn out and the weld repaired clutch drum had the outer part of a modern small bike’s rear sprocket welded on for the primary chain drive. The original primary drive parts used on the M33 were also made for the M20s, which had the same single spring pre-war design clutch. So, New Old Stock cush drive and clutch parts were found through Ebay.
2015-08-04 15.51.32.jpg
2014-06-30 09.41.44.jpg
2014-06-30 14.45.09.jpg

Found a complete New Old Stock clutch. Lovely parts indeed. Made in the 1950s, never used. Will clean up easily.
2015-02-28 13.53.00.jpg
2015-02-28 17.12.49.jpg

Some cush drive parts still available wrapped up with the original "Cosmoline" preservative grease. Gorgeous.....
2015-06-11 10.04.32.jpg

Made a new cush drive nut on the lathe, the original had been a little abused.
2015-06-04 17.15.47.jpg

I found a replacement Smiths Chronometric speedo, also in Mae Sai. This was operating, apart from the odometer trip reset. These are wonderful instruments and very reliable. Full of little gears and ratchet wheels.
Smiths 1.jpg

2017-03-27 11.25.10.jpg
Santas visit to the Viang 2011 7.jpg

I made some special tools, in wood and steel, to open the speedo as the bezel was screwed on tight.
2018-02-04 14.18.06.jpg
2018-02-04 14.59.21.jpg

The trip reset block was loose on the speedo main frame, so it needed a slightly larger screw & dowel, forming a new thread in the aluminium frame, to hold it securely, so it now operates.
2018-02-06 16.23.14.jpg
2018-02-05 16.28.41.jpg
2018-02-05 15.11.58.jpg
2018-02-07 12.09.24.jpg
2018-02-07 16.23.31.jpg

Brazed 2 M6 studs to the back of the case for bracket mount, epoxied in a new brass bulb holder collar and re-sprayed the case black. I didn’t want to refit the original “cowpat” cowl over the headlamp, so I re-formed an M20 speedo bracket, to fit the M33 fork top crown.
2018-07-07 08.09.53.jpg

The bike had an incorrect Wico magneto fitted and no means to charge a battery. This magneto may have been from a standing engine or an old tractor.
2014-07-07 11.55.33.jpg

From Paul Dunn in UK, I bought a rebuilt Lucas MO1 manual timing adjust magneto and a rebuilt Lucas EL3 dynamo which fit together to make the correct Magdyno, fitted at the rear of the engine. This critical instrument can give reliable service if rebuilt correctly. Fortunately, this magneto gives a “fat blue” spark when turned, as is required. The beauty of magneto ignition is that you not need a battery to ride.
2015-12-23 16.57.49.jpg

Later, I cut the wires to the older capacitor inside the magneto armature and fitted a BrightSpark Easycap circuit board capacitor that fits under the points block. Also the paper armature bearing insulators were breaking up, so those were replaced. Brightspark UK sell many parts for Lucas magnetos.
2015-12-18 14.08.47.jpg

2015-12-04 11.06.04.jpg
2015-12-04 11.17.56.jpg
2015-12-20 12.59.45.jpg

Made a little locking tool to lock the fibre dynamo drive wheel when tensioning the star spring clutch on the end of the armature.
2015-12-22 14.57.06.jpg

Trying to check the locking tension.
2015-12-22 14.54.44.jpg

Brass and paper spacers are used between the points housing and the main body to achieve minimal end float.
2015-12-20 11.10.06.jpg

Replacing worn pins in the magneto base. A new timing cover oil seal went in and due to wear on the armature shaft I ground the lower face off the timing pinion to ensure it ran clear of the timing case.
2015-06-18 16.49.00.jpg

2016-09-10 14.00.35.jpg

2016-09-10 11.33.44.jpg

Draganfly Spares in UK do an exchange deal for rebuilt oil pumps. These are a gear driven pump and new gears restore the correct oil pressure. It fits up in the crankcase above the sump plate.

The saddle (possibly from a scooter) had open frame mount brackets at the nose. This was so you could remove the seat without removing the petrol tank. Occasionally, if I moved back while riding, the seat nose would come apart from the frame and rear up in the air, which was a little alarming. Welding closed the open brackets fixed that.
out and about in CS.jpg

AO Services in UK supplied a 12V V-Reg electronic voltage regulator, which I fitted inside an original Lucas mechanical regulator box. The holes in the mudguard below the saddle are to secure that. This modern regulator is designed to work with a fifties dynamo DC output.
2017-03-27 11.22.20.jpg

Off came the Mikuni carburettor and on went an Amal 930 Concentric Mk1 (these were originally made 1967 – 1973). The original carb when new, was an Amal 289 Pre-Monobloc with remote float bowl (originally made from before WWII until 1954, when replaced by the Monobloc series). Although these are lovely instruments, they are a bit leaky and expensive. Amal now make the full range of carburettors for classic bikes. A late sixties style Amal pancake air filter fitted nicely on the Concentric carb.
Below is a new Amal Premier Concentric (an improved version of the Mk1) and below a standard Mk 1 to go on the M33.
2015-10-19 10.02.36.jpg


But by March 2017, getting tired of breakdowns, it was time for a full stripdown.

The engine bottom end felt fine, so not necessary to split the cases. A new Alpha Bearings big end pin and caged roller bearing is about $350.
I took the barrel and cylinder head to Richco Powdercoating in Chiang Mai for a matt crinkle black powdercoat finish. This should be tough enough to last a while on the engine.
2016-09-22 15.46.43.jpg

Bought a cheap Indian valve seat cutter, good enough for a few jobs, while sharp
2016-12-29 16.44.15.jpg

2019-05-11 14.47.42.jpg

2017-06-22 22.21.24.jpg
2017-06-22 22.23.53.jpg

In went a new STD size Gandini Italian made piston and rings. The piston found in the engine was a +0.060” item that had been hand filed and sanded to fit the bore!
Made a new small end bush on the lathe. The barrel was resleeved, then bored and honed back to STD 85mm size.
2016-09-22 16.29.56.jpg
2016-10-06 09.55.00.jpg

Replacement Hepolite sleeve.JPG

I put new iron valve guides in the head and fitted new valve springs, original springs over compressed. The valves appeared OK.
2018-06-16 13.14.02.jpg

Decided to make a better valve spring compressor tool, which looks a bit Heath Robinson but works well.
2019-05-11 16.21.15.jpg
2019-05-11 16.21.32.jpg

The primary chaincase was being used dry. The idea being that you spray chain grease on the chain through the filler hole, regularly. Not a great solution. The chaincase was made to hold oil (AFT works fine), to lubricate the chain. The inner case gearbox mainshaft sliding seal was missing and the case was distorted (due to someone sealing the chaincase with sealant and then needing a screwdriver to lever open the case). A friend repaired and squared up the inner case and I fitted the sliding seal. Now the case contains oil, as it should, even though the primitive cork seal leaks a bit… Also fitted a missing stop bracket, for the brake lever.
2014-01-12 11.30.39.jpg

New sliding seal and clutch hub.
2017-04-15 13.23.40.jpg

2019-02-03 08.09.29.jpg

2019-02-04 09.54.07.jpg

I was missing what I realised was an essential part; a cover to go right over the clutch to keep it dry. After some searching I bought one from Australia.
2019-06-17 11.50.54.jpg

The rest of the bike was stripped down and parts taken to Chiang Rai for 2 pack epoxy paint spraying. Khun Odd, who does paintwork for Golden Sun HD shop, does a great job.
2017-03-29 08.01.33.jpg
2017-03-29 08.02.05.jpg
2017-03-30 16.57.17.jpg
2017-03-30 16.57.49.jpg
2017-04-01 15.12.07.jpg
2017-04-09 17.00.56.jpg

The original ugly cowl was not used and instead fitted an oversize Indian made 8” headlamp (7” standard) with ammeter and light switch panel. Put in an LED headlamp bulb from Goffy’s in Wales. The incorrect enclosed battery box was not used and a frame with rubber battery box is fitted. The home-made western handlebars were replaced with the correct item from Armours in UK.
2019-02-14 16.13.50.jpg
2019-02-18 12.51.40.jpg
2019-03-07 13.55.53.jpg

Top is an LED, H4 Halogen bullb replacement and bottom is an LED with British Pre-Focus fitting for the M33.

I machined a new set of kingpins and bushes for the plunger rear suspension. Needed to make a special tool out of threaded rod to assemble the plungers into the frame.
2017-05-31 10.07.42.jpg
2017-06-05 17.01.33.jpg
2017-06-06 13.57.09.jpg

2017-06-26 11.35.11.jpg

The forks were rebuilt with new head bearings, steering damper friction discs, stanchions, stainless cap nuts with brass filler plugs, seals & seal holders and bushes. One of the lower stanchion nuts was from a foreign bike with different thread and had come loose, getting wedged in the bottom of the slider. Had to make a special gouging tool to remove it.
2017-04-04 13.51.00.jpg

2017-04-19 15.08.56.jpg
2017-05-11 17.57.10.jpg
2017-06-24 08.03.48.jpg

I rebuilt the wheels with powdercoated rims, stainless steel spokes and nickel plated brass nipples (Thanks to Mark for carrying them over from Central Wheel, UK). I prefer to true and balance the wheels on the bike, if possible to check any offets are correct. The tyres are easy to find in Bangkok, as all the classic guys use these; Dunlop Gold Seal K70, 3.50 x 19”, front and rear. The bike has a folding rear mudguard for easy rear wheel removal. Also the front mudguard lower stay doubles as a front stand for front wheel removal.
2017-12-01 13.59.56.jpg

It's great to see the newly painted parts going together.
2018-03-07 15.16.21.jpg
2018-03-12 17.51.41.jpg
2018-03-13 16.49.01.jpg
2018-04-17 11.57.52.jpg
2018-05-20 15.13.17.jpg
2018-05-21 17.20.10.jpg
2018-05-22 17.01.29.jpg

I replaced the cracked and leaking gearbox outer cover, after finding one in the States on Ebay. Although corroded and pitted it cleaned up nicely with varying grits of sandpaper.
2012-06-02 13.42.54.jpg

Made up a new wiring loom and fitted that with an original Ducon handlebar horn/dip switch. Didn’t like the look of the horn, so cut the centre out from the facia and glued some split electrical cable insulation around the cut edge. The added acorn nuts to the facia, to make it look a little like the original Lucas Altette horn (now hard to find). Made a bracket out of plate steel and fitted the horn where it should be, below the saddle.
M33 original horn.jpg
2018-01-14 04.10.50.jpg

Bored and sleeved the rear brake lever spindle hole to give a snug fit again. Footrest, gear lever, kickstart and handlebar rubbers were fitted and transfers applied.
2017-04-19 10.19.57.jpg

The petrol tank was not in great shape, with many repair pieces having been welded in. The original hole for the fuel tap had been blocked and a new hole made with tap fitted. But this was directly in the way of the underslung tyre pump, fitted as standard (I bought a new 14” correct type pump on Ebay). So, I opened up the old hole, fitted a new fuel tap and blocked the incorrect hole. The tank was coated inside with POR-15 sealant and then my wizard friend in Chiang Rai did a marvellous job filling and painting the tank with a traditional BSA green colour and silver (instead of original chrome). The colour design is more like a late pre-war tank.
2017-05-25 15.54.20.jpg
2017-05-25 15.54.44.jpg
2017-05-25 15.54.50.jpg
2017-05-25 15.55.00.jpg
2017-05-01 17.12.22.jpg

Also needed to weld up and re-drill the worn forward mounting holes.
2018-01-26 15.11.38.jpg

Super result.
2018-10-23 16.59.54.jpg

So, the bike took about 2 years to strip, repaint, refurbish and rebuild. Many many other jobs done during that time.
2019-02-04 11.36.26.jpg
2019-02-14 16.10.57.jpg
2019-02-14 16.11.27.jpg


After the rebuild was complete, on the first test ride, the engine seized after 800m riding. This happened twice, due to my lack of knowledge. The exhaust valve was too tight in the new valve guide. Then I realized the engine had the wrong valves…. So, with new valves in and more new guides, clearance at; IN 0.0018” & EX 0.0028”, the engine ran fine.
2019-02-17 13.57.22.jpg

Luckily, bent pushrods straighten easily...
2019-02-22 14.25.16.jpg
2019-03-07 13.36.02.jpg

Except that, I then realized the major reason for all the past breakdowns, was poor timing control. The timing of the spark is adjusted manually, with a lever on the handlebar. A cable then moves a face cam under the rotating points block at the end of the magneto. Due to wear, there was a lot of play between the movement of the lever and the movement of the face cam. I took up the play on the perimeter of the face cam with a curved strip of 0.008” shim steel. Then made a new actuating pin in the lathe, with a taller mushroom head. Now, the timing control is snug and fairly precise with the movement of the lever.
2019-05-18 15.41.22.jpg
2019-06-01 11.43.49.jpg

Terry's spanners, essential set for magneto maintenance. Includes a screwdriver, various small BA sizes and 0.012" feeler gauge to set the points.
2019-06-08 08.11.03.jpg


The bike is now complete, running and usable with a canvas roll toolkit & puncture repair kit in the toolbox.

There is still a lot of fettling to do. The front brake is weak, so a new pair of shoes are on their way from UK. The rear brake is just fair.

Its starts easily with the timing control set on ½ advance and the carburettor tickled to fill the float bowl. Once the engine is running, if the timing is set to full retard, the engines ticks over with a slow chug. When you want to ride, select a gear first (right side shift, 1 up & 3 down) and then increase the timing halfway, when the engine revs increase. Once out on the open road, set timing to full advance for a flying top speed of…… maybe 50mph…. I’m running it in at the moment, so 30 – 35mph is enough. With just 23bhp, it’s definitely a bike for quiet country roads and enjoying the scenery…

It was a long, interesting and enjoyable project, bringing a 67 year old bike back to good usable running condition.



Staff member
Jan 16, 2003
Chiang Khong
On man, what a true passion you have for old bikes. A Craftsman as Canthai says.
Amazing work & report. What an effort - 5-stars.

I trust you're ready for a drink Wednesday night now.


Dec 9, 2008
Thank you for the compliments, Gents. It's the 7th old Brit bike I've restored or built and find it a most satisfying hobby. These old machines are history in engineering and look wonderfully mechanical.
Done 200 miles on the bike now and it's great fun, as long as you have plenty of time....
Yes, I'll be testing the lights on Wednesday evening, riding up to Mekong Pizza.


New Member
May 26, 2019
Interesting read and well worth all your hard work and skill, hope to see(and hear it)in the flesh sometime, cracking bike.

Wish I had half your expertise !
Jul 20, 2013
Ian as someone who was at one time accountable for aircraft flight controls repairs take my hat off to you – a very thorough restoration! Just as a point of interest do you think the design of the BSA was a generation away from your Triumph twin?


Dec 9, 2008
Hello John,
Yes, they are very different machines. Although the Triumph and the BSA were made in the same decade, plus both were based on pre-war designs, Edward Turner's vertical twins were far superior in many ways.
The 1952 BSA is a 23hp thumper, low geared and slow. Lower revving than the Triumph with a longer stroke. But solidly built and reliable. An ideal tug for a sidecar, which many were used for.
The first 34bhp 650cc Triumph twin, came out in 1950 and was capable of 100mph. By 1959, the Thunderbird 650 was the touring model, low compression but high geared with dual seat instead of saddle. The 46bhp T120 twin carb 650cc came out that same year.
The M33 is fine for plodding around locally, but is a smaller machine. The footrests are a bit far back and the saddle is only comfortable for about 45 minutes without a break.
The Thunderbird feels bigger but has a lovely lazy feel and can cruise unstressed at 65mph. An easy starter with a wide comfy seat and footrests well placed for me.
Yes, worlds apart really.
Jul 20, 2013
Ian thanks for this - yes your interesting summary does match my non expert impression of the two bikes .

Eoin Christie

Jul 16, 2019
Great job! Restorations are, indeed, a labour of love - Discovering the mass of the iceberg that sits beneath the surface.
I very much appreciate the pictorial account that shows the 1,000 words-worth.
Happy riding.


Dec 9, 2008
For a bit of icing on the cake.... although from a much earlier era. I just love these...

I bought a Lucas No.60 "King of the Road" bulb horn, for 65 quid on Ebay.
2019-07-04 14.40.34-2.jpg

It looked a bit of a mess but did work when blown and no large dents. The flyscreen has a couple of holes but can live with that.

Joseph Lucas made these horns in Great King Street in Birmingham, England in the 1920s & 30s. Actually the Lucas factory was about 6 miles away from where I grew up. Desmo were another Brummie company making horns in those days.

It is all brass construction, with a brass reed, the same as a trumpet. They made several models and all extremely well made. The No.60 was the smallest and sold to fit cars or bikes. The sound tube after the reed goes inside the main horn and turns around twice before the sound emits from the main horn. So, although the main body is only 8" long, the horn tube is about 24" long. The same principle as in a Tuba or suzaphone. This gives it a lovely loud deep sound, quite different from the local quack-quack ice cream man horn.

Made up a paste mix of 1/3 Salt, 1/3 vinegar & 1/3 flour. Plastered this over the horn, wrapped it up with clingfilm and left it for some hours.
2019-07-07 09.18.30-1.jpg

This paste took off all the dross and about 90 years of patina, paint, lacquer, horse poo and whatever else.
2019-07-07 11.18.36.jpg

It was then just a case of rubbing it down with increasing grits of sandpaper; 240, 320, 600 & 1200. Metal polish to finish off.
2019-07-07 14.53.51-3.jpg

These is a small company in Aussie repairting old radiators, run by a vintage car guy. His wife runs a business selling various rubber horn bulbs to all the vintage car & bike guys around the world. I bought a quality item to handle the uv here.
Now, you wouldn't think you could run a profitable business selling rubber bulbs for vintage horns......
2019-07-15 13.27.27.jpg

Next had to make a clamp to hold it onto the handlebars. A time consuming job on the lathe, making two rings and then rivetting them together with twopieces of 1/8" steel rod
2019-07-22 15.50.35.jpg

Now mounted. I kept the original brass extension tube on it for now... can operate the horn with my teeth.... haha.. May take that off later.
2019-08-16 14.30.02.jpg
2019-08-16 14.30.17.jpg

Eoin Christie

Jul 16, 2019
Excellent, Ian. It makes me happy that a woman in Australia has found such a vintage niche, no doubt assisted by the globalisation of information and logistics access - A great mix of old and new.

The de-patina mix looks like it did a great job.