A  History of Classic Racing  50cc Motorcycles


The Mk 5 Water-Cooled ITOM

Ian Ager (50cc Racer and Tuner of the late 1950’s and 60’s)

Written for the Small Torque Magazine of the "Racing 50 Enthusiasts Club"

After watching the 250 mile Enduro race at Snetterton in 1961 a decision was made to water-cool the ITOM engine: “playing” about on the air-cooled engine showed that as the engine heated up the performance dropped off, by water cooling the expansion rate of the cast iron barrel could be controlled therefore the engine could be kept working at its maximum performance.

The water cooling was to work on the thermo-siphon system, so I searched the automobile technical publications of the period for any articles on water cooling as to the operating temperatures at which to run the engine at etc, but none were found so it was a case of checking on a friends old pre=war Ford 8 and Austin 10 car engines and record their operating temps, 

I did have a quick word with Dave Simmonds about the operating temperature of his W.C.S. ITOM, and he said he was running with the water just about to boil, from the recorded temperatures of the car engines and Dave’s information, a water temperature of 90 degree C was chosen as a starting point.

A standard ITOM Astor barrel was purchased, all the cooling fins with the exception of the top and bottom fins were removed, this then gave access to the transfer tunnels, which then had the top edge cut off on the angle to give access to the ports.

The shape, position and sizes of the ports were copied from the air-cooled engine; the exhaust port was raised 1mm (this saved having to modify the piston local to the exhaust port). Brian Woolley in one of his chats had said that he had reduced the angle at which the gases enter the barrel via the transfer ports which seemed to be an improvement, so these were made with a 15 degree angle knowing that I could increase this angle with the use of “Araldite” at a later date if required back to that of the competition barrel, and the inlet stub replaced to suit the 18mm carburettor as per the air cooled engine. 

After all the ports had been filed and polished, scrapped pieces of the removed fins were used for sealing the tops of the transfer ports, these and the water jacket which was made from a soft copper sheet were silver soldered onto the barrel, silver solder was chosen over “Sif” bronze welding due to its lower melting temperature, thus avoiding any distortion if possible by the heat applied on the already finished sized barrel. Two 1/2” water inlet stubs were fitted one either side of the exhaust port, and one 3/4” outlet at the back of the barrel above the carb. 

The hunt was now on for a suitable radiator there was not much to choose from in those days, it was only the top of the range cars that had heaters fitted in the late 1950`s/60`s where the grille could be used, and they had not yet worked the way into the car breakers yards, the only water cooled bikes were the Scott and L.E. Velocette. On inspection the Velocette radiator was found to be two radiator grilles separated by a air box, with common water chambers top and bottom. With using one of the grilles, the water chambers were modified to suit the required water connections top and bottom.

With only two days before the race in 1962 the engine was rebuilt using an old standard thick ring piston that was originally fitted in the air cooled engine for safety, and the ignition reverted back to the standard ITOM system, as I had no knowledge if a battery ignition would last a 250 mile race. In the morning practice session before the race the problems showed up, the engine would only rev to 11000 rpm and it was misfiring, the condenser and points were changed and the square expansion chamber exhaust replaced with the standard 'megga' all to no effect. 

With no practice time left it was a case of starting the race and see what transpires, we had agreed to change riders after each 15 laps completed (40 minutes), at each pit stop the engine temperature was checked and part of the radiator grilles blocked off at the bottom to raise the water temp, by the end of the race we were still there, the engine was revving out to 11500 rpm but it was still misfiring, We were so pleased when the results were pinned on the board, saying we finished in 8th place. The little WCS (right) did well.

In hindsight I should have put the square expansion chamber back on the engine before the race, as all we had was a buzzing in our ears for a couple of days after, just ask the likes of Chris Walpole and Vic Dedden what it was like. George Ashton had shown an interest in the engine at the Enduro and meeting up with him again at a Saturday open practice day a few weeks later at “Brands” he had a ride on it, by this time I had the battery ignition back in place, fitted a standard sized thin ring piston and the square exhaust, and my lap times were down to the 1min 18secs, within a couple of laps he was down to that and under, this is where his weight factor came into play. 

A couple of weeks later he rode the air-cooled engine bike at Snetterton and finished 4th behind the Simmonds brothers, from that day I was plagued with phone calls from George Ashton senior to sell the air-cooled or water-cooled engine to him. A deal was struck that I would do the necessary to George’s engine and fit the water-cooled barrel etc for the sum of £20 plus his old air-cooled barrel. At the next race at “Brands” guess who was two places in front of me at the finish of the race? From being in the last dozen finishers he was now in the first dozen.

With the information that I had obtained from the air-cooled engine in regards to the fit of the piston in the bore, that basically the piston has two duties to perform. 1) to compress the gas in the top half of the cylinder prior to ignition, 2) to reduce the pressure to below atmosphere in the crankcase whilst it is rising to pull in a new charge ,then to compress that charge up via the transfer ports to the cylinder on the descent. If the skirt clearance is excessive, with the piston rising it could pull in positive pressure gases from the exhaust port down the side of the skirt, also when descending could blow some of the new charge back out of the inlet port so therefore upsetting the tuned resonance of the inlet tract. A serious look at the piston showed that it was more than just a plug of aluminium with a couple of ring grooves and a hole for the gudgeon pin.

On inspection I found that the piston was .008” smaller in diameter across the piston skirt than from front to back this seemed to be the normal practice on pistons when I checked an old piston from my 125cc Villiers engine. A new thin ring piston.40.2mm was machined down to give a true.0015” fit in the bore, I was working on the assumption that material technology had moved on a bit with the inclusion of silicone in the aluminium material, plus there were new formulated oil additives now available for two stroke engines. After the engine was run with the re-machined piston it was checked and the only signs of seizing were either side of the gudgeon pin these marks were quickly removed with a file, I can’t ever say if there was an improvement in engine performance over the stock machined piston but it was worth the try, as nothing ventured, nothing gained. 

One other thing I tried whilst on pistons was to re-machine the top of the head, on the early pistons they had a contoured top to help the gas flow, to a conical shape and reducing the dimension from the top ring to the top of the piston head by .062”. This had the effect of increasing the exhaust and transfer ports total opening time, again I had no time to evaluate the work as it promptly burnt a hole in the top after only a few practice laps at Brands, 

Why the hole? I never did find that out, I put another piston in and all was well, one possible cause was that there was not enough metal left on the head after the re-machining. One of the small standard cylinder heads was machined on the inside to a top hat shape, with the dimensions that Herman Meir had used on the Ariel Arrow but scaled down to suit the 40mm bore, again there was no noticeable improvement in performance.

No other serious tuning work was ever done on the engine after it was sold to George. I had the engine back a couple of times to give it a quick once over, about a year later George purchased the CR110 Honda and sold his ITOM on to a chap called Vince in the Midlands, he for some reason had it re-bored, and at Church Lawford racetrack he was finishing at the back of the grid where it should have been at the front. 

A quick look with the barrel off and I soon realised why, you could throw the piston into the bore it was so slack, who ever had done the re-bore had machined it way oversize to the piston, the only way of saving the barrel was to chrome line the bore but he would not spend the money on it, after a few other race meetings at Church Lawford languishing at the back of the pack disillusioned, he disappeared from the scene. 

As a foot note to this article, regarding the fit of pistons in the bore, whilst in the island for the 50cc parade in 2000, I took over my 1968, 80cc Suzuki as a run-about. The Suzuki motor cycle company were doing free dyno tests for owners of Suzuki motorcycles, and they were quite happy to test the 80cc, (it made a change from the GSX-R’s etc). The engine produced 4.8 bhp with a top speed of 62 mph., when checked against the original Suzuki performance figures they quoted 8 bhp with the same top speed, but did not state if their readings were taken from the back wheel or straight from the gearbox. 

I would guess that the drop in power can all be put down to the fit of the piston, which is still the “as fitted” original, it now can be thrown in from 25 yards what you call a very rattley good fit, (new pistons for 80cc`s are now like rocking horse droppings). The top speed of the engine is the same, but it just takes so much longer to get there. 

Ian Ager. 

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