The Mystery of the Seized Cylinder

Scroll down for earlier stories
  • Magnetos – a care free alternative
  • Art Seniors’ very fast JAP
  • A short history of JAP speedway engines
  • With and without glory
  • Skirrow progress too slow!
  • Starting trouble?
  • Great Cooper pics on-line

MYSTERY of the SEIZED cylinder

Most enthusiasts are familiar with the 8/80 V-twin JAP engine. It is essentially two of the famous 500 speedway JAP heads and cylinders bolted to a common crankcase to give 1000ccs of brute power. Made by the English company JA Prestwich, variants of their V-twin engines went into countless Morgans, Brough Superior motorcycles, speedway sidecars, and early Cooper racing cars.

Around 1990 I acquired a 1955 Mk9 Cooper fitted with an 8/80 JAP. It was an exciting car to drive with an excellent power to weight ratio, and went like a jet. For about a lap! Then, as soon as the engine reached operating temperature and ran on full throttle for a few seconds, the front piston would either seize and or melt. This was a real puzzle, because with identical carburettor and ignition settings the rear cylinder showed no signs of distress.

The obvious cure seemed to be to guarantee adequate fuel flow to the front carburettor, and fit larger and larger main and needle jets. All this achieved was to upset the carburration and allowed the engine to run a little further up the road on full throttle before the front piston failed again. Similar experiments with ignition timing, various grades and makes of sparking plugs, and substantial air scoops directing air onto the front cylinder, all failed to achieve a result. Over many visits to the race track for private practice, the engine just kept eating pistons and plugs on the front cylinder. Nothing I did made the car driveable flat out. Something was basically wrong. But what?

I started to ask around. One old retired sidecar speedway rider with a long memory knew what I was talking about straight away. He’d had the same problem with the front cylinder on his 8/80 sidecar outfit running hot, but not as bad as mine because his race lengths were only four laps, and his motor was exposed to the breezes. His cure was to have his sidecar passenger pump an extra shot of oil onto the thrust face of the front cylinder when he had a spare moment between corners! This idea had some merit because the rear cylinder of a V-twin gets most of the oil flung out of the big end and off the flywheels, and the front cylinder gets very little. However trials in this direction made no real difference to my engine.

Then I read something in an old 1954 Motor Cycle magazine that sounded interesting.

The article, (see below) by well-respected rider and journalist Vic Willoughby, was the story of a similar V-twin JAP used for record breaking at Brooklands and ridden by a Ted ‘Barry’ Baragwanath. I quote,……”The front carburettor jet was about 2 sizes larger than the rear to combat an INEXPLICABLE tendency to overheating of the front cylinder.” I kept reading and found more….”Early tests at Brooklands revealed  a pronounced tendency for the front cylinder to overheat… so much so that it was impossible to complete a FULL LAP ON FULL THROTTLE.”


That was exactly my experience, so I was not alone! But what to do? I was tempted to say, “OK. Just let’s use the car for hillclimbs and forget about circuit racing. The head is cast iron, has virtually no cooling fins, and is only meant for the speedway anyhow.”

Now, from the 1920s and even earlier, JAP made thousands of engines for all sorts of applications and one hesitates to question the design of such a well-known manufacturer, but after all the work we’d done, something had to be basically wrong somewhere. So the question remained unanswered. Why did only the front cylinder overheat when both cylinders were the same? But were they…… the same?

In desperation both front and rear cylinder heads were removed, stripped to the bare castings, and examined side by side. They were identical…. EXCEPT FOR ONE THING!

What follows needs a brief explanation. When we look at a standard 500 speedway JAP engine’s cylinder head, we find it has the carburettor and exhaust pipe ports located on the right hand side of the head, (like the typical English BSA or AJS.) This 500 speedway head will bolt straight on the rear cylinder of the 50 degree V twin. However if we try to bolt a 500 head on the front cylinder of the twin, we find the front carburettor pokes into the rear cylinder’s exhaust pipe. So Mr. JAP got over that problem by using front heads for the twins identical to the rear heads, but with THE INLET STUB FROM THE RIGHT SIDE TO THE LEFT. Now the front carb pokes out on the left side of the engine, well out of the way of the rear exhaust pipe.


‘Barry’ with ‘blower’ – not the usual view

But what does this have to do with our overheating problem? The answer is everything, because when we take a quick look down the inlet ports of both front and rear cylinder heads with the valves removed, something is obviously different! It’s all to do with the location of the spark plugs! On the rear cool running head the plug is clearly visible, and the electrodes are in the direct path of the incoming cooling air and fuel. But looking down the inlet port of the hot running front head, the PLUG is hidden around the corner and NOT VISIBLE at all, and therefore does NOT enjoy the full blast of cooling air and fuel on its electrodes on every intake stroke.

Consequently, on 14:1 compression on full throttle, and with skimpy finning on a cast iron head, it was reasonable to assume that it would not take long for the electrode of the hardest plug in the range to become incandescent, pre-ignite the charge, and destroy one piston after another.

So, working on this theory, we drilled and tapped the front head to 14mm, and transferred the spark plug to the opposite side of the combustion chamber where it would be directly in the path of the incoming charge, just like the rear head. The results were immediate!

Now with sensible jetting, the engine will hold full throttle down the long straight at Mallala, and do six flat out 1.25 laps before something else breaks … but that’s another story!

Chas McGurk

Editor’s Note: As best we can work it out, about 1933-4 JAPs moved the inlet port on the speedway engine from the left (as shown in the drawing  below)

Image (435)11237

to the right hand side, as described by Chas. The original head design then seems to have been used on the front cylinder of the 8/80 which went into limited production for the Skirrow in 1936. This same engine, but with the crankcase extended to provide a sump, was used in the 1949 Cooper twins and was followed by what JAP called the ‘Mk1 ’in 1950 with aluminium heads and barrels.





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