Triumph GP Trophy type race engine with Square barrel & head.
I am not saying this IS a GP engine, or even a Trophy engine. That’s clear from the 5T engine number stamp and the date of 1946.
It’s far more interesting than that.
The previous owner has clearly used this Triumph GP Trophy type race engine. A considerable amount of work has gone into matching an old 1946 5T 500 twin left hand engine case, with a later, 1950’s, big bearing timing-side engine case. Presumably, this was so that he could have bearing shells that he could replace.
This engine is similar to the ones used in the factory works Triumph Grand Prix bikes. They were supplied between 1948 and 1950. In that time Triumph produced less than 200 Grand Prix motorcycles.
The story behind the development of the Triumph GP Trophy type race engine.
The story of the Grand Prix engine development is vague as much of the early work was done in secrecy by the experimental department.
The story goes that Ernie Lyons, a top racer, got hold of the all-alloy square barrel and head from a WW2 bomber generator and bolted it onto his 1946 500 bottom end. Of course, he was looking for weight reduction and this was a considerable weight saving over the original iron barrel and head.
Because Ernie Lyons was essentially a privateer and Triumph didn’t want to be part of a failure, the head of Triumph’s Experimental Department, a chap called Freddie Clarke, worked in secret with Ernie Lyons on the development.
I understand that one of the main differences between the later Grand Prix cylinder heads and the generator heads is the larger valves in the grand prix motors.
There were many upgrades over the standard 500cc engine. Freddie Clarke developed the separate inlets for the carbs that were fed by a single remote float chamber. The valve gears were standard but were lightened and polished. This was the theme with the rest of the engine. However, heavier conrods were used together with roller main bearings and alloy pushrods.
As Ernie Lyons went on to win the 1946 IOM Senior TT, Triumph came out into the sunlight and openly developed the concept into what are now the highly sought after Grand Prix motorcycles.
About this engine
This engine has been used for racing. There is evidence of lockwires on the extra-finned rocker caps and the carb float chamber cover. It is fitted with a pair of 376 Amal carbs and, as you can see, they have rubber caps over the tops to prevent water ingress.
It is also fitted with a BTH magneto. It has a rev counter drive coming off the magneto drive.
The timing cover is a special Triumph timing cover with tacho drive. These were supplied on the Trophy TR5 and TR6 models between 1957 and 1960. The speedometer drive has a 2:1 ratio. It can also be used on many of the pre-unit Triumphs between 1946 and 59. The part number was E2218. You can get reproductions of these today.
Whoever built this engine for racing knew what they were doing. The engine case halves don’t have matching numbers. However, they are very well fitted together. The reason that they don’t match is that the engine builder wanted to use a big bearing bottom end. If you look under the timing cover on the case, you will see a small elliptical bulge. That is how you tell a big bearing engine at a glance. Triumph didn’t make big bearing engines like this until the 1950’s. So, this is a later rebuild.
If you haven’t already clicked on the picture to view the video of tis engine, here is another way to view it: Go to my YouTube channel and see it there.
Can you build your own Triumph GP Trophy type race engine?
Of course. You just need to source the square barrel and head. Occasionally, you see the heads and barrels come up for sale. They aren’t cheap but they do make a great looking pre-unit engine. The rest of the parts are fairly easy to source.
You can let me know if you need appropriate parts and I will contact you if I find any. Or, you can keep an eye on the Triumph spares for sale here.
I was sad to see this engine go. To counter balance the sadness though. I was very happy that it was going to a chap who is a real Triumph enthusiast and was going to pair it with the genuine Grand Prix gearbox that I had.
A good place to start looking for a complete one is in the Triumph Motorcycle Owner’s Club.
