Brad....the big decision for you is not so much what you're putting the Trumpy in but
which Triumph motor to use. The way I see it it's down to three distinctly different Trumpy motors, the pre unit 500, unit 500 and unit 650. I'd cancel out the pre unit engine (like the one VMX60 posted that's on eBay) for the need to find and adapt through plates a BSA or Norton gearbox. The end result is a rather large lump that needs a big engine cradle. The pre unit is also a 60 year old motor with 60 year old engineering that's just plain not as good as the unit engines in my opinion.
The 500 T100 unit motor is a good choice because its so comparatively small and adaptable as shown by the previous posts......it fits in a lot of places. The third choice is the 650 T120 unit construction engine which is decidedly larger than the 500 but the tradeoff is that its the horsepower king. In pre 65 it's arguably one of the best choices....most definitely the best twin cylinder engine. The American flat track guys have squeezed up to 75 bhp out of the 650/750 but for motocross stock is more than adequate.
If you've watched Vernon Grayson on his little Cheney 500 Triumph you'll know that the unit motor revs like all buggery and has rather 'modern' power characteristics and
sound. For pre 65 you can use Cheney, Metisse, Mojo or even a B40 frame to make a TriBSA....the choices are infinite. The big 650 has the same Cheney/Rickman choices but you'll need a Goldy/A50/A65 BSA frame to make a TriBSA. In the end the engine choice is down to the light weight and revs of the 500 or the grunt and potential horsepower of the 650. I've got both engines in the shed and I'm still tossing around ideas as to what to do with them. Plan A is to fit the 500sc unit 500 in my Beeza C15 roller for a different than the norm pre 60 TriBSA but after I came back from the Nats for fun I stuck it into the Petite Metisse frame I'd intended to fit my oval Maico engine into for pre 65 and it looked pretty much at home so I'm a tad undecided now
. The 650 engine is destined for a chro-mo BSA replica frame I bought off eBay about 10 years ago that is now legal for pre 65. However, I'm soon to receive an Australian complianced Bonneville roller from a mate who's using its engine in a C&J so I might build it into a street tracker. The third choice is to fit it into my Trackmaster flat track frame for some flat track action but now that I've got the OW72, the third option is getting slimmer. I only mention my dilemma to show what choices owning a Triumph can evoke.
