Triumph/Norton/BSA bastard engine

Ran across an interesting ad today, someone is selling a 1971 T120R with a "stroked T140 engine with a Norton crank, BSA 650 rods and 830 cc". Anyone here done anything like that? Does the Norton crank fit easily in the Triumph case?
Tommy

I've done it, not with BSA rods though.
Made a good strong motor.
Weak point is the stock cases if pushed hard.

Fast Eddie said:

I've done it, not with BSA rods though.
Made a good strong motor.
Weak point is the stock cases if pushed hard.

Click to expand...

Interesting, what was the size of your engine? How many cc:s?
Tommy

840cc if I recall correctly.

I have heard of it being done with a pre-unit Triumph. The problem is that in our historic racing there are very few pre-unit Triumphs due to engine blow-ups. A main cause of these is failure of the crankcases. When you spin the Norton crank to 8,000 RPM which is what a good 650 Triumph spins at due to the cams, the game becomes expensive. One way would be to buy a set of Puma cases. - Nourish probably sell them these days. The next step is the Weslake head. So perhaps you should buy a Nourish motor and start where you would finish with the Triumph motor ?

Thunder Engineering in Leicester make strong pre unit Triumph cases.

Depends a bit on how you intend to use the bike. Of you want to race, and there is a class for it - the Nourish engine would be a better option. Twice as many valves means a higher Brake Mean Effective Pressure. Also most of the problems have been solved already. It would probably be cheaper in the long run.
As a road bike a Triumph with a Norton crank might be OK as long as you don't stress it.

It makes a lot of sense to do this if you're looking for a race bike where Nourish aren't allowed (plenty of those here) or a hot rod road bike on a budget.

A Norton crank needs some mods to fit, I've forgot the precise details, but its quite easy for a competent tuner. A lightweight Maney crank would be an ideal starting point!

If you use Norton rods, and T140 pistons and barrels, the piston deck height all lines up perfectly.

As with Nortons, a big motor allows you to run a big cam, so Spitfire profile inlets and exhaust are the order of the day.

This makes for a really good, torquey, fast engine that is awesome on the road. I raced this set up in a few variations of Triton and ended up putting the last one on the road. I had the crank dynamically balanced to 80% and it was a really smooth bike (relatively speaking).

All in all, highly recommended IMHO.

Good advice !
The lightweight Maney crank might better suit the power characteristics caused by the hot Triumph cams. Most hot Triumphs seem to operate between 4,000 to 8,000 RPM, the lightweight crank would probably spin up better. If you used the heavy crank, the Triumph motor would probably not be as good as a Norton because the head is not as good. The big advantage in building the Triumph motor using the lightweight Maney crank is that you would have separate inlet and exhaust cams to play with, so it's easier to get the best out of your motor. A squish mod to the Triumph head and twin-plugging might also make things better.

I have a problem with developing old motors. In this case the better option is probably a Nourish, however the race rules often preclude such things being used in historic events. Historic race classes are now the major development classes for the average guy. My feeling is that original parts for old bikes should be used for restoration purposes, and the historic race classes would be better if they became pure development classes for old designs. I love old twin cylinder and single cylinder four-stroke bikes and I cannot see why races should not include Jawa and GM speedway engined bikes along-side Manxes and G50s. At least the races would sound right !