I what are the pros and cons of the unit construction models of BSA and Triumph. I am looking for a project English 40 cubic inch bike. I am a conservative rider and rarely run at the 90 and up area. Just want to build a good oil tight keeper.
Pre OIF, the handling of the BSA is reputed to be better. Both mine are OIF, so I cannot comment on that.
BSA pros: push rod tunnels are part of the cylinder casting= no push rod tube leaks.
The gearbox is a 'cassette' design, the entire box contents can be removed when the engine cases need to be split.
Only one camshaft, definitely less noise from the timing side.
The kick-start lever hangs down below where it rubs on your leg while riding.
BSA cons: At least one set of push rods are too damn long.
You must remove the entire gearbox, and strip the primary, just to replace any single part in the gearbox. And, you must remove the entire timing cover to get to the gearbox.
The kick-start return spring is a PITA to put back on.
The life of the engine depends largely on the fit of one bushing on the timing side of the crank. This must be ground to a clearance of 0.0015" in line with the crank. Rebuilding these engines requires precision machining by someone familiar with these engines, and such people are rare and expensive today.
Ratio of second and third gears leaves a big space between the two.
Triumph pros: Better parts availability.
The gearbox is behind a separate cover, and single parts can be replaced without the need to take the whole thing out. Primary only needs stripping to replace the main-shaft high gear.
Engaging dogs on the gears are more robust.
Available as, or convertible to, a five speed. Better spaced ratios even as a four speed.
Better oiling system with proper bearings on each end of the crank.
Less expert machine work required to rebuild.
Triumph cons: Push rod tubes can leak if care is not taken to check seal crush on assembly.
Gearbox must be disassembled any time the engine cases are separated. They don't stay nicely assembled to the end cover as with BSA.
If you find each with a well built engine, it's all matter of personal choice. The BSA engine revs a little freer and can run smoother if properly balanced.
However, if starting from scratch, the Triumph engine is easier to build and parts are more available.