1971 BSA Thunderbolt

[fredintoon]

phased twins

Hi James,
the XS650 engine has no twirlythings, it's just massively overdesigned with an all roller and ball bottom end, it's two singles splined together.
The normal 360º XS650 engine is phased to 277º because it's the nearest you can get to 270º when you rotate two flywheel sets on the 13-spline central main bearing spigot.
Balancewise a 360º vertical twin is just a big single.
Rephasing puts one piston at it's highest piston speed when t'other is at TDC & BDC.
This arrangement gives a partial balance so the engine runs smoother.
BTW, Yamaha dealt with the XS650 engine's vibration by rubber mounting everything on the bike except the engine
FWIW, I vaguely remember somebody in the late 1940s rephasing a Triumph Speed Twin and trying to interest Triumph in it but they rejected the idea.

[James England]

I vaguely remember somebody in the late 1940s rephasing a Triumph Speed Twin and trying to interest Triumph in it but they rejected the idea.

We were good at that. We rejected..electric starts, indicators, counter rotating cranks, roller bearings, electronic ignition.

Gestetner rejected the first photocopier with the immortal words "nothing will ever replace the wax stencil". The inventor took it to the USA....to Xerox........

I won't go on.....! For once...

[mharrington]

Its ironic- that 1960-70s Harleys, Triumphs and BSAs were terrible quality built bikes but today they are worth a fortune!!!!! The Japanese built superior bikes but in such a high volume and affordable price that those bikes have low low value. But its OK cuz people like me can afford them!

I will say the BSA twin cyl is a magnificent looking motor! James I found your recollection of 1970s Brit bikes very informative and interesting.

I love the look of the "Not avail in the US" Kawasaki W800. But its power output is way too low!!!!!