Triumph Speed Twin

[john.k]

Triumph Speed Twin ...........whats the interchange of components between a 500 and 650 1954 pre unit twin............can a Speed Twin be fitted with 650 barrels ?..........and a 650 crank?

[R]

Without anything to hand to test this out, I'd suggest almost certainly NOT.
This goes completely against manufacturers principles, that you must prevent upgrades,
at all costs !!

If you look at the tight grouping of the base studs/nuts around the 500 cylinder base,
its difficult to imagine a 650 cylinder could possibly be shoehorned onto that.
Comparing the cylinder base gaskets for the 500 overlaid onto the 650 one,
I'll bet you find they are all half a spacing different, if not more.

Anything is possible if you use a large enough hammer, however !!!!

[iansoady]

Without anything to hand to test this out, I'd suggest almost certainly NOT.
This goes completely against manufacturers principles, that you must prevent upgrades,
at all costs !!

I'm not sure I agree - in fact most manufacturers (at least British ones) tried to have as much commonality as possible to save on tooling costs. My only experiences are with Norton and Velo singles and small unit Triumph twins but with these capacity changes were relatively easy.

[R]

Except that I have a variety of Norton singles and twins (mostly projects), and the possibilty of upgrades
to any of them seems to have been (deliberately) designed out ?

The 500 and 600 early dommies are designed around a common crankcase, so that is possible.
But if you put an ohv engine say in a sidevalve frame, then the bloomin footrests can't fit.
And you have to then change a whole heap of parts to make this all work.
And a sidevalve single tank won't fit over an ohv engine, naturally enough.
Changing a 650 to a 750 has studs all over the place half a hole out.
750 to 850 has different stud spacings in the head.
etc etc etc.

Enfields have different engine mounting lug widths on the 350 and 500 engines.
So a simple engine swap is not so simple. etc etc etc.

Dunno if you can unbolt a Trumpy 500 engine and bolt in a 650.
I have heard of folks accidentally (?) competing in a 500 class with a 650 engine.
"Why is that bike so much faster out of the corners ?"

[iansoady]

Mostly true. However years ago I put an ES2 engine in a 16H rolling chassis and had no issues with frame or footrests - but had to have someone chop out the underside of my freshly painted and lined tank!

[R]

That might depend on the year of your 16H. ?
The location of the footrest bar through the primary cases varies by model, quite a bit.
It goes up and down, significantly, for some years at least .

We diverge from Speed Twins ere ....

[iansoady]

... and a good thing too. Always better to talk about Nortons.

[john.k]

Actually I thin k the change from the old style gearbox to the laydown box had more to do with the footrests .......the WW2 bikes had two footrests the same,laydown box has the RHS footrest  with bends and loops to clear the gearbox.

[john.k]

Anyhoo,the Triumph question is now moot .....after saying there was no way he'd drop below $6k ,he s sold the bike to a dealer for $4k....so thgats the end of that ............or as her indoors says .....why arent you happy with the bikes you've got.......you dont need any more ...you need less............of course ,no one needs a 70 year old bike ...or do they?

[iansoady]

Actually I thin k the change from the old style gearbox to the laydown box had more to do with the footrests .......the WW2 bikes had two footrests the same,laydown box has the RHS footrest  with bends and loops to clear the gearbox.

The upright gearbox top mounting was a "fork" that engaged with a rearward lug from the seat tube. These were notorious for fracturing if the bolt wasn't kept tight. The laydown was much better in this (and other!) respect(s) as it had a solid top lug that sat between two engine plates.