Old twinshock Cota fuel tank/seat unit interchanging and metal tank queries

[iconic558]

Did a search but came up with now't.

What's the situation with the fuel tank/seat unit interchangablity between the older Cota's of all flavors, say 1974 onwards?

I notice that 247 have single downtube frame whilst the 348 have a twin downtube so the frames can't be the same, so my thinking is that 247's don't cross over to 348?

Also, did all UK spec' Cota's have false 'tank tops' with a metal tank under the cover, was there a cut off year? My own 247C is 1978 yet has a glassfibre tank.

I know a metal fuel tank was mandatory for road use until the late 80's (a SSDT question there) I believe but then again, not all UK spec' trials bikes had metal tanks, Fantic for example were all plastic when SWM's ran a metal tank if UK spec' unless I'm mistaken which I often am.

cheers as always guys

Edited May 16, 2017 by iconic558

[cleanorbust]

Well, a bit of a history lesson from memory but I know at least one TC member who'll correct any mistakes:

Any other 247 tank/seat units should be interchangeable as the frame didn't change from the original design. Not that you'd want to use a fibreglass unit if you could help it with today's petrol. It's unfortunate that your bike has the fibreglass unit, mine is a '79 bike and has the alloy undertank as standard: no issues with modern fuel. So, until 78/79 247s ran fibreglass tanks. Sammy Miller did an alloy tank seat unit for the 247 back in the day. I don't think the 348 or later units would sit properly on the 247 frame due the different top rail and rear loop which the unit rests on.

Fibreglass tanks were banned for road use on new bikes in the UK but the Fantic polypropylene ones were permissible because they were tough enough not to disintegrate in an accident, as were, for example, the plastic tank units on the first TY 250 monoshock Yamahas.

I'll wait to be shot down if my facts are wrong...

Edited May 16, 2017 by cleanorbust