What makes a classic bike tax/MOT exempt?

[SumpMagnet]

Was pondering this the other day, as you do.

If I was to buy a CB750 frame, for example....get it properly UK registered. and then build a bike around it from a more modern vehicle. Lets say, a 900 Hornet or late model CB750 DOHC.

UK registration is based around the frame number from what I recall, so I guess it could legally stay registered with it's 1970's numberplate, and be considered a 1970's vehicle. But as you swap the engine, suspension, wheels, brakes ... does it still get considered as a 1970's vehicle in terms of these things? Is there a point at which your vehicle ceases to be a tax, MOT and emissions exempt vehicle and becomes something else. Or does it just stay as a 'modified' classic vehicle.

I know it would involve an interesting conversation to insure it .... but thats another question.

[taysidedragon]

I believe you can only change it to historic class and declare mot exempt if the vehicle is not substantially modified. A modern engine and brakes/suspension is almost certainly a  major modification.

[Bryanj]

You would have to start with a complete bike already historic then start modifying, if you do too much it becomes a heavily modified vehicle requiring an sva test and possibly ending up re registered on a Q plate which will never been allowed historic and is just about impossible to get rid of