The dvla guidlines state that:-
"If a new monocoque bodyshell or chassis from a specialist kit manufacturer is used (or an altered chassis/bodyshell from an existing vehicle) together with two major components from a donor vehicle, an age related mark will be assigned. The mark will be based on the age of the donor vehicle. An SVA test will be required to register the vehicle."

What constitutes as a "major component"? I am using a motorbike engine and gearbox (but don't have the vehicle details), I have the details (and logbook) of the car that the diff, driveshafts, prop shaft and brakes came from. Is that enough?

Engine and gearbox I believe are the major components. If you have the V5 for the car. I suggest you register the bike engine to that car, then build your locost and put it thru the re-reg process.

Although I have heard that if you want to downsize the engine capacity on a car too much. They may want to inspect it. In case you are trying to get round tax implications.

FWIW,
Ian

Isn't there a point scoring type system with this? I would venture to say you probably won't be able to get an age related plate. Having said this, if it is two major components, could you not count the engine and gearbox from the motorbike as the original vehicle and get a presumably much newer plate? Nothing wrong with a Q plate anyway in my view.

With regard to reregistering the Sierra with a motorbike engine, my understanding is that if you drop into a lower tax bracket, the chances are they're gonna wanna see it, and they're gonna wanna see it at their place, meaning you'll have to drive it there. I think that pretty much rules it out.

Kingr

Jon Ison did this and I think all they required was proof from an engineer/mot testing station I don't think the DVLA needed to see the car.

spot on Chris, i had to send a report from an MOT station, (with that crimp'd stamp thingy on it) saying the engine was fitted and comfirming the engine number, was around this time last year, don't know if anything as changed.

My father has a knackered Lada 1300 on its last legs, so will re-register the engine in that before it goes to the scrappers

worth a try, let me know how you gerrron