what propshaft should i useto mate sierra 1.8td engine to capri rear axle in the locost ron champion chassis

The choice of propshaft (or at least the ends of it) will be determined by the gearbox you are using, not the engine. But whatever, its gonna have to be custom made for the job. IMO it's best to have one professionally done.

Cheers, Stu.

If, like me, you are going to make your own prop, you will need a capri, or transit item as the sierra has the donut at the gearbox end which we cannot use. The sierra diff end of the prop does fit the capri axle though