We have just purchased a reconditioned BMC 1.5 engine with a gearbox attached. We have since discovered that the coupling which joins the gearbox to the propshaft is a different size so will not fit ): Our previous gearbox was a PRM160VR. The gearbox purchased with the engine is 2.1 as was the PRM. Can we get a coupling to join the new gearbox to our prop and if so where would we purchase this?
As the coupling on the gearbox almost certainly has a sleeve that goes into the box for an oil seal to run on, I think it highly unlikely that you will be able to get a different gearbox coupling, but if you measure the outside diameter, the diameter of the land/recess on the face of the coupling, the diameter of the holes and the distance a hole id from the centre, PRM (address of dealers in mags.) just might be able to supply you.
Otherwise I fear its changing the coupling on the shaft. If its a fairly modern one it is likely to have some form of clamping device. If its older (and you are lucky) it will be on a taper with a nut hidden in the end of the coupling, or if you are unlucky it may be shrunk on – in which case a hydraulic puller may be required to shift it.
If its one of the later two types any half decent engineering shop should be able to make you one up, otherwise start by asking PRM for suppliers of suitable couplings and also try any engineers/millwrights suppliers.
