Have been asked about replacing a 12/4 with original type 6 cylinder engine recently, prompted following reply:

Things to consider are gearbox, clutch, and why change.

Things to check are cracks around centre main,cracks on sides and back of water jacket and cracks around cam follower holes inside and on top face studs to followers and waterways. Also that all main carriers are correct type for the block/oil ways. Size of cranks and soundness there of, rods probably not worth reusing… cams likely same.. Starter motor, Dynamo, inlet manifold, water pump and dissy?

Check head very carefully to see all the oil /water holes match up…..this is a minefield on Riley six cylinder engines.

Rebuild costs hard to quote, but beware of “New” Folk in market place who appear to be singing from the Hymn sheet of Adam and the Ants……
“Stand and deliver”…..after the event and now even, before.

We would recommend new rods on shells, as rods are the weak point of the assembly…plumbing in of modern oil filler. If using a Pre selector box, we do make a flywheel to take a 12/4 2nd type clutch as the original type is more like a Claymore mine than a clutch….Not sure if there are any bell housing available anywhere! Several needed in the last couple of years following explosions. Manual All helical box coming into stock soon, can supply lightweight flywheel and Borg&Beck clutch.

Main thing to remember is to join the 6 cylinder Riley club there is an oath/joining ceremony to preform :

Open your wallet and say after me “Help yourself”