Might be helpful if you told us what engine you rebuilt !!

Anyway, looking at the part, it has a keyway on the inside suggesting it fits onto a shaft. The only place I have seen something similar is at the front of the crankshaft, where the oil seal fits around the crankshaft. The oil seal goes around this spacer, and there is sometimes an oil thrower behind to keep oil to a minimum at the oil seal face.

 

I agree with Fraser Mitchell, that looks loke the spacer from the front of an XK engine, though they didn't have a groove in the middle, the seal ran there, used to make it's own groove. If you find a major oil leak from the front of the motor, problem solved.

 

I doubt that the OP will read the replies.

He is no longer an active member.

"Guest" with N/A posts is the clue.

 

I'll bet he still has it on his desk and ponders over a good malt of an evening.

 

Perhaps this is why he left us?