The MGB front crankshaft seal (Moss #120-000) tends to leak engine oil because there isn't a built-in device for ensuring the seal is centered with the crankshaft centerline when the seal is replaced. Engines for most other cars utilize dowels to correctly position the timing cover and thus center the seal. The official MGB service manual recommends that the seal be centered with a special service tool that is slipped on the crankshaft and into the seal after the timing cover is installed but before the timing cover bolts are tightened.
I have never seen the tool offered for sale in the United States, but I have found something to use in its place. That "something" is the MGB single row timing chain crankshaft sprocket (Moss #460-425). The sprocket hub is tapered on one end and when slipped on the crankshaft and into the seal, will center the seal perfectly.
I obtained my "centering tool" from an extra MGB engine which I have for parts. If you don't have an extra engine with a single row timing chain, you may be able to borrow the sprocket from a friend who does have an extra sprocket.
This "centering tool" is also useable on the late model MGAs which utilize the same seal (Moss #120-000). I have not verified its application to the early model MGA since I don't have one.
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