What is this?

[vincereynard]

Just above the crankshaft gear?

Cheers

[KevJ+2]

Oil lubrication for chain I think.

[englishmaninwales]

Correct. It matches up with an oil drilling in the backplate. Care needed on engine assembly not to allow sealant to block the oil way.

[vincereynard]

Thanks chaps. I shall be most careful!

Toad's engine is now stripped revealing more knackered bits.

Perversely this has made me more determined to get it right. Although I thought it ran a bit "flat"
before, considering the state of it, it's a wonder it ran as well as it did.

Once up and sorted it should go well!

[vincereynard]

Correct. It matches up with an oil drilling in the backplate. Care needed on engine assembly not to allow sealant to block the oil way.

Here is the tiny drilling in the back of the Burton that lines up with the hole.
(Shame it was blocked with mastic!)