I don't know if others have had this issue, but my Comet TAV2-30 Torq-A-Verter has arrived with a bag of fixings but the bolts do not fit the mounting holes on my clone engine.

I am assuming this is because my engine is metric and the TAV imperial? That said, my engine (Lifan Q Eco) has a 3/4" shaft, but I guess that is about compatibility with other hardware rather than an indication that the engine is itself imperial?

So, that was a bit irritating, but I have just looked up Adrian's guide and it looks like I should be using M8s instead of the 5/16ths. But then, isn't 8mm the equivalent of a 5/16th?

I am inclined to say almost that you shouldn't try to explain, because this is the sort of thing I never fully understand! But presumably the threads are different?

To avoid me having to take my engine to Suffolk Fasteners, can anyone confirm that I need M8s? Don't clones for sale in the USA have the same size holes - and if so, why would Comet supply TAVs with 5/8-25x1" bolts?

In (very) brief. There is no "standard" for the engine bolts.
Some have imperial  bolts in the case, some metric standard, some metric fine.

I am a bit confused here myself. Are we talking about the bolt that holds the clutch onto the end of the shaft?  I have one that is imperial and one that is metric.
I>{