Originally Posted by Carosel43

Sorry, 4ba is the thread size of the bolts used in the engine. ba was a british thread standard which is now obsolete. We switched to metric threads just after the turn of the century. That's why the 2003 engine is marked with M for metric as there was a period of crossover between the only and new thread sizes. All current engines use M4 bolts for head screws and either M4 or M5 bolts for holding the front housings and backplates. Only the 70 uses M3 backplate bolts. All crankshafts are either M8x1.25 or M10x1.5. Again the 70 is the odd one out as it retains a 1/4x28 unf crank. I think this was due to M6 being a slightly unusual size and potentially difficult to source prop nuts locally for customers over seas.

In your photo it looks like you might have M4 bolts, they could have been retrofitted at a later time. To test it find a 3/32 allen key and see if it fits. If it does then you have 4ba, if not try and 3mm allen key as this will fit M4