Threading and then bolting into aluminium can be troublesome. Also, the cylinder design of this poppet valve engine means there is no space to get the bolts in between the cooling fins and the cylinder annulus that mates up to the crankcase.
The first 3mm stud fitted in the crankcase. I marked, drilled, tapped and fit one stud before marking the second.
There isn’t even enough space to get a scribe, centre punch or really anything in to mark the hole on the crankcase.
So I had to make this tiny centre punch that fits snuggly in the 2.5mm diameter hole (tapping drill size for 3mm bolts).
The centre punch was just machined from mild steel and this is fine as it’s really only going to be used to mark 4 centres.
small centre punch located in hole underside showing centre punch ready to apply pressure to the punch the faint centre punch mark
The centre punch marks that result are quite faint and so I used a very small centre drill to align with the punch and ensure that I didn’t just remove the punch mark before finding the centre – this can easily be done if the punch mark is faint and you’re not careful.