Hello guys

I had this 3W 75i for the past year or so on my plane.
The engine is great so far. Powerful and easy to start.
Engine is in very good condition. Never crashed, although I had a couple of rough landings.

Recently, while I was warming up the engine for a flight, the engine stalled suddenly on the ground.
Upon closer inspection, I realised that the prop bolts have sheared (all 6 of them!).

Luckily, I managed to get my local machinist to unscrew the 6 bolts that were stuck.
Then I installed another prop Menz 24 x 12, drilled with a 3W Drill Guide.
It worked fine for 10mins or so .... then the engine stalled again.
The prop bolts have sheared again (5 out of 6 this time).

I don't really know what I have done incorrectly (screwed up somewhere).
I do notice that the holes that I have drilled on the props are slight bigger than the screw diameter..... to make it easier for me to install the screws .........

Does anyone else have the same problems?
Is the bolt shearing thing commonly seen?

Thanks!