The poppet valve crankcase has been bugging me and I wanted to fix the brackets from inside the crankcase with countersunk caphead bolts. There just wasn’t room or the right angle to get a bit in to make the countersinks.
The bit extension was made in two parts plus a grub screw.
The shaft on the extension is 2.5mm diameter and designed as such to go through the pilot holes in the side of the crankcase.
The countersink then fits into the extension and the grub screw locks it in place.
You can see that you need to be very careful with the length of the countersink bit, the length of the extension and the length of the body of the extension. Add to that the fact that the extension bar has to go into the crankcase and align then with the hole it will be driven through.
Once it’s all assembled the drive bar has to extend out through the crankcase far enough that the drill chuck can drive it. Plus it need to be able to drive it when the countersink has been machined to depth.
So, I had to carefully line up the parts, then using a handheld drill I could machine the countersink.