A very sketchy and risky operation. Drill bound up and twisted on me twice, though final pass was good.
Tapping the threads went badly, finally used a 6 inch extension with ratchet , did it in one pass.
Thickness of the case is about 1/4"
Used paper towels lined with grease to keep grinding debris out of the engine
Used a Mikita grinder; disc about the size of the bore,there was some cosmetic damage to the bore which will be invisable, and of no consequence.
I'm a pretty good carpenter and used to these tools, and still, disaster almost ensued.
If anyone is tempted to try this mod, please, use a lot more preparation and planning before drilling and tapping, and sealing off the innards of the engine then I did.
For example, perhaps a cardboard cut out to seal off the inside bore of the engine, sealed with silicone, then you could just vacume the sand from the grinder.
Fabricate a holding device for your drill.
Better yet, wait 'til it's time for a bottom end rebuild and do it right.
But I am so happy with the results... Can't wait to ride a cooler , smoother running AVL!