I believe it was an 8-32 tap and I'm pretty sure the hole is the right size for the tap. You buy an 8-32 muffler nipple and use that after tapping the hole. Remove the plate before tapping and after you screw the nipple in use a Dremel to grind it off flush with the underside of the metering plate so it can't contact the diaphram and puncture it. I drill the hole in the cap of the bottle large enough to slide the tube in slightly loose. That way I know it's not airtight. There is another method where you solder a piece of tube into the hole and bend it so it's right next to the carb intake (a 3W design I believe) but I have never used it. There are those who have to great success, just not me.