They are not threads they are csk head self tappers. They are gauge sizes- 8G, 10G, etc. This is because they are just screwed into sheet metal.
I'd go get some S/S ones at a boat shop so they don't rust, take one of yours and get the next size up.
Bias Boating at Parramatta (cnr Barney and Church St) has a good range.
Put some windscreen sealant (butyl mastic) on them before you screw them in.
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
