Hi,
The tap size is 16mm x 2.0 (per the factory service manual). But, it would be best to measure and double check the threads on the isolator switch you've got, to be absolutely certain before cutting. If you don't have the tools to measure it, take the switch with you to a good store that sells good quality taps and dies. A good tool supplier will have what's needed to accurately measure the threads and set you up with the right size tap.
The tap will be labelled right on its shank with the drill size it needs. All taps are supposed to be labelled this way. (Although it can wear off or might be very small and difficult to read, so there are separate references such as Peter mentioned. And there are special storage cases that to hold taps along with their respective drill bits.)
Try to get a high speed steel (HSS) tap, not the garden variety steel that many hardware stores and Home Despot might sell you. HSS taps are sharper and will cut much cleaner threads.
By all means, whenever cutting threads in aluminum you *must* use a tapping lubricant that's made especially for use with aluminum. Ask for it at the store that sells you the tap. If you use other types of tapping lubricant or none at all, you stand a good chance that the alu will gall (sieze) on the tap and the threads will be damaged right from the start. In aluminum, the tap will typically gall when you stop turning and try to reverse it to clear the cuttings. The galled threads will pull out and that material sticks in the tap's threaded areas and will damage other threads in the alu as the tap backs out. A tapping lubricant formulated specially for aluminum is the best way to help prevent this.
Dave's suggestion to practice on a scrap of alumium is a good idea, too. Hardware stores sell aluminum stock in various thickness, try to get something about 1/8-1/4" thick. It will probably be a softer alloy than the gearbox top cover, but that's okay as it's more likely to gall than the harder cast aluminum. So, if you can cut good threads in the soft, sample piece, the top cover will be easy by comparison.
Also, you'll need a proper T-handle or similar for the tap (don't try to use an adjustable wrench). Ask at the store.
By the way, the isolator switch is fitted with one or more fiber washers underneath it, possibly to help prevent leaks, but mostly to act as shims that adjust the depth of the switch's probe within the gearbox cover.
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