Nah, it's metric... 2.54 mm... in any case, you're cheating... using a metric division of an imperial unit. Should be one tenth of an inch.I bet you wish you still had the "black watch", unless you haven't seen what they fetch nowadays. Did yours actually work? I believe most didn't. The only Sinclair I built from a kit was the Micromatic radio. Most of Sinclair's designs were actually very good, technically - often ingeniously so. Then usually let down by cheapest-sourced components, cheap, flimsy cases and cost-cutting on things like heatsinks. The ZX81's "slow-mode" performance can be boosted by about 15% by adding one transistor and a couple of resistors that the bean-counters removed. Mine has a sensibly-sized heatsink fitted on the regulator and (so far) the over-utilised ULA chip has held out too.
Good old Uncle Clive... I started out with his 'build it yourself' Hi Fi system, then it was the black calculator followed by the white scientific one and the 'Black Watch' My first computer was a ZX81 before veering off into Memotech and PCs. Now, for fun, I sometimes run a ZX81 emulation on a PC... trying to remember the keystrokes is great fun...
EDIT: just popped into my head that electronic stuff tends to be based on imperial sizes (e.g. 0.1" pin spacings on DIL chips)
Yes, the black watch did work for a bit but it went through batteries like they were going out of fashion and eventually, the buttons stopped - well - buttoning.