Thanks for the rapid responses. I found wikipedia, but there seemed to be some disagreement. I eyeballed the pitch at 0.024" using a caliper (which is why I though 40tpi), but I reckon it is probably a standard one.
Not sure I've got 1/4" by 32 ME anyway, but I have a suitable threading tool – a homemade tap should be fine in the aluminium head.
Thanks again,
neil