What size Allen key required for Rad Valve extensions?

The other way is a flat steel strip filed to fit across opposite corners of the hexagon.

Bob

There is a pukka tool to do the job it has a tapered hexagon at one end and a square with a recess at the other with a tommy bar in the middle. I think Monument make them and sell through Wolsely Centres plus other outlets.

Hi there thanks for the replies. I've now seen the Monument tool (search for "universal radiator key".

I might measure the size properly (when I find my vernier) and buy a corresponding imperial Allen Key, if that's what it turns out to be - there are plenty on eBay. I recently inherited my late Father's extensive tool collection, including boxes and boxes of Allen keys, but surprisingly don't seem to have one to fit.

Regards Jon N

The proper keys also usually have an end to deal with those that have a square section rather than hex.

yes, my 'non-universal but standard' key has one of those. It was that key which doesn't fit, to my initial surprise.

I've measured the size as a 'just a whisker under 12mm' AF. My 12mm Socket head just doesn't fit. Bob minchin's suggestion of 7/16" sounds like it might be a bit small, but probably worth trying. I'm still surprised they make these adapters with such an unusual size hole...

Thanks Jon N