steering tube size

[rogerzilla]

They were made by YST and called Impact or Control Center (it varied). The adjustable race had a T-shaped cut in it that allowed it to grip the steerer when the clamp bolt was tightened. They work ok - they have bigger than normal 1/4" balls in the lower race - but water sealing is fairly poor. The later types have an O-ring on the crown race but early ones have a big rubber boot. The rubber gasket in the adjustable race is usually perished by now but can be carefully recreated with black silicone.

 

[chriscross1966]

1 inch.

My understanding is that 1 1/8 inch steerers are all threadless. Am I wrong, vintage gurus?

Brompton use 1 1/8 threaded, its not massively common, but its hardly rare, you can get headsets from a 7 quid VP cheapy to a "hide the credit card bill from the wife" Chris King.

 

[Nigelnightmare]

Measure the stem.
As suggested above use an adjustable spanner and a tape measure.
If it's 7/8" then you've got a 1" steerer.
If it's 1" then it's a 1 1/8" steerer.
SIMPLES!

HTH