What Size

I have acquired a Stuart Turner 500 boiler the boiler is in good condition but does have a small leak on the screw on the underside of the water gauge bottom fitting, Ive removed the screw and it's slightly corroded needs to be replaced as it's only 4/5 threads long. Does anyone have one of these boilers or know what the thread size could be it measures 0.158 so I would suggest it's 5/32 but I have a 5/32x40 ME taps and it's not that thread also I have 5/32 whit which looks to be close but not quite, the hex size is that of 3BA 0.282 It will not be a problem to tap it 3/16 but I would prefer to keep the size as is.

H

Looking at an old catalogue they only give the thread size of the connections to the boiler and glass size of 3/16" not what the end plugs or gland nuts are.

Tubal Cain's "Model Engineer's Handbook" quotes 5/32 BSW as 32 tpi and 5/32 M E as 40 tpi.

3 BA is a little bigger at 0.161" ( 4.1 mm ) diameter and approximately 34.8 tpi, so it might be worth checking that

Howard

The odd Stuart fittings I have looked at have the blanking bolts all threaded 40 tpi by either 1/8 or 5/32 or 3/16 and the threads that fit to boiler bushes appear be 32tpi. I think I would double check your 0.158 measurement as it sounds very unlike Stuart !

An alternative idea might be to mill a 5 mm slot from one of the boundaries of the workpiece and then silver solder in a 5 mm thick piece of mild steel leaving a 5 m square hole, rather as George Thomas did to make a square hole for aligning number punches when making his staking tool,

Rod