Water Gauge

Modelling the Kerr-Stuart Brazil Class 0-4-2ST locomotive in 16mm scale. - Keith Bucklitch

Water Gauge and Safety Valve.


1. Water Gauge


Small diameter sight glasses can be difficult to read. They are easily furred up by deposits in the water, and surface tension can give misleading water levels. However, I am a strong believer in the value of fitting water gauges to my models, and provided they are cleaned out from time to time, find them very useful.



The top and bottom fittings are fabricated as the regulator body. Water gauge glass dimensions are only nominal. Check your piece will easily fit in the 4.5 mm hole specified. (You may be able to grind it to fit a little.) The glass is retained with gland nuts and seals cut from a length of silicone tube. It is unlikely that when the fittings are screwed firmly into the boiler that they will be in line. This however is essential as the glass does not like being forced around a corner! Put some Loctite retaining compound on the threads and use the 4.5 mm drill to align the fittings. Allow this to cure before fitting the glass, the seals and the retaining nuts.


2. Safety Valve

Safety valves are essential. The purpose of a safety valve is to relieve excess steam pressure and thereby prevent a boiler explosion. They must be set carefully, and checked from time to time for accuracy.


The spring used in the safety valve comes from a tyre valve. Ask you local tyre stockist for a few old valve cores. You may also be able to utilise the spindle of the tyre valve. Half way along the spindle there is a cup. Cut off the spindle below the cup. Grip in the lathe and tidy up the cup to sit neatly on the valve ball. The spring will need to be shortened later to adjust the valve pressure. Steam escapes from the valve through holes in the side of the body. Four of these 1.5 mm in diameter should allow all the steam to escape that we need for a boiler of this size.

  1. Valve Body.
  2. Start by chucking a piece of 10 mm (3/8) hex brass in the lathe. Face the end, centre and drill 2.25 mm dia. for a depth of 20 mm. Open out to 3.2 mm (1/8") for a depth of 9.5 mm. Replace the drill with a 3.2 mm D-bit and cut a seat for a further 1.2 mm. Open out again to 5/32 for 9.5 mm and tap 3/16 by 40 tpi. Turn the outside diameter to 6.5 mm diameter. Part off at 18 mm from the end. Reverse in the chuck. Turn down to 1/4" diameter for a distance of 5 mm. Thread 1/4" by 40 tpi. Finally, make 4 1.6 mm holes in the body, equally placed around the circumference. Clean up the internal thread after drilling these holes.


  3. The adjustment piece
  4. The adjuster is made from 3/16" round bar. Chuck this in the lathe, face the end, centre drill and drill 1/16" for 5 mm. Open out to 1/8" diameter for 3 mm. Thread the outside 3/16" by 40 tpi. Part off at 5 mm length. Make a saw cut across the top of the adjuster to provide a screwdriver slot.


  5. The spindle

The spindle is made from 3.2 mm stainless steel. Turn down the shaft to 1.6 mm diameter until it fits in the adjuster, for a length of 13 mm. Part off at 16 mm. Reverse in the chuck, face the end, centre and drill 7/32" for 1.5 mm.

6. Place a 1/8" rustless ball in the body, cut a tyre valve spring in half and slide over the shaft of the spindle. Insert in the body, and screw in the adjuster. You will need to adjust the valve to the lift-off pressure later. Fit an 'O' ring to the bottom of the valve to form a seal with the boiler bush and the valve is complete.


Next Pipework.

If you have any questions, or comments or find any errors in these notes please contact me by email. Email Keith Bucklitch.

Copyright © 1998, Keith Bucklitch
Last Edited - November 1998