Gauge Glass Fitting

[Thomas ClarkeParticipant @thomasclarke78224]

I am replacing  the broken gauge glass on my 5 in gauge locomotive. The glass that was was on the locomotive was 5.5mm in diameter. I am replacing it with an improved 6mm one but it is a tighter fit. How much of a gap should I allow in the union nut and the top and bottom valves and should there be a gap at the ends of the gauge glass when it is seated between the valves. Thanks.

[noel shelleyParticipant @noelshelley55608]

Nothing should be tight. I would open up all diameters .5mm or 20 thou. The end clearance, the important thing is that the glass and fittings can move, so not to far through the O rings or seals but enough to ensure a good seal hot. Good luck.  Noel.

[Bo’sunParticipant @bosun58570]

Not wishing to hijack the original post, but a related subject.  What’s the most reliable way to cut 5mm OD Borosilicate Glass for a water gauge.  I say reliable because I don’t have much to spare.  Plus, how to finish the ends.  Bearing in mind the glass has to pass through two O rings.

[noel shelleyParticipant @noelshelley55608]

Use the corner of a triangle (3 square ) file to make a small groove then snap it, try to wrap in cloth or use gloves to protect the hands. As for the ends clean up with a diamond file or emery cloth just to take the sharpness off the corner. Assemble with care to avoid the O rings being damaged.    Noel.

[Dave WoottonParticipant @davewootton]

I’ve tried several different methods over the years, including buying a little glass tube cutter which did not go well for me despite other people getting good results with them. The method that has had a good success rate is with the tube held in either a collet or if I’m feeling lazy in the 3 jaw (carefully!) in the lathe and scoring a ring around the tube with a cheapo triangular diamond file, some very careful tapping with something like a plastic screwdriver handle and the tube breaks off quite cleanly. A rub on a diamond lapping stone takes off any sharp or uneven bits. This method even worked on some 10mm tube for sight glass lubricators, there was a lot of tapping before the tube cracked off but it gave a very clean break.

I’ve not tried it yet but a friend beds his gauge glass into the gauge using high temp RTV silicone sealer, and has never had a problem with breakage or leaks, i’m sure there has been a post on here or Proboards about using it.

Edit Noel obviously types much faster than me!

[bernard towersParticipant @bernardtowers37738]

the ends melt to a nice smooth finish with a small butane torch, guaranteed not to cut your O rings.

[Bo’sunParticipant @bosun58570]

Thanks all, let’s get cracking.  Hopefully just once.

[DalboyParticipant @dalboy]

This came up on my U tube Just at the right time for you

[Dave HalfordParticipant @davehalford22513]

A red hot steel wire used to cut Pyrex test tubes otherwise if you have a dremel style tool a cheap 1″ diamond grinding discs will.

[RRMBKParticipant @rrmbk]

I use my Dremel, mounted horizontally in the vice with a very thin grinding disc . Rotate the gauge glass around the spinning disc, creating a nice groove all round the glass, they then just snap off cleanly for me. You can also use the dremel to grind a slight bevel on the ends,

[duncan webster 1Participant @duncanwebster1]

The Utube link to Luker is very good, all he doesn’t tell us is how thick he makes the rings, or perhaps I missed it

[Bo’sunParticipant @bosun58570]

I think he says they’re “square section”, so that makes the thickness half of the difference in the diameter’s.

[noel shelleyParticipant @noelshelley55608]

If one was using O rings one might well use 1mm section. Using that as a guide I would be tempted to use 1mm wall tube  and for width use 1.5 or 2mm. Luke sometimes appears on here may be he will answer the question ? Taking this a step further the scale of the model or size of the gauge glass fittings will dictate the size required.  Noel.

[LukerParticipant @luker]

Hi Gents, (Dalboy) thanks for posting that vid here. There’s many ways to do this; and if your method works, I would stick to it. Personally; I don’t use O-rings simply because a normal boiler operates outside the recommended operating temperature of Nitrile (in fact the only place I use O-rings is on my mechanical lubricators and that system’s one way valve). Viton/ silicone o-rings are less common here (nitrile, Viton also typically has a higher shore hardness than silicone). Nitrile tends to go hard after a couple of years and leaks, and/or the glass breaks. I prefer not to use o-rings, in general, because it creates a nice contact stress line on the glass, which again, over time could cause the glass to break on a rough track.

My seals are a square cross section (thickness depends on the tube I have available) with the OD and ID machined to suit size-for-size, compression is in the direction of tightening. Clearance is no more than 0.1mm (on diameter) glass to nut and sight glass seating. A size-for-size drill is used to tighten and align the sight glass body before fitting the glass.

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