Find a gauge with the right crossing/check rail details. If you have access to lathe, drill a hole through the centre. Cut the gauge in half. Tap both holes and put a bolt through. You can screw one end in and out until the gauge is right.
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I have some gauges which are a bolt with various washers on it held apart with a tube spacer. I think they were C&L but that's not the ones I've seen recently. If you can locate a source, just needs a longer bolt and spacer.
I am building a short 3 foot [21mm gauge] line as a pilot project. I'm using the Slaters wheels.
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Today I'm building the first point. I can't find any data on the track or wheel standards?other than measuring the wheels. What I need to know is the flangeway clearance to use in the crossing.