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I'd like to put a cap on this steam pipe, but the standard cap sizes don't seem to fit -- a 2" cap is way too big and a 1.5" cap is slightly too small:

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There is a very faint "1 1/4" engraved on the T-joint. That doesn't make sense to me, because if a 1.5" cap is too small, then a 1.25" cap should also be too small.

Is there a trick to matching pipe sizes (or to getting the almost-right 1.5" cap to fit) that I'm missing?

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1-1/4 is probably the thread size on the bottom (standard pipe threads.) The threaded part that's currently exposed is the radiator union connection, and may fit a 1-1/4" union, or it may be an odd size specific to the radiator, depending on how that company was playing the game at the time.

If its a 1-1/4 union thread, you'd need to buy a 1-1/4 union and a 1-1/4 plug (or cap, depending on the flavor of the union), remove 1/3 of the union corresponding to the part on the valve, and attach the remaining 2/3 of the union to the valve, and plug or cap the hole in the union. If it's non-standard and specific to the radiator you probably need to remove the valve and cap the pipe.

If you have not thrown out the radiator, that should have the fitting you need attached to it.

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