Union Nuts & Fittings

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Union Nuts & Fittings

Bill Van Brocklin

The North American Live Steamer, Volume 1, Number 9

This union chart has been worked up to keep nut sizes as small as possible and consistent for easy tightness. When properly made they take very little torque for pressures up to 125 pounds. The 40 pitch threads allow for smaller size nuts in relation to tube sizes, and taps and dies are easily obtainable. A boiler backhead using these fittings looks ever so much neater, besides giving more wrench clearance. The most difficult part is in silver soldering the ferrule to the tubing. Just a small amount of solder is needed here, for if it creeps over the F diameter, you will have to file it off to get the nut over.

For tubing, use between 0.020 and 0.030 wall thickness, anything thinner is difficult to bend 90 degrees or more without kinking. The smaller sizes up to 3/16 inch O.D. can easily be bent by your fingers, annealing the tube first. After annealing, shine up the straight tube before bending. A piece of soft iron wire bent to suit the location of fittings will aid in getting straight length. Leave ends of bent tube long and cut off to suit fittings, then silver solder ferrules on, not forgetting the nuts!

Incidentally, there is nothing new in these union nuts and ferrules, as they have been used for years by L.B.S.C and others in the Model Engineer. Sometimes the 40 pitch threads are known as the "M.E." series taps and dies.

Bill Van Brocklin, Jr
Pine Street
Dover, Mass.
BillVanBrocklin UnionChart.jpg
Union Chart
Tube Dia. A Drill B Length C Depth D Thread E Hex F Dia. +0, -0.005 G Drill H Length Tap Drill (Nut)
1/16 #51 3/16 5/32 5/32-40 3/16 1/8 1/16 3/16 #30
3/32 #41 3/16 5/32 3/16-40 1/4 5/32 3/32 3/16 #21
1/8 #30 7/32 3/16 7/32-40 1/4 3/16 1/8 7/32 #12
5/32 #21 1/4 3/16 1/4-40 5/16 (9/32) 7/32 5/32 1/4 #2
3/16 #11 9/32 7/32 9/32-40 5/16 1/4 3/16 9/32 F
1/4 F 5/16 1/4 5/16-40 3/8 (11/32) 9/32 1/4 5/16 L
5/16 O 11/32 9/32 3/8-40 7/16 11/32 5/16 11/32 S
3/8 V 1/2 7/16 1/2-24 9/16 29/64 3/8 7/16 15/32

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