Re: Preferred truck mounting on resin cars

Dennis Storzek

Coming in late... A little bit of background on why HO scale trucks have 1/8" holes in their bolsters:

In the beginning...  models had wood floors and wood screws were used to hold the trucks on. This was ideal because wood screws aren't threaded all the way to the head, and that left a smooth shank for the truck to pivot on. A No. 2 screw had a shank diameter of .086" and NMRA RP-23 specified a .089" hole in the bolster (#43 drill) to provide a running fit. Still, to this day.

But Mr. Athearn had a problem. He wanted to use a two piece plastic floor and underframe to trap his steel weight between, so he increased the hole in the truck bolster to a running fit on the 1/8" diameter boss he molded on his underframe, and had the screw that threaded into the floor tighten against this boss to hold the sandwich together. Other manufacturers saw the advantage of having the screw tighten against a boss, whether they needed it to hold the UF to the floor or not, and it became the defacto standard.

But some trucks remained that had the .089" hole, and resin manufacturers left the boss off to allow the use of these trucks... aside from the fact that the thin walled boss is hard to fill.

If you are dealing with Accurail trucks a 4-40 screw won't work, because the counterbore in the bolster is too small for the screw head; I specifically sized the counterbore  to be a running fit on the HEAD of a 2-56 pan head screw, but other brands of trucks are different, so check before you drill.

And, someone mentioned the Proto:87 Stores car stabilizers... These also have a clearance hole to accommodate the 1/8" boss.

Dennis Storzek

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