Scale and Gauge: A Response

by Bruce A. Metcalf

Library Director, Kalmbach Memorial Library

In the August 1992 NMRA BULLETIN, Kenneth Mortimer proposes a set of names for the variety of scales now being modeled with what is variously called 3/8" gauge, #1 gauge, and G gauge track. All measure 1.766" [44.9-mm] between the rails.

I quite agree that a consistent and universal set of names for these different scales is necessary if we are to discuss "large scale" model railroading intelligently. However, I would like to propose a set of names for these scales that is based on the principles previously established for NMRA Standards. These names are what we use at the Kalmbach Memorial Library, and we believe they are a preferable alternative.

NMRA Standard S-1.2, Standards for Scale Models, defines the names for the various scales (and subsidiary non-standard gauges) for all recognized modeling scales. In each instance, the names of scales are letters or inch dimensions (Such as HO and 3/8"). These names without suffix refer to North American standard gauge of 4'-8½" [1435-mm].

Where S-1.2 recognizes non-standard gauges, a gauge indicator is added as a suffix. Presently, 3-foot and 2-foot gauges are listed, with suffixes of n3 and n2, respectively, and 2'-6" gauge, with a suffix of n30. NMRA Standard MS-1.1 also recognizes 2'-6" gauge with a suffix of n2½. (One is obliged to wonder if anyone has previously compared these two conflicting approaches.)

While only a few of the non-standard gauges built and modeled have been established as NMRA Standards, the NMRA gauge suffix naming system is based on a broader system that uses other letter/number combinations to denote other non-standard gauges. In this system, gauges wider than standard are given with a w suffix, such as w6 for 6-foot [1829-mm] gauge.

Meter gauge is indicated by a simple m suffix, and other metric gauges use an m plus the gauge in millimeters, as m760, m750, and m600.

Any other non-standard gauge is easily accommodated by this system, with appropriate and unique gauge suffixes to denote each.

The importance of maintaining a constant scale designation, regardless of the gauge modeled can be seen from the fact that the size of a structure or detail part is unrelated to the distance between the rails. In order for such items to be given a simple and unambiguous label, a uniform name is required. Conversely, it is necessary that each different scale ration have a different name so that all products labeled for a given scale will be in correct proportion to each other.

Table 1 below includes the scales appropriate to the prototype gauges Mortimer mentioned. A few NMRA Standard scales are also included for comparison purposes:

Table 1: Scale and Gauge Names
scale
name
ratioprototype
gauge
scale
reduction
real
gauge
errorpercent
error
3/8" 3256.5"1.766"1.766"0.000"0.0%
Gm 22.5meter1.750"1.766"0.016"0.9%
Gn3 22.536"1.600"1.766"0.166"10.4%
¾"n2½ 1630"1.875"1.766"0.109"5.8%
¾"m760 16760mm1.870"1.766"0.104"5.6%
¾"m750 16750mm1.845"1.766"0.079"4.3%
1"m600 12600mm1.969"1.766"0.203"10.3%
1"n2 1224"2.000"1.766"0.234"11.7%
½" 2456.5"2.354"2.500"0.146"6.2%
O 4856.5"1.177"1.250"0.073"6.2%
Nn316036"0.225"0.250"0.025"11.1%
3/5"n3 2036"1.766"1.800"0.034"1.9%

While the errors present in some of these scales are not insignificant, the three examples as the bottom of the table show that they are not entirely out of line with previously approved NMRA Standards.

One of the more hotly debated errors, the 10% oversize for 3-foot gauge, could be remedied by the creation of a new scale. A ratio of 20:1, which could be named 3/5" scale, would produce an error of less than 2% when used as 3/5"n3 on 1.766" [44.9-mm] gauge track.

The NMRA has, since this article was first published in 1991, essentially adopted this proposal in NMRA Standards for Proto Models S-1.1, through recognition of Proto20.32n3. (Using Proto20n3 for a name wasn't close enough?)

While I must applaud any serious effort to clarify the nomenclature of "large scale" modeling, I must also resist any effort to redefine scale labels on the basis of gauge rather than scale, as the NMRA has done in their NMRA Standards for Scales with deep flanges S-1.3. This way lies only confusion and incompatibility. There are now two "offical" scale designations for many parts that might be used by scale modelers, Proto modelers, and Hi-Rail modelers.

The Kalmbach Memorial Library has been using the above system for labeling scales and gauges since opening, and we have found the system to be sufficiently comprehensive to fully meet our needs. "Large scale" models are not a special case, and they should be handled within our existing naming system.


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Updated 11 March 2006 by Bruce A. Metcalf, who would very much appreciate your comments.