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HO Scale Conversion Factors

 

  • HO is 1:87 real size - Approximately 1/8" = 1 foot

  • 3.5mm = 1 ft. in HO (1mm = .03937 in.)  That's a tad over 1/32"

  • 1 ft. or 12" in HO scale = .1378 in (decimal equiv. in thousandths)

  • Therefore 1 scale in. = .01148 (decimal equiv. in thousandths)

 

Just what good is the above information ?  Well, Let's say you want

to replace the wheel sets on a freight car or diesel locomotive, and

are unsure as to the scale size or diameter of the current wheel.

 

To get an accurate measurement of the wheels you are replacing, you

should have use a dial caliper or micrometer, in a pinch you can use a

quality HO scale ruler that's graduated in scale inches, usually at

one end of the scale, such as General's HO Scale # 1251.

General's MG-JDC6 Dial Caliper. [click image to enlarge]

 

General's 12" HO Scale Ruler # 1251

 

** HO Conversion Factor Examples **

Millimeters to Thousandths to Scale Inches

1mm = .03937

3.5mm = .1378 thousandths (12" scale)
1" scale = .01148 thousandths

12" = 3.5 x .03937 or .1378 thousandths


Example #1:
.383 divided by .03937 = 9.7282194mm
9.7282194 x 3.5 = 34.049" scale inches


Example #2:
.375 divided by .03937 = 9.525019mm
9.5250 x 3.5 = 33.33757" scale inches

 

Example #3:
.3712 divided by .03937 = 9.4284mm
9.5250 x 3.5 = 32.99" scale inches

or - basically 33" scale inches


Working Backwards: Wheel Diameters


Example # 1:
48" wheels >> 48 x .01148 = .551 thousandths

 

Example # 2:
42" wheels >> 42 x .01148 = .482 thousandths


Example # 3:
36" wheels >> 36 x .01148 = .413 thousandths


Example # 4:
33" wheels >> 33 x .01148 = .379 thousandths

 

NMRA RP-25 Wheel Contour

 

In the examples above, the diameter

shown in thousandths (i.e. - .379 diameter) is

ideally located at the R1 point on the wheel.

 

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by C. H. Waters.  All Rights Reserved.

 Revised: June 10, 2008 09:58 AM.