Model Railroading > OO scale & HO scale.........what's the difference?

Alright, educate me. What's the difference between these two scales. Different track gauge, size of the rolling stock....etc? Just curious! Thanks much! Pat

Oh boy, this will be opening a hornets nest, but here goes. This is my understanding of it, other Brits / ex-Brits etc may be able to add more detail:

Nominally OO scale is 1:76, HO is 1:87. The early British model mfrs chose 1:76 scale for a reason I'm not aware of. However, in the interests of compatibility with continental Europe and US track, the early British model mfrs decided to gauge their track at 16.5mm. The correct gauge for 1:76 scale would be roughly 18.8mm, and in fact there are a couple of fine-scale British standards which offer this (EM and Proto4). OO track looks kind of narrow underneath British outline models. (The effect is even worse when you run Irish outline models on OO or HO track - the Irish standard gauge is 5'3").

So in practical terms if you buy OO track it will be the same gauge as HO, although the tie spacing and length may be different to track from continental Europe or North America. I personally use PECO OO track with Atlas and Microengineering track and I'm happy enough with the results. If you buy OO rolling stock it will run on North American HO track quite happily (sometimes you have to watch the flange depth on track with profile lower than code 83).

More details at http://www.emgs.org/

Doesn't it have to do with one being based on the Metric system and the other being based on the Imperial system (of measurements)?

however their were several 1:76 american manufactures, including Lionel & Scalecraft

Mike Slater

This is what I've heard over the years:

HO Scale is 3.5 mm to the foot.
OO Scale is 4.0 mm to the foot.

I've also heard that early motors were too wide, thus a slightly larger scale made things fit better. Before you youngsters laugh at that statement, just ask Grandpa why old Athearn diesels have wide bodies. This explains the selling point Athearn mentions when they state such and such a model has a scale width hood.

Larry

Larry,

You are right, OO is 4mm to the foot and HO is (half O) 3.5mm to the foot. It is quite possible that OO models simply adopted existing HO track work as a standard and thus 16.5mm became the gauge. Purists have then gone on to EM gauge (18.2mm) and Scale four/P4 (18.83mm) for a better and more scale look to track and of course the wheels (including wheel profiles) on that track.

I do recall that before N gauge became popular we had something called OOO, made by Lonestar in the early 1960s over here in the UK. So you can see there is a graduation, rather like paint brush sizes, from O down to OO and then half again to OOO (or roughly N as now is).

Mike Arnold

... someone came up with the idea of numbering all of the then popular sizes, like back in the 1920's. All models were larger, then, and the largest was given No 5 (somewhere around 4 1/4 inch gauge, IIRC), then down to the smallest, 0 (as in "zero", not the letter between "N" and "P").

Originally, in England, 0 gauge was 7mm to the foot (which is a curious mixture of metric and English ratios, by itself). The exact correct rail spacing for this is 33mm. When North Americans started using this size the metric scale was rounded to 1/4" = 1' and the gauge to 1.25", which has plagued the scale's purists ever since.

When smaller motors could be made, the obvious next size down from "0" (pronounce "Oh") would be "00" (pronounced "Oh, oh"), and it was established with a scale of 4mm = 1' and a correct gauge of 19mm. Incidentally, 19mm is exactly 3/4", which is perfect gauge for 1/4" scale models running on 3 foot gauge track - Thus, OO track is On3 track.

Then, some Brit came up with the idea of making track half the size of 0, and calling it "H0" for "half-0" with a gauge of 16.5mm (half of 33mm); but the Brits continued to use the 4mm scale of 00 running on their H0 track. When North Americans started using H0 they corrected the scale to 3.5mm, half of the original 7mm scale of 0 gauge.

"000" was mentioned. This obsolete scale is "Oh, oh, oh", or more easily handled "Treble-oh".

Also, the next size up from 0 is No 1 gauge, 1 3/4" which is also 45mm. An intermediate scale between 0 and H0 is 7/8" gauge, which is half of No 1, but instead of calling it H1 it came to be called "S". No 1 gauge was obsolete from about 1940 to 1980, but now is very popular today - renamed "G", and used by large scale garden railroaders.