How to correctly measure axle gear on an HO Brass Steam Locomotive

[Locojunkie]

Hello everyone, I recently ordered a new chassis assembly for one of my Brass HO RF&P 4-8-4. I got a new gear box but the new gear box will not fit the axle gear on the replacement chassis. The gear box is not wide enough. I do know that the axles are 3 mm. What is the proper way to measure the gear so that I can order the correct gear box. Or if anyone can tell me the gear size on the OMI replacement chassis. Thanks.

[gmpullman]

Have you visited the NWSL's 'old catalog pages'? There's a wealth of information there. This article deals with gear sizing:

https://cdn.shopify.com/s/files/1/0250/4032/2669/files/Gears_1_2.pdf?263

Good Luck, Ed

[CandOsteam]

First issue is the worm gear in the gearbox must properly mate with the spur gear on the driver axle and the number of teeth on the spur gear, and therefore the diameter of this gear must also be correct for the gearbox you have.  The other issue is the thickness of the spur gear itself.  

The bottom line is you have to get an entire set that has the gearbox housing, worm gear, and matching spur gear.  NWSl has a range of sets depending on chasis configuration, etc.  It is nearly impossible to find OMI gearbox parts, but American Scale Models do carry a few gearboxes, but for Boo Rim manufactured steam locomotives and even in these cases you need to know specifics. Greenway Products use to sell replacement gearboxes for steam imports, but again need details to see if any would work in your case - a long shot.

I have replaced poorly made gearboxes on brass steam with high quality NWSL gearboxes and I am sure they can help guide you, BUT you will have to remove the original spur gear to put on the gear that comes with the NWSL.  This means requartering the driver and possibly doing all the drivers if the quartering jig you use does not put the same offset angle done at the factory when drivers were first manufactured.  Not only that, the new spur gear must be installed square and centered on the axle.

Hope this was at least informative, wanted to give you a realistic picture what is involved for a total gearbox swap if original gearbox parts cannot be found.

Joel