OTHER MODELLING WORK: Replacing Split Gears on a Farish Class 20


27 August 2014

This article is based on a thread in the N Gauge Forum, and has been compiled from conversations that helped to repair the split gears on a Graham Farish Class 20 diesel locomotive from the Poole-era.

Purchasing a cheapish Class 20 diesel because the chassis was needed for a new kit body may not have been my brightest idea. Especially as it had split gears. So before it could be used, the split gears had to be replaced, and this particular idiot needed an idiot's guide to achieve that. And a lot of help from other people. Thankfully various members of the N Gauge Forum were there to provide that, and between us we managed to get the job done.

This article is a compilation of all the best bits.

Tools you will need at the very least: a flat-bladed screwdriver, a nail punch or similar, and long-nosed tweezers. Recommended tools: a desktop magnifying glass, preferably with a light.

Here's the Class 20, which was relatively cheap because of those split gears:

Photo 1

I understood from various sources that if one gear is split then all of them should be replaced, but other than that, this was all new to me. I didn't even know how to take apart the loco safely, so here's my learning curve.

To begin with, the loco's yellow front panel was loose, so a fingernail could be used to prise it off, leaving one visible screw (see next picture). If the panel isn't loose you need to take some care in prising it off, as I've heard that the plastic prong that holds it in place can be snapped off if you're not careful.

Photo 2

With a small, flat-bladed screwdriver, loosen and remove the front screw.

Then use the screwdriver or a fingernail, placing either between the blue bodywork and the top of the metalwork at the front (immediately underneath the front screw in the photo above).

Twist the screwdriver or push downwards with the fingernail while pulling the blue bodywork in the opposite direction. The two should slide apart (shown in the next photo).

Photo 3

The bodywork and chassis are now separated. It would be a good idea to photograph each stage of the work so that you can be entirely sure how to reassemble the loco, especially when it comes to the chassis frames and bogies.

Photo 4

On top of the chassis block (the grey metal section that houses the motor) are two long screws, located in diagonally opposite corners of the chassis. Unscrew these and draw them out.

Photo 5

Once you have done that, the metal blocks at the front and back, which hold the bogies in place, will come loose, and perhaps even fall out of their own accord.

Photo 6

Now you can access the bogie frames and bogies which hold the wheels and gears.

Photo 7

Now you can test each bogie to see which one has the split gear(s).

Gently push each bogie along a short piece of track. If there is a tiny bit of give in the gears so that each bogie moves a fraction before the gears lock then you have a diagnosis.

You can also try manually turning the large gear at the top of each bogie (shown above) to see if they will rotate all the gears. In this case, one did so for a few turns (about three), while the other (the bogie with the red spot, on the right of the photo) wouldn't move at all. This meant that both bogies had at least one split gear. If one of the bogies had moved freely without any sticking then the gears would have been okay.

Photo 8

So you've diagnosed at least one split gear per bogie. Now take out the drive axles of one bogie (don't touch the other as you can use this as a guide to reassembling the first one).

This stage is best done with everything inside a large, clear plastic bag - the type that comes with mail order clothing or well-packaged kitchen equipment. Or, if you work in a maternity unit, a spare incubator would do the trick. This cannot be made clear enough. A plastic bag (or maternity unit) is your only hope of surviving this stage with your sanity intact.

Each bogie has a slot at the back (see photo above). This is what locks the bogie frame to the bogie itself. Insert the screwdriver vertically between the plastic of the frame and the plastic of the bogie and gently twist. This will force the frame away from the bogie and it will eventually detach.

Take the frame away from the bogie. This exposes the coupler spring, which is when it's likely to make a dash for freedom. This is why the plastic bag is so vital. The wheels are fairly secure in the bogies so they won't fall out. Once you have the coupler spring safely to one side, you can dispense with the plastic bag until it comes to reassembly.

Photo 9

If the spring somehow remains in place, a piece of tape will help it stay there.

Now you can dissemble the bogie itself.

Starting with the back wheels, ease each axle out of the frame. The axles can be pushed to one side a little so that part of them is exposed and can be pushed out with a careful finger. Don't be afraid of applying a little firm, steady force here, as long as you have a firm grip on the frame itself. The bogie's plastic body is fairly flexible, so you're unlikely to damage it.

Photo 10

The brushes will fall out of their own accord so you might want to save them the bother and remove them all.

Carefully store all the disassembled bits in the right order.

As you take off each of the axles, test the gear. A good set of long-nosed tweezers is handy for this stage. If the gear moves sideways or rotates while the axle is held firmly in place, then the gear is split. Sometimes you can clearly see the split, and sometimes you cannot, but it's there.

Photo 11

The gears shown above are 12-tooth (nearest each of the wheelsets) and 16-tooth (in the centre), and this is the order in which they sit on the bogie.

If any of the gears can easily be slid off the axle, then you definitely know it's a goner. Replace the gear. It's also a very good idea to replace the wheelsets, especially if you're working on a Poole-era Graham Farish loco. You'll be able to exchange the shiny pizza-cutter wheels with the new-standard, darkened, smaller profile wheels, and as they're relatively inexpensive, you can get them at the same time as you order your replacement gears.

Photo 12

Having presumably identified which gear was the problem one on this particular bogie, I put the other axles and wheels back in place, starting from the back and using the other bogie to help identify which way round each axle was supposed to go.

That done, I tried rotating the bigger gear on top of the frame to see if it would go round smoothly now. It did, mostly, but continually stopped every rotation or so. With a bit of force it would start again, but the same block was there on each rotation.

The same effect could be seen when turning the back wheels. The cogs all went round in unison but there was still a point at which something got stuck, and it took a bit of force to push through it. That meant more than one split gear.

To try and narrow down which of the remaining gears it was, first I took off the wheels at the coupling end of the frame (the rear end, as far as this guide is concerned, as this is usually the far end of the frame in the photos).

I tested the gear on the wheel axle by hand and it seemed okay. I fitted the front wheels into the same rear slot on the frame and the same sticking action was still there. So the wheel axles were probably innocent.

Taking the wheels out again, I tested the top gear, with just the central drive gear below it and one of the idler gears and its axle sitting between the drive gear and the rear wheels. Still sticking, so it had to be one of these two. I removed the idler gear's axle and took a closer look at the gear. Although it seemed okay at first, I was able to grip it and start sliding it around on the axle, so this must be the one.

That meant that both of the minor gears on this bogie were split. It seemed likely that others were too, or could go the same way sooner rather than later, so the best solution was to replace them all, and also to replace the old Poole-era shiny wheels in favour of newer Bachmann wheels. Not only would the bogie look much better all round, but I would be spared the rather complex task of disassembling and reassembling the wheelsets with limited tools and checking back-to-backs to ensure proper running.


(See additional section, below, which covers replacing gears on an existing wheelset.)

Go to Replacing Wheelsets >>


It seemed to be safe to assume that the bigger 25-tooth gears, the single gears that sit on top of the bogie, were okay. Replacements in this case were not needed.

The items I needed for a complete upgrade were as follows, all from BR Lines:

  • GF7101-1 - Drive Axles (Bachmann Type) for Class 20 x 4
  • GF2503 - 12-tooth Gears (pack of 4 latest, thicker gears)
  • GF2504 - 16-tooth Gears (pack of 6 latest, thicker gears)

The replacement parts soon arrived from BR Lines, and I had a pretty informative chat with Bob himself when placing the order. A very knowledgeable chap!

I knew what my evening's work would consist of...

Photo 13

The next trick was to get the new gears on the old axles. The Bachmann Chinese 12-tooth idler gears (the only ones that are now available) are wider than the Poole-era gears and, unlike the larger 16-tooth axle gears, can be fiddly when placing them on the axle shafts.

When replacing any gear, you're advised not to use force, otherwise you may inducing stresses that will lead to another split quite soon. However, that leads into the reaming debate, which may or may not be necessary (see boxout, below).


[To ream or not to ream, that is the question] Reaming involves placing the appropriate tool into the central axle hole in each gear and shaving off a little of the plastic so that it will fit much more easily onto the axle. The tool required is a cutting broache, a very popular tool with guys who scratchbuild etched brass kits.

However, not everyone is convinced that the gears need to be reamed at all, so this seems to be one of those situations in railway modelling where it's really down to individual preference. General consensus seems to be that reaming isn't essential but it can reduce the chances of a repeat split.

One expert in the field certainly does strongly suggest that the reaming is absolutely essential, as placing the replacement gear under any stress while fitting not only deforms the gear, thereby altering the tooth spacing (sometimes unequally), and swelling the gear to a different size but can also cause the gear to split immediately (seen at first hand). His suggestion is never to use a drill. Instead always use a rat tail file or a diamond reamer that is an abrasive not a cutter. Broaches are poor for this type of work as they are for use on metal and just weaken the gear. Gears should be reamed so that they are a gentle, easy push-on fit - there must be some tightness to transfer the drive.


Lacking more sophisticated tools for this part of the operation, I used a large diameter counterpunch as a cradle for the axle end while using my other hand to hold a pair of long-nosed tweezers to ease the gear onto the axle. It's a bit of a balancing act, and far from ideal. You have to keep a pretty tight grip on both counterpunch and tweezers to avoid things falling all over the place, but it is viable. Some kind of vice to hold the counterpunch would be handy, but don't worry if your beginner's workshop doesn't stretch to that.

Unfortunately on the very first attempt to push a new gear onto an axle, one of the teeth on the gear went missing somewhere between opening the bag and joining it to the axle. One moment it was there, the next, poof! Gone. Only without the puff of smoke.

Photo 14

That seemed to be the end of the adventure. I had four split 12-tooth gears and three surviving replacements. Luckily, a generous dose of help was available from the N Gauge Forum in the form of two very kind offers of replacement gears so that I could finish the job - and the idiot's guide.

The replacement gears duly arrived, along with several much-needed doses of encouragement and advice, and I had another go.

The next stage took significantly longer than it probably should have. One piece of advice I was able to follow was to place the plastic gears in warm water to gently soften them, and the metal axles on ice for a time (to contract the metal, I presume). It didn't seem to make any difference for me, but perhaps I didn't leave them for long enough.

Fitting the gears onto the axles required a good deal of careful but insistent pressure. It took me a long evening of work to complete the process with my makeshift repair shop tools, and unfortunately, there are no photos as my spare set of hands were in the wash!

Make sure that you push down on the body of the gear (not the teeth - I've already proved that these are prone to breaking or being damaged). It takes quite a bit of targeted force, but sooner or later each gear moves down the axle. The trick is to stop pushing when they get to the middle, so that they rest over the teeth on the axle. All four 12-tooth gears went on and centred pretty well, but I had to replace one of the central 16-tooth gears too (that's a lot of split gears on one loco!), and this one was much harder to centre. Got there in the end, though.

The next stage, once your fingers have recovered from all of that, is to reassemble the bogies. Put the central 16-tooth gear back first, snapping the axle back into its slots. Then the 12-tooth gears on either side, and then the brand new Bachmann wheels and axles. All of them simply snap into their respective axle slots.

Test that they work by rotating the top gear on each bogie. If all gears turn smoothly then you've done it right. One of mine was catching, and I found a slight nick in one of the teeth on a 12-tooth gear that I'd caused when pushing it onto the axle. I was able to file this smooth and the problem was gone. You should now have something like this:

Photo 15

It was here that another - and highly unusual - problem reared its ugly head. One of the wheelsets wouldn't move even though it was properly inserted into the bogie. It's probably a very rare problem that in most instances shouldn't arise at all, but if it does, you'll need to see the Axle Stiffness boxout in the problem-solving section below for more details.

Go to Axle Stiffness boxout >>


A new wheel set supplied by a friend solved the problem at a stroke, and the faulty set was duly returned. It just had to happen to me on my first attempt! On the plus side, I've probably greatly increased the odds of anyone else suffering the same problem for quite some time.

So, now the fresh gears and wheels had been slotted into place, and the brushes had been correctly reassembled, matching them up exactly to the photos made beforehand. The latter stage was trickier than it looked, and took more time to complete than I anticipated. A pair of tweezers was invaluable for this work as it's quite fiddly.

Next, join the bogies to their respective bogie frames, making sure you handle this stage on the inside of your clear plastic bag so that the coupler spring doesn't make a break for freedom. Place the spring inside the coupler box (if it's not already there), ease the bogie down on top of it to provide a protective cap and then, before snapping it in place, push the coupler into the spring (there should be a slight locator nodule on the back end of the coupler that should be targeted at the centre of the spring). Slot the coupler into its box and snap the bogie into place in the chassis. Repeat for the other bogie and frame, and assembly is complete.

Attach each bogie frame back onto the motor, taking extreme care that the brushes don't get caught in the bodywork. Make sure each chassis has full side-to-side movement and isn't caught in any way before you insert and tighten the motor body's screw (one for each end of the motor).

Now you're ready to test. I didn't have that impressive a test track, but the Class 20 itself was very impressive. It sounded smooth and quiet, purring its way along the track without any rolling or wobbling at all. In fact, it looked like it had been returned to full health:


Although I hit a few snags along the way, the basic process was relatively simple. Wheelset production error aside, most of the 'problem' areas were down to my lack of knowledge and the proper tools. Hopefully, I've proved that it can be done relatively easily, even with the most basic tools and a photo or two for reference.





[Replacing Gears on an Existing Wheelset] I didn't attempt this myself, having opted for brand new wheelsets, but these notes are compiled from advice received just in case you would like to try it.

To replace axle gears on wheelsets you might want to use a GWS pinion gear puller. Model shops that sell radio-controlled cars are one potential source for this bit of kit. It's a pretty precise tool for pulling wheels off axles and then putting them back on. In the latter process, after initially lining up the gear with the axle using fingers, the gear can be aligned to the centre of the axle using the puller.

To push the wheel back onto the axle, you could use a small hand vice. Gently twist the wheel onto the axle, making sure that the plastic insulation in the centre of the wheel is not caught, then place the axle into the vice as squarely as you can get it and press the wheel into place.

Before fitting the reassembled axle onto the bogie, slowly roll it across your work surface with a small flick of the finger to check if the wheel is true, with no wobbles.

Having used the puller and vice, you do not need to adjust the back-to-back measurement to ensure correct alignment. This is because, when the axle is pressed back through the centre, it stops flush with the outer edge of the wheel (bearing in mind the fact that you should be using a vice with a smooth jaw face so that the axle does not protrude beyond that point). It's a guess, but it seems likely that Graham Farish would have had their axles made to a certain length for this very reason so that when their machine assembled the wheels, the axles set the width.

When running locos quite fast back and forth across the points, after having used this process, no problems have ever been experienced.





Problem Solving

There are a few things that can potentially go wrong during this process, and this section tries to cover them.

Only the problems that arose in this specific instance are covered for the moment, but I'm happy to add any others. Just get in touch with the details.


[Axle Stiffness] One of the new axles seemed to be very tight - so much so that I couldn't get it to rotate at all. I reassembled the loco and tested it on the track.

When power was applied, I could see the motor trying to turn, but the 'locked' wheel set refused to turn, so the whole chassis was pulled upwards, so much so that the back wheels left the track. With some considerable assistance and support, a manufacturing fault was found in the Bachmann axle.

The sleeve onto which the gear was pressed was too long, and also the gear was sitting off-centre on the axle shaft. Normally these sleeves are short and the length only just protrudes slightly on either side of the gear. This indicated a manufacturing fault. To quote N Gauge Forum member, Mr Sprue: "The fact that this sleeve was sitting to one side made the shaft a tolerance fit in one side of the bogie block and thus restricted free movement."

The offending part is shown in red in the diagram below. Replacing the entire wheelset with a fresh one cured the problem entirely. (In all fairness to BR Lines, their analysis of the fault didn't entirely agree with the one that was supplied to me, but the wheelset was accepted back and replaced, so I must thank Bob at BR Lines for his advice and cooperation.)


Photo 16


[Trapped Brushes] When reassembling the chassis, one thing you really must avoid is getting the brushes trapped somewhere in the bodywork. It will mean that the bogie will lose its manoeuvrability. Ensure when attaching each bogie back to the chassis and motor housing that the bogie retains full mobility. It will mean that you need to take a little longer on the process but it is certainly worth it.



Main Sources

N Gauge Forum: Replacing Split Gears Farish Class 20

Main Contributors

Users 'Mr Sprue', 'StufromEGDL', 'RusselH', 'AndyGif', and 'Calnifoxile', with thanks