Modifications to the Airfix/Hornby 61xx Prairie Tank

This article describes the process of fitting a DCC decoder to the Airfix GWR 61xx prairie tank locomotive, together with a few additional detailing modifications.


The Airfix model as supplied

The Airfix GWR 61xx prairie tank is a model of a very successful prototype introduced by the GWR in 1931 for the purposes of commuter and empty carriage stock workings around the London area. They were mainly shedded at Old Oak Common, Southall, Slough, Reading and Aylesbury throughout their lives. In the early 1960s, the advent of the first generation diesel multiple units made them semi-redundant, resulting in their last few years seeing employment on more menial goods duties.

The Airfix model dates from the early 1970's and comes in the original 'blue box'.
It was actually a good model, featuring a 5 pole motor which at the time, was a new revolution for smoothness, however, it was a bit noisy. The model appears to have been upgraded a few times during its life as very early versions did not have flanges on the centre driving wheels.

The model covered in this article is one of the later versions which does have flanges on the centre driving wheels. In fact, it is so late on that the underside of the chassis keeper plate actually has 'Mainline' (as in 'Palitoy') moulded, indicating that it dates from the period when Mainline took over the former Airfix range until they too folded and the range was taken over by Dapol and Bachmann, with the Dapol share later going to Hornby.

Of particular note is that the model is in BR green livery which was not one of the Airfix liveries - to my knowledge, Airfix only ever did the model in GWR green and BR black. Note in these links how the GWR model is the earlier version with no flanges on the centre driving wheels, yet the BR version is a later model with them.

Back in the 1980's, I'm sure that many of us can probably remember that models in pre-nationalisation liveries always cost more than the BR livery version because no-one wanted the latter. How things have changed!

When I converted to DCC in the early 2000's, the old Airfix chassis was unsuitable for conversion because it simply wasn't up to the smoothness of modern mechanisms, so the loco was laid up for a few years, out of use. When Hornby re-introduced the model, they upgraded it and fitted a proper can motor and the opportunity was taken to purchase a replacement chassis.
This article describes fitting a DCC decoder to the new Hornby chassis, together with a number of modelling modifications to bring the model up to 'scratch'.

Please note that Hornby have recently introduced a brand new tooling of the 61xx class. This article is NOT that model.

The Hornby Chassis

The original Airfix chassis had an open frame 5-pole motor driving the leading driving wheels via a worm gear arrangement:

Hornby modified this to move the drive onto the centre axles, making the whole mechanics much more balanced. The smaller can motor drives the centre axle firstly, via a worm drive, and then by a tower of a few spur gears. The whole mechanism runs a lot more smoothly than the original. Note that the second picture shows additional pickups having been fitted.

Where the Hornby upgrade doesn't seem to have faired well is in the are of the pickups. The original Airfix model pickups protruded from the underside keeper plate and were positioned vertically against the rears of the wheel treads - this can be just about seen above in the second picture of this article marked with red rings. This worked well but potentially could suffer the issue of the pickups shorting against the frames, although I never experienced this. Insulation would solve this anyway.
Hornby modified the pickups to be horizontally mounted, consistent with the way they do all other locos these days. I'm really not quite sure about this because for me, the new mechanism always seemed to stutter and loose electrical contact. I'm not entirely sure that the material the wheels are made from hasn't got something to do with this either as it seems to be different to all other wheels I have seen - a very yellow appearance.

The Ashprington Road layout has had its track laid to a very high standard and is very flat with no undulations other than those introduced by proper canted transitions into curves, therefore, there shouldn't be problems with pickup but alas, this new chassis kept loosing power. A lot of experimentation later found that the leading and trailing driving wheels have no rocking capability because they are completely constrained by plastic protrusions on the keeper plate which are designed to stop axle movement such that any uneven track immediately lifts a wheel off the rail head instead of allowing it to drop. In the following picture, I have marked where these protrusions have been removed:

Fitting a DCC Decoder

Initially, I fitted my decoder of choice: an ESU, in this case a Lokpilot micro. While this gave extremely good running characteristics, because the loco kept loosing power, it kept stopping.
Personally, I'm not a great fan of 'stay alive'. I hail from the 'old camp' where we believe that such things are a band-aid to a problem and that if the problem is fixed, then the 'stay alive' shouldn't be needed. This loco is the only one I have ever had which has eluded me and I was simply unable to fix the mechanical pickup problem, even after adding extra pickups, so I fitted a TCS KAT22 decoder which comes with built-in stay-alive. This immediately resolved the stuttering problem, however, I don't find that TCS decoders are as good at motor control as ESU. To that end, I looked for CV adjustments. The KAT22 has the ability to switch on/off an 'aggressive algorithm' for 3 pole motors. I don't know whether the Hornby motor is 3 pole or not, but selecting the 3 pole setting made a considerable improvement. But the setting only works for speed steps 1 to 3 and guess what ? This loco needs the adjustment from speed steps 1 through 9. I have posted an enquiry to TCS to find out if this is adjustable. I never received a reply.

Modelling Modifications

Dating from the period that it does, this model is fairly primitive by today's standards. To that end, I made the following modifications:

  • Replaced the printed bunker number plates with brass offerings from CCW
  • Made up a number plate for the smokebox door
  • Fitted vacuum pipes according to my article here
  • Removed the plastic coal in the bunker and fitted real coal
  • Replaced the front and rear pony truck wheels with Gibsons wheels as the originals had wide treads. The original Airfix wheels didn't even have see-through spokes
  • Weathered the couplings rods and slide bars
  • Fitted Romford screw couplings
  • General all-over light dusting of weathering using Carrs powders

Further modifications will be made in due time:

  • Paint the sides of the footplate in green
  • Replace 'handrails' with proper wire equivalents
  • Replace the cylinder drain cocks with brass fittings
  • Replace the smokebox handles with brass fittings
  • Fit wire sanding pipes
  • Tidy up the chassis mounting holes at the lower side of the rear bunker
  • Fit some lamps
  • Tidy up the roof moulding, particularly on the rear of the cab - this seems to have its herritage in the old Airfix kit!
  • Add pipework detail underneath the cab

Why not buy the brand new Hornby model ? Well, I actually like this loco. I've done a lot of work on it over the years. And besides, I enjoy modelling.

Further DCC Decoder Changes

After spending a lot of time modifying CV's, I wasn't able to get the KAT22 decoder to 'tame' the motor of this loco, so I decided to abandon the TCS KAT22 decoder. It found a new home in the Heljan 7mm scale GWR 43xx. I find that TCS decoders work well with good motors, but unlike ESU and Zimo, they are not able to improve lesser-quality motors.
It was at this time that I found out that the reason for the continual loss of power was due to the rear pony truck lifting the loco off the rails, meaning that only the front drivers were fully in contact. Adjustments to the mounting of the rear pony truck resolved this issue and made 'stay alive' unnecessary.
Following that, I decided to try a Zimo decoder in this loco. I've seen numerous 'ESU vs Zimo wars' on online fora, so I thought I would find out for myself. I purchased, and duly fitted, a Zimo MX600R, after installing a proper 8 pin DCC socket in the coal bunker. Performance and slow running was superb, straight out of the box, with no adjustments to CV's necessary. The decoder 'tamed' the motor perfectly. Was it better than ESU ? Let's just say that it was certainly 'as good as' ESU, maybe marginally better in that it didn't require a single CV change whereas the ESU required one CV change. Zimo certainly has many configuration options which no-doubt make it a very advanced decoder, but for what I was trying to achieve, I can't say that I noticed a significant improvement over the ESU decoder as some online fora would have you believe. It is difficult to place either ahead of the other as they are both extremely competent products, both of them right at the top of quality and control. I would not hesitate in purchasing either in the future.

The Completed Model

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Graham Plowman (10/09/2019)

Replacement chassis kindly provided by Simon Kohler of:
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