New chassis for the Hornby 2721
by F.C. Roberts
Detailed Hornby 2721 Class, with Bachmann chassis
The Hornby 2721 class is a nice old stager from RTR land, and although the dimensions are not perfect, it gives a reasonable representation of one of the numerous panniers that pre-dated the later 57xx and 8750 classes.
On this page, Fred Roberts gives an account of how he has fitted several 2721s with the Bachmann 57xx chassis.
This will give you a chassis that (a) has better slow-running abilities and (b) looks better: Unlike the Hornby model, the Bachmann chassis features fully-flanged center driving wheels and brake rodding.
Note that the project does involve a trade-off: A small section of the rear motor housing will be visible in the cab, although fortunately this is not particularly noticeable from most viewing angles.
Rear view of modified loco
You need a Hornby 2721 body, and a Bachmann 57xx chassis. Note that the 57xx chassis has different dimensions from the more recent Bachmann 8750 class. The chassis on the latter version is taller, so may not do for this conversion.
The 57xx chassis has been marketed as a separate item, but is getting hard to come by as new. Fortunately, Bachmann 57xx locos can be obtained secondhand without too much trouble.
1. Dismantling the body and bunker
Remove the 2721 body from chassis. This is done by inserting a small screw-driver in the small hole at the rear of the bunker, thereby pressing back the lug that holds the chassis in place. Now, remove the bunker and cab-floor from the body by pressing the two holding lugs beneath the bunker. Be careful not to damage the inside of the buffer-beam while doing this.
2. Widening the motor housing channel
Increase the width of the motor housing channel (i.e. the width of the inside of the firebox) to 20mm by cutting from the front of the firebox to point B (figure 1). The cut should end up in line with the moulded strapping just behind the smokebox on the outside of the body (point C, figure 2).
3. Removing the firebox backhead
A square section in the backhead of the firebox needs to be removed from the cab to allow the motor housing to extend into the cab.
Working from inside of the firebox, cut along the edges of the backhead moulding to the start of the top curve, then cut straight across – resulting in a square hole in the backhead (figure 3). The total length of the motor housing channel from point A to point B should be approximately 62mm (figure 1).
4. Testing the chassis
Now tidy up the cut areas, taking care not to damage the tank Connector Pipe (point D, figure 2) which will now be exposed. It may be necessary to chamfer the inside edge of the front splasher, to ensure that the front driving wheels on the chassis do not touch the body. Test fit the chassis to the body to make any adjustments necessary.
5. Adapting the cab floor
On the 2721, the cab floor forms an integrated part of the detacheable bunker.
The floor needs to be adapted in order to accomodate the new chassis. This involves increasing the size of the existing rectangular cut in the cab floor to 20mm wide by 12mm deep (figure 4). This should now be a firm fit around the end of the Bachmann motor housing.
6. Fitting chassis to body
Securing the new chassis to the modified body is a bit tricky, and I am still working on the ideal solution. As a temporary measure, I have put a piece of thin plastic between the chassis motor housing and the inside of the body. This has improved the fit and the chassis does not fall out. To disguise the new chassis motor housing, I painted the exposed area in the body colour.