Modifying a High Level gearbox
Entry posted by Captain Kernow
775 views
My current project is the conversion of a Bachmann 64XX pannier tank to a 74XX (almost identical, but not auto-fitted and thus much more suited for a goods-only branch in the Welsh borders), which will be used on my new OO layout 'Bethesda Sidings'. I've been posting occasion updates on the project on my layout thread here -
Being generally far too fussy and anal about smooth, slow running for my own good, I have once again found that the Bachmann RTR chassis just doesn't 'cut the mustard' in terms of controlability. I appreciate that others have had better luck, but (and I suspect part of this is just me), I frequently seem to find the RTR chassis unsatisfactory.
What I have done in the past, is to replace the RTR chassis with an etched kit, normally using Romford or Markits wheels. I've done this a number of times, in some cases where the old Mainline split chassis has given up the ghost or, more recently, because the RTR chassis runs poorly and no amount of tweaking and running-in seems to improve it. Two recent examples have been my Hattons/DJM 14XX and (a few years ago) a Bachmann Standard Class 3 2-6-2T.
I had run the Bachmann chassis in for several hours back in 2017:
I had got it just about running well enough, to consider it suitable for slow speed shunting etc. on the new layout.
A few weeks ago, I started the body modifications to convert the 64XX to a 74XX, at our Scalefour area group's 'Indulgence Day' (the most obvious work involves modifications to the bunker and cab roof).
A little while later, having more or less completed the layout, I gave it another trial, which was when I encountered what I considered to be the final straw. The Bachmann chassis didn't like running through the A5 crossover, which had been built (most carefully) to OO-SF standards. This corresponded with an emerging theme, namely that some RTR chassis with 3 axles (or more) didn't like running smoothly through the crossover (oddly enough, a Bachmann 8750 pannier and an 08 diesel shunter are fine).
What did stand out, though, was that all my locos fitted with etched chassis and Romford or Markits wheels ran through with no problem.
As such, I decided that there was now no alternative but to replace the chassis with an etched kit. In terms of the smaller 54XX, 64XX and 74XX panniers, there is really only one option, namely the excellent kit by High Level, which gives you the option to build the variants suitable for any of the above three types of pannier loco.
The kit is highly detailed and certainly the most complex chassis kit I have yet assembled. However, the detailed instructions are extremely thorough and the fit of the etched components is excellent, a tribute to the quality of the product. I found that I just had to take my time and methodically follow the sequence outlined in the instructions to the letter (which is what Chris Gibbon of High Level recommends) and the chassis slowly started to take shape.
Once I had fitted the hornblocks, with their sliding brass bearings and the twin-beam compensation, however, it began to dawn on me that there might be a problem.
The chassis is designed to be built in either OO, EM or P4, of course, and the issue I encountered would not affect EM or P4 modellers.
The kit comes with it's own gearbox, the parts for which are included on the main chassis etch. In all other respects, it is typical of the excellent gearbox design that High Level have been producing for many years now, etched in nickel silver, with plastic gears, brass worm and brass final drive gear (with grub screw). It is also quite a narrow gearbox when assembled, being approx 7.5mm wide (some of High Level's other designs can be up to 11mm in width, in my experience).
When looking at the amount of space between the hornblocks and brass bearings for the drive axle (rear axle), it seemed to me that the distance available would mean the gearbox was a very tight fit indeed, which might compromise the ability of the compensation to work properly.
Once I had folded up the main gearbox and offered it up to the chassis, the problem was confirmed, the gearbox was too wide:
I felt that I would need to reduce the width by at least 1mm or so, in order to give sufficient free movement to the compensation.
The first action was to remove one side of the gearbox, easily done by bending the side to and fro a little:
I then fitted a couple of 10BA bolts, with nuts set to the new spacing:
Once I was happy with the alignment, the gearbox side was soldered back on, with a small piece of scrap etch for additional strength:
Once the nuts and bolts were removed, the gearbox now fitted between the brass bearings, with about 0.5mm or so clearance, which should be sufficient:
A small bit of scrap etch was soldered in place to provide some additional strength:
The gearbox with the motor and gears fitted:
In hindsight, it would have been easier to thin down the sliding brass bearings. These do come with a raised, circular ridge on the inner face, that High Level recommend you remove for the front and rear axles, to clear the dummy valve gear and the gearbox respectively, but with an OO chassis, further brass would need to be removed to allow the gearbox to fit.
I have shared my thoughts with Chris Gibbon of High Level.
-
7
-
8
4 Comments
Recommended Comments
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now