This is a double issue. First, the Bachmann boiler is made a a plastic shell and a pair metal weights. You'll will see the seam between both part and slots for mounting screws. Also, the weights are undetailed (no boiler straps, etc.).
The second problem is related to mounting the running boards. You need them to be sturdy enough to survive normal handling conditions and to be locatedd in a such fashion they won't be nuisance when dismantling the locomotive for maintenance. This means you have to mount the running boards on an incomplete shell and this is the big challenge.
|The milling in progress. The smokebox will need to be thinned down too.|
After discussing the problem with Doctorwayne, we came up sith a set of different solutions. I decided to use one of them and will explain how it works. I suspect this is a solution similar to the one Marty McGuirk and Iain Rice came up in a 1999 MR article which unfortunately I couldn't locate at the local model railway association library.
The basic idea is to simply extend the plastic shell as low as the running boards will be mounted. It means adding extra styrene sheet at the bottom of the shell. However, this simplistic solution require you to mill the metal weights to remove a equivalent amount of metal. In my case, 2 mm thick of styrene must be added to the boiler. This is exactly the thicknees of the metal weights walls encasing the motor.
Milling the frame took about 90 minutes using unspecialized tools like a tool grinder, a Dremel sanding drum and a large flat file to finish the work.
When done, styrene sheets were laminated against the boiler and left to dry overnight. At this point, the metal weights slide easily inside the shell as it should be. It must be noted you must mill the weights on all their height to make sure it will be possible to insert it. You also need to mill it up to the fron too.
To attach the running boards, small brackets will be pinned and cemented into the new shell extentions. Brass wire, music wire or phosphore bronze wire can be used: choose a material that will be strong enough to keep its shape under normal handling conditions. The new running boards will be them glued on the brackets, exactly just like the real prototypes. If wanted, the running board could be made out of brass and soldered on the bracket to make a single and sturdy assembly.
Sure, there is more than a way to tackle this problem, but I feel this option is manageable even for a not-so-skilled kitbasher.