Step 2 in building Tamiya’s M8 Howitzer Motor Carrier is titled “Drive Sprocket”, and deals with exactly that: the assembly of the track drive sprockets. In theory this is an easy step, with the modeller required to assemble two drive sprockets, each consisting of two parts. Unfortunately Tamiya got the track drive sprockets a bit wrong, so although this chapter is still fairly simple, it is not as simple as gluing two parts together.
As I mentioned above, Tamiya got the look of the track drive sprocket wrong: they provide an open sprocket, when in actual fact the M8 HMC was fitted with a solid drive sprocket. Fortunately the Eduard exterior PE set supplies four solid sprockets which can be fitted over both the inner and outer sprockets to make them look the part.
Above are the Tamiya kit instructions relevant to this chapter. The inset shows the Eduard instructions.
As you see from the above instructions, each track drive sprocket consists of only two parts, the inner sprocket (B9) and the outer sprocket (B8). After removing the parts from the sprue and cleaning up any blemishes, my first task was to remove the moulded bolt heads from the outer sprocket. I did this simply using a chisel blade fitted to my hobby knife, but I guess you could also use the grinding head of a rotary tool if you really fancy. Now the kit parts were ready to receive the PE sprockets.
A note to newcomers to photo-etch: photo-etch really can be quite sharp, so you really should practice caution both when removing it from the fret (the frame the PE is attached to) and when handling it.
Aligning the PE parts with the kit parts was simple enough – when dry-fitting it, that is. Why do I say that? Well, if you think about it, when aligning the PE part with the kit part you have to align each point of the PE part with the underlying plastic part – so you have fourteen points to align! Although fairly simple, it can be time consuming. Now try to do it with CA glue applied!
So here is how I handled this problem – and I must confess I only realised this method on a second go at aligning a set of parts. What I did was broke the task down: I only applied CA glue (note that I do not apply it directly from the tube, I put some on a piece of plastic card and then apply with a sharpened match stick or toothpick) to half the plastic part, and then quickly placed and aligned the PE part. Once the glue had set, I gently prised the unglued section of PE up, as seen in the below photo, applied more glue, and then pushed it back into position and clamped the whole part.
Above left you see one of the outer sprockets completed, while other has half the PE bent up for ease of gluing. Above right you see one of the sprockets being clamped.
The whole purpose of the PE parts is to transform the open track drive sprocket into a closed on. However, placing the part does not complete the transformation. When looking at the sprocket, one could see the gaps: one could see the gap in the centre of each sprocket between plastic and PE, as well as the open gears themselves were visible. And so, I decided on the route of least resistance: filling the gaps with Tamiya putty. Once this was done and the putty sanded smooth, the two parts of each track drive sprocket could be glued, and clamped, together.
Above left is one of the drive sprockets being clamped. Above right you see the two completed drive sprockets. Note the use of Tamiya putty visible inside the sprockets and in the centres.
This chapter is not quite over though. You may recall in the first part of this chapter I removed the bolt heads from the outer drive sprockets, and thus these need to be replaced.
Using some (approximately fourteen per sprocket) of the lengths of 0.5mm styrene hex rod which I cut while making the fixes in M8 HMC: Step 1 Errata I replaced each of the bolt heads on the outer sprockets. Each one was glued into place (there is a slight dimple on the PE part denoting the positioning of each bolt) using CA glue. Once all in place, I lightly sanded them all to approximately the same length.
The above left photo shows the front, or outer, of the two completed drive sprockets, while the above right shows the inside.
The next chapter is another short one, and covers the fixing of the drive shafts.