The gears fit straight into the Hymek gear casing:
First thing to note is that the gear mesh is quite soft, and everything revolves loosely.
But it's when you combine these Hong Kong cogs with the original Heljan ones, that you notice a slight difference:
I've put one HK cog in the middle of some Heljan ones. Spot anything here?
In fact, the HK cog has slightly thinner teeth than the Heljan, so the mesh is a little looser and, when combined / compared with Heljan, seems to have a marginally smaller overall diameter. It is so slight, yet makes quite a difference for the purposes of the experimental Sulzer bogie, because it means that the HK gear teeth do not impinge upon the space required for the new centre axle, and we really need to reclaim as much room as possible here.
For anyone wanting to experiment with these generic 0.5MOD cogs, they are available from Hong Kong eBay seller "sellerbible". This particular pack cost just £1.49, and the seller does not charge for international postage, which I think is quite remarkable.