Having measured mine the HPI rear pod is 3mm wider than the F103 pod, but the HPI pod is offset 2.5mm further. A standard F1 diff will be centred if you use a 10mm spacer on the non diff side, but to end up at the same width as a Tamiya car you need to add a 2mm thick spacer on the diff side and use a 12mm spacer on the non diff end of the axle to get the alignment right.
Gearing isn't much of a problem as both Tamiya and 3Racing make a spur gear adapter to fit standard spur gears in the F1 diff, but the amount of movement in the HPI motor mount means it can cover the normal F1 gear range anyway.
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