As I've mentioned before, I've used the 13.7:1 planetary gearboxes with the 18V 775 motors that Gimson sell as a single unit to great effect on 6S though I'd say that was the upper limits of its capability, 26.4V on 8S LiFe is OK but it can't stand the pounding for too long. Pretty sure I am going to put a couple of those in a new FW I planning.

As Al says, for the price, you could simply afford to get a new drill/gearbox if one breaks given the economics but if the 16:1 gearboxes Danjr1 work, then they are less than £20 a box. Tempted to get 4 of those (or the 20:1 variant) for a replacement Kaizen...