Core RC do the serrated nuts in tamiya blue for not alot of money. I use them on all my cars now
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I genarally carry all the plastic parts trees for the 501 and DB01, a couple of sets of wishbones, all the shock spares, hinge pins, bearings, all the hubs, pullys, belts, spur gears and a front shock tower for both cars.
The only thing Ive broken on my 501 is a rear hinge pin, a shock shaft and a wishbone but I would recommend getting a set of hinge pin pivot balls too, I lost one at bury and had to wait a week to get some from HK as no one in the UK stocked them, Good job my DB01 and 416 use the same ones or I would have been stuck.