When we build custom motorcycles at the shop, special care is taken to prevent grounding problems. On custom frames, or frames that have been powder-coated, I'd suggest you remove the paint from the engine mounting surfaces for grounding purposes, but also because it will compress eventually and your engine mounting bolts will back off. We also remove any paint from beneath the trans plate mounts for the same reason. I also run an additional ground strap directly to the starter on big engines so the ground path isn't through the frame. When we paint heads/cyls we also mask the head bolt seat because powder paint melts and again the bolts come loose. Another place to be careful in the grounding department is on the front forks. If you don't have a wire in the harness that connects the fork ground to the frame, the current from the headlamp, turnsignals, etc will have to pass through the steering neck bearings and can cause problems there. I've actually seen stainless cables get hot and begin to melt when the ground path goes through them instead of the greasy steering neck bearings. On motorcycles with rubber mounted engines like Harley Davidson FXR and FLHT models, always use a fresh mesh ground strap under the engine, remove paint where they attach, and use good star washers to ensure good grounding. On rear fenders made of fiberglass or on lighting mounted to saddlebags and trunks you must provide a ground wire as well, and also on trailers and hitches. Size the wire appropriately for the load, small 18 ga. for most lamps, larger wire for headlamps, and large cable for starter and chassis grounds. Ground your motorcycle correctly and prevent most common charging and starting problems!