EditSanding and Shimming Slot Car Motor Magnets
EditSanding & Shimming
Drop a pair of Magnets into your chassis - how do they fit? Is one looser than the other (one is usually a little tighter than the other). Do they sit flat? Do they sit at the same height? You'll want your Magnets to be relatively tight, close to the armature to maximize the use of available field strength and minimize the air-gap turbulence between the magnets and armature.
EditShimming
If you are going to shim, consider cleaning the Magnets with grease cutting soap (like dish soap) and using adhesive backed labels, masking tape or similar to use as shims. Shimming is something that is done to the backside and/or underside of the Magnet. Shimming the underside of one or both Magnets may possibly help align the Gear Plate, while shimming the backside is typically for getting the Magnet faces closer to the Armature's lams... Think about both aspects while you're trimming & shimming!
EditEpoxy
Another way to accomplish proper placement is by filling the rivet hole directly below the Magnet with a drop of epoxy (or similar), align & level the Magnets and let 'em dry. This method will ensure your Magnetic field won't 'shift' once you've decided on a pair of suitable Magnets. This is usually only needed on very high performance applications like drag racing.
If we're going to epoxy Johnny Lightning (JL) Magnets (which are a hair taller & wider than Aurora Magnets) we'll will put a little dab of epoxy in the rivet hole, then install the JL's so the tops are above the chassis line - then flip it over on a truly flat surface (glass, tile) and gently press the chassis down flat to dry. The T-jet JLs are taller, so having them "flush" with the top of the chassis line is about right anyway depending on your Gear Plate.
A consideration with Epoxy is whether or not you'll need to retrieve the Magnets later for another project. We're generally discussing Road Racing, but retrieving the Magnets becomes a bigger issue when Drag Racing & using Polymer or NEO Magnets. At $20-$50 per pair (and a higher incidence of chassis burn-ups) you may find yourself moving the Magnets from chassis to chassis more often. Paper shimmed Magnets are obviously a lot easier to remove. Paper shimming is also good for tuning, as you'll be able to change out Magnet pairs until you are satisfied with a set, then making your Epoxy decision.
Above are 2 pair of Johnny Lightning HO Magnets. The top pair is untouched, however the bottom pair have had their 'faces' sanded. Place a pair on on a flat, solid 400-600 grit sandpaper surface and rub till they fit an Aurora chassis. We recently began using white 'aluminized' paper (available from home depot) and find that the paper lasts much longer for this task. 5 min's to better a fit - the real '5 minute abs!'
EditSanding
The last thing worth mentioning is sanding of the Magnets. I have heard about sanding the Magnets when I was a kid, but I never knew where 'they' were sanding or why. A decade or two later, we now know that sanding is done to the 2 small vertical faces on each Magnet in an attempt to bring the Magnets closer to the armatures spinning outer edge... Taller X-Traction Magnets can also be sanded to reduce their height, allowing for fit into T-jet Chassis's . Remember to Match or Re-Match your Magnets after any alterations.
More and more Organizations are allowing sanding of JL T-jet 500's in this area - since JL's are thicker, sanding helps them fit into an Aurora chassis without incurring chassis 'twist' or other distortion.