Chassis Fitting: Another notch on the screw post

I have mentioned elsewhere that the Auto World Ultra-G Thunderjet chassis is infinitesimally larger than the original Aurora chassis. I tend to use the word “infinitesimally” a lot when talking about slot cars, because when it comes to issues of clearances when assembling, modifying or customizing a T-jet, you are working with fractions of millimeters. Here, then, are some of many ways I have found to get around some of the frustrations that can come of working in this scale.

When looking at an original chassis in comparison to a newer one, you can see that the notch in the back of the baseplate for the rear screw is deeper on the original. This allows more flexibility in moving the chassis fore and aft when mounting in order to center the wheels in the wheel wells. I believe that the reason the AW chassis has a smaller notch is that the rear motor gear is just a hair larger than the stock original as well. This is probably influenced by the new gears being plastic, while the originals are much more durable metal, but it may also have been intentional to generate a tiny extra amount of torque. Either way, this can cause problems when mounting if you find your front wheels are too far forward and are either rubbing the front edges of the front wheel well, or they just look wrong.

So once again we turn to our trusty Dremel tool.

One easy solution to the notch problem is to simply deepen the notch. I find I have to do this on about 30% of my builds. I use a small ball-ended grinding attachment on the Dremel to grind some extra plastic out of the notch, almost always going back about as far as the originals.

However, this modification does not come without cost.

If you do this you may find that your fore/aft wheel positioning is now fine; you may split the difference between front and rear if things don’t like up exactly right, and it may never be perfect (these are plastic toys, not “scale models”) but you can usually get it to look pretty good. Before you turn those screws down too tightly, though, turn the back wheels by hand and make sure they turn freely. If they do, put it on the track and take for a spin and make sure that you don’t hear a loud “ZZZZZZZZZZZZZZZZZZZZZZZZZ” that sounds like something is rubbing somewhere. If you hear that, something is rubbing: the teeth of that slightly larger rear motor gear are now rubbing against either the side of the rear screw post, or the underside of the body…and sometimes its both.

Which leads to the next solution…

Using a small fine stone attachment on the Dremel-which is marketed as a knife sharpener-give the front side of the rear screw post a shave. You can usually tell where its rubbing after just once or twice around the track, as the gear teeth will make a mark on the screw post or body where it has contacted that you can see under a magnifying glass and strong light.

If your rear gear is contacting the body itself this can be even more delicate to fix; I have had several bodies where the lower edge of the window insert protrudes down a little farther than the body where it tucks under the window frame; in these cases it is necessary to shave the lower edge of the window insert very carefully in order not to marr the “glass” badly enough that it looks bad when the car is reassembled.

The last trick I’ll mention requires the most care of all; using the same fine grinding stone to slightly enlarge the wheel wells. Most bodies have a bead around the wheel openings just the real cars they are modeled on, so only a fractional amount of material can be removed from them before its obvious they’ve been enlarged and the look of the car suffers as a result, but with enough care you can take a micrometer off that front edge and its usually enough to get the wheels to stop rubbing against the body in the turns.

Care and patience are required here! Never take too much off; the bane of the Thunderjet design is cracked screw posts, and you can easily see how removing too much material, or in the wrong places, can weaken the post. When shaving the body itself, be cognizant of how thick the plastic appears to be and how much you can safely take out; you don’t want to grind a hole through your beautiful new or vintage T-jet body! Done correctly, however, these changes will not affect appearance or structural integrity.

In addition, bear in mind that there are some clearance issues that no amount of grinding can compensate for while still producing an acceptable result; in these cases, use shims to provide extra clearance; modify the body only when you absolutely have too; remember, its the body that’s valuable!