The Magazine For Slot Car Enthusiasts

Thunderjet Chassis Mod - Cook Up a Low-Profile Pancake
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By Mark Owyang
This article was originally published in the Fall '99 (v.13, no.2) issue of HO-USA.

When I returned to slot racing a few years ago, I didn't expect to be working much on t-jets. I had fond memories of running and modifying t-jets as a kid, but remembered giving them up when Aurora introduced the AFX. The AFX was the latest, so I figured it had to be the greatest, right? Like the t-jet, the AFX was an ideal platform for modification and improvement, so I was soon drilling holes, cutting brass, and trying out ideas to make it run faster and handle better. Not that I ever had much success! My parts box was always littered with the hacked up carcasses of pancake chassis experiments gone awry. But for me, this process of experimentation was perhaps the most enjoyable part of the hobby.

I missed out on the Magna-Traction chassis somehow. With high school in full swing, I was devoting less time to the hobby. But I sure do remember seeing the first G-Plus cars. Though slot cars had slipped down on my priority list, that beautiful Aurora Ferrari 312 body drew me back in. And man, were they fast. It was clear to me then that the pancake motor era was over, once and for all. Or so I thought.

Fast forward through college, marriage, and the birth of two kids. Slot cars were just a fond memory. Then in 1995, I discovered through the Internet that slot cars were still very much alive. I was drawn back in again. My first new cars were the Super G+'s included in the race set I bought in 1996. Snapping off the body and studying the chassis for the first time, it was obvious the design had evolved from the original Aurora G+. While learning to tune the cars, I found that the hobby was still fun.

Yet the cars were so fast, any major modifications seemed unnecessary. I found myself missing one aspect of the hobby I'd really enjoyed from my youth: innovation and experimentation in chassis development.

The Fray in Ferndale got me back to running and enjoying t-jets. It was just a matter of time before I wanted to build a modified car again.Remembering the rewound arm, drilled gears and wide brass pan on racing pioneer Carl Dreher's creation from Auto World's 1970 HO Racing Speed Secrets, I thought I'd try to recreate a vintage "pro" car. However, as is often the case, I got sidetracked with another project, and my first modified t-jet ended up being a quite different car altogether.

One of the most common goals when modifying a t-jet is to lower its center of gravity. Brass pans, drilled gears, and vacuformed bodies all contribute towards this goal. On a hard bodied car, trimming the body posts and thinning the plastic allow the body to be mounted as low as possible. But the stock t-jet chassis puts strict limits on how low you can go. Staring at the venerable t-jet chassis one night I had a brainstorm: what about lowering the gear plate into the chassis to create more clearance for the body? A low-profile pancake. A flatter flapjack!

I made the low profile t-jet by sanding the gear plate and the inside floor of the chassis to about half their stock thickness. The magnets sit lower in the chassis, allowing the gearplate to drop down as well. Though the chassis is lighter than stock, the center of gravity is now lower. The chassis is nearly 1/16" lower than an unmodified chassis. The body, of course, can now be lowered that much more. The car handles better, and it looks a lot better too. Check out the effect on Aurora's Porsche 904 (bottom). Now that's closer to the stance that a Porsche should have! Bodies with a flat rear deck or trunk lid benefit the most from this modification.

It's a lot of work, but I think these changes lead to a mighty cool t-jet. The car is great fun to drive, retaining but improving upon the characteristics of a stock t-jet. Of course, now I want to build one with a rewound arm, stronger magnets, and a brass pan!

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Follow along to build your own flatter pancake chassis!

1. Start by removing the armature pinion from the armature and the driven gear from the drive gear shaft so you can work with the bare gear plate.

2. Sand the bottom of the gear plate is down to its original thickness, removing about 1/32" of material. I used a Dremel grinding bit and an emery board/ sanding stick (from the drugstore's beauty supply fingernail section) to do the work. The collar for the cluster gear shaft is reduced in height by 1/32" as well. At this point, do not modify the gear plate locator tabs at the front corners and towards the back of the gear plate.

3. After the sanding is done, be sure to clean out the armature and cluster gear shaft holes in the gear plate. It's easy to accidentally leave a bit of plastic in there, which will hurt the car's performance significantly.I found it helpful to twirl a #52drill in the hole to help clear out the debris.

4. The gear plate rail must be removed around the driven gear to allow the plate to sit lower in the chassis. Note that the entire gear plate is narrowed at that point, with the exception of the rear gear plate locator tabs. I left the top of the rail above the locator tabs, but that is not really necessary.

5. Replace the armature pinion and the driven gear. It's a good idea to use new gears to ensure there is no slippage. Note that the armature and gear shafts will protrude through the arm pinion and driven gears by about 1/32". Before installing the driven gear, trim that much from the top of the cluster gear shaft so it doesn't interfere with the car's body.

6. Moving on to the chassis, start by removing the wheels, axles, pickups and springs so you can work with a bare chassis. The inside, bottom portion of the chassis must be thinned to allow the magnets and the armature to sit lower. This was done with the trusty Dremel. Thin the floor of the chassis up to the point where the rivets come through the chassis. To prevent the grinding bit from damaging the commutator brush springs, I temporarily inserted a small sliver of card stock in the indentation between the bottom of the chassis and the brush springs to gently bend them out of harm's way.

7. Test fit the magnets in the chassis. The top of the magnets should be 1/32" below the top of the chassis. The outside edges of where the magnets sit in the chassis were more difficult for me to thin down than the rest of the chassis floor. I ended up sanding down the bottom edges of the magnets themselves to allow them to sit low.

9. As with the gear plate, the process of grinding the chassis can leave bits of plastic in the armature shaft hole and in the brush holes. Check to make sure the holes are clean.

10. Reinstall the magnets and test fit the gear plate on the chassis. Trim the bottom of the gear plate locator posts as needed to get the gear plate down nice and low. The top of the gear plate should sit nearly flush with the top of the chassis. Make adjustments as necessary to get the gear train turning smoothly. I ended up grinding down a bit of the chassis in the area where the the cluster gear shaft rides to make sure there was no interference with the drive pinion gear at that point.

11. Cut and re-bend the gear plate clamp to fit the new lower fitting gear plate.

12. Check the brush spring tension, and adjust if necessary. With the lower arm, you may need to decrease the tension slightly.

13. Trial mount the body, and trim the body posts. The chassis modifications should allow you to trim nearly 1/16" more away from the posts. Grind out the inside of the body as necessary to ensure clearance for the gear plate and to maximize the amount the body is lowered. Trim the wheel wells as necessary.

14. Go racing!