How to: Dial-in your Rebuildable Stock Motor

Trinity’s Jim Dieter shows you how

words: Kevin Hetmanski

Rebuildable stock motor Now that rebuildable stock motors have become the standard for stock-class racers, it’s more important than ever to know how to dial in your motor for maximum performance. Just about all of the tricks the mod-motor guys have used for years can now legally be used with stock motors, and none of them require any special skills—just a little time. For the skinny on stock-motor tuning, we turned to Trinity’s motor master, Jim Dieter. Here’s how Jim makes a stock motor scream!

Above right: To disassemble a motor, you’ll need only a Phillips- head screwdriver and the items shown here. It’s that easy to give your motor the Dieter treatment.

Dissasemble your motor Disassemble the motor Remember to remove the brushes before you pull the endbell off. Remove the phenolic washer and shims from the armature, and make certain there aren’t any shims left inside the can or endbell.

Right: To tune a motor, you first have to disassemble it. If you’ve never had the chance to see what’s inside an electric motor, here you go.

Jim Dieter “It may seem strange to tear down a new motor before you run it, but the idea is to make it even better than new. If you want to win races, it’s all part of the game.“

Seat the bushings Seat the bushings The bronze bushings in the armature and endbell must be completely seated so their bores are perfectly in line with each other. If they aren’t, the armature will bind in the bushings, and the motor will not achieve maximum rpm. Place the endbell and can on a sturdy, hard surface, and use a hammer and a section of dowel that has the same diameter as the bushing (or is just slightly smaller) to tap the bushing into place. A couple of relatively gentle taps will be enough to seat the bushing if it is not already fully seated.

You can press the bushings into the endbell or motor can with a wooden dowel. A light tap or two will seat the bushings in the motor and will ensure that they sit flat.

Jim Dieter “Generally, the bushings are correctly installed at the factory, but this step is worth doing ‘just in case.’ If the bushings are at all misaligned, it will affect a motor’s efficiency and power.”

Cut the comm Cut the comm You’ll need a motor lathe for this step, but don’t worry if you don’t have one; someone at the track who has one is bound to be willing to cut your comm for a soda or a slice of pizza, or your hobby shop may offer comm-cutting as a service. If you do have a lathe, just skim the comm to true it; try to reduce its diameter as little as possible so that you’ll get maximum wear from your motor. If you have a “Pro” motor, this step has already been done for you at the factory.

A quick turn on a lathe will ensure that your commutator is perfectly true. Trinity’s new Tru Lathe 3 is pictured; look for a review in a future issue.

Jim Dieter “When the comm segments are pressed onto the armature at the factory, they bow slightly. The skim cut ensures a perfectly true comm, and that reduces the likelihood of brush bounce and guarantees that the brushes will have the best possible contact. I usually coat the comm with black marker so I can see any ‘high spots’ still left after the first pass. I make the next pass only just deep enough to get rid of the high spots, so I keep as much of the comm material as I can.”

Center the armature in the can
Center the armature in the can It’s important for the armature to be centered in the magnets’ magnetic field; if it isn’t, your motor won’t be as powerful or as efficient as it could be. To align the armature, reassemble the motor without armature shims or spacers, and leave the brushes out. Spin the armature; it will center itself. Pull the armature shaft out of the can’s nose, and note how much play there is. Remove the armature, add shims to take up the play, and repeat the process until there is just the slightest free play when you pull on the armature shaft after centering it by spinning it in the can. Then add shims to the commutator side of the armature until there is just a tick of play when you push on the armature shaft; if there’s no play, the arm is pressing against the bushings, and that causes friction. Make sure you can feel just a tick of play! If you need extra shims, buy Trinity’s shim kit (part no. 4027)

The number of shims required on each end of the armature varies with the motor. Determining how many shims to install can be tedious, but it’s well worth the time.

Jim Dieter “If the armature has no shims on the nose bushing side but you need the armature to sit lower in the can, you can shave the nylon off the armature shaft with your motor lathe. Remove as little material as necessary. When you reassemble the motor, try using Trinity’s Teflon shims (part no. 4030) in place of steel shims where the shims contact the bushings. Friction is the enemy, and the Teflon shims are another way to fight friction.”

Align the brush hoods Align the brush hoods The “hood” is the part of the endbell that the brushes slide into. To align the hoods, you’ll need a special tool; Parma PSE* makes a nice one. First loosen the brush eyelet screws and the spring posts. You can avoid scarring the posts with pliers by using Trinity’s 4528 spring-post wrench. If you use the PSE tool, just fit it over the hoods and then retighten the endbell hardware. To use the Trinity tool, you’ll have to remove the endbell from the motor and slide the brushes out of the hoods. Loosen the endbell hardware, slide the tool through the hoods, then retighten everything. Properly aligned hoods offer two benefits: first, the brushes will make maximum contact with the commutator; second, the brushes will be correctly positioned relative to the magnets. Both are important for maximum power and efficiency.

Above left: Use a brush-hood alignment tool such as this one from Parma/PSE to align the hoods.

Jim Dieter “Like the bushings, the hoods are probably OK when they come from the factory, but it’s best to check them to be sure. The P2K’s brush hoods have small dimples that fit into the endbell to ensure correct alignment, but it never hurts to manually check the hoods. Stock-motor tuning is all about finding the numerous little tweaks that make a real difference when you add them together. Properly aligned brush hoods and bushings are a must.“

Replace the stock brushes Replace the stock brushes The full-face brushes included with most stock motors take time to break in, and they don’t allow the motor to develop as much power as it’s capable of. Jim recommends the Trinity E-brush (part no. 4499) and suggests that you solder the brush shunts directly to the endbell’s tabs. “Pro” motors have soldered E-brushes from the factory.

Above right: You can skip this step if you prefer the convenience of screw-on brushes with eyelets, but for the absolute lowest electrical resistance, soldering the brush shunts to the endbell tabs is best.

Jim Dieter “The E-brush has more silver in it, and that increases its conductivity for greater power; its serrated face also breaks in more quickly. If you solder the brush shunts, be careful not to overdo it with the solder; you don’t want to saturate the shunt and make it inflexible. Use a good iron that gets very hot, so you can attach the brush quickly with less chance of solder traveling up the shunt.“

Spring tuning Spring tuning By installing brush springs of different rates, you can tune the strength of the brushes’ pressure against the comm. There isn’t any trick to installing the brushes; it’s the spring rate you choose that matters.

Left: Here, Trinity’s “heavy” red/purple spring combination is installed for maximum torque—a good short-track setup.

Jim Dieter “When I’m setting up a motor for sedan or off-road racing, I replace the stock brush springs with Trinity 11-ounce red springs. The heavier tension gives more low-end power, which is important for the heavier vehicles in these classes. In 1/12 scale and other cases in which you need higher peak rpm, I use a lighter spring such as blue [7-ounce] or green [9-ounce]. The lighter springs press the brushes against the comm with less force, so friction is reduced and rpm is increased.”

Break-in Break-in Now that the motor is completely aligned and trued up, it’s time to break in the brushes and bushings. Place the motor shaft in a drill or rotary tool, apply a drop of Trinity Bushing Buster to each bushing, and spin the armature at low rpm for a few minutes; that’s all there is to it. After break-in, clean the motor with motor spray to remove the cutting oil and any bushing particles, then put a few drops of bearing oil on the bushings before you run the motor.

Above right: It isn’t difficult to break in your motor bushings: put the motor’s output shaft in a drill or Moto-Tool, and spin the armature for a few minutes. Trinity makes a special bushing-cutting fluid that will help you to break in the bushings faster.

Jim Dieter “The Bushing Buster compound contains abrasive particles that speed up break-in. It’s very effective at freeing up new, ‘tight’ bushings, but you shouldn’t use it after the bushings have been broken in. If you overdo it, the bushings will get sloppy, and in extreme cases, the armature may buzz in the bushings at high rpm. After break-in, just use light oil on the bushings; I like Trinity Zero G oil.”

NOW GO RACE! “Are you surprised it was that easy? It really doesn’t take much time or money to give your stock motor an edge, so there’s no excuse not to do it. If you don’t follow these tips, your competition will! Special thanks to Jim Dieter for the inside info.”

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