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Slot Car Gear Lapping Tutorial

You already know what 'Lapping In' a set of slot car gears means and can correctly pronounce 'Simichrome' along with the best of 'em...

But have you ever actually done it to one of your cars?

It's much easier than you'd think and not really that messy.

The first time we did it we didn't get so much as a drop on our track - probably because we used a power supply that was nowhere near it at the time!

The reality is that even a poorly done Lapping Job will still net good results. A properly done Lapping Job will net even better results! No two people do it the same way, so you really can't mess it up. Benefits includes Quieter Operation, reduction in Overall Friction, Better Oiling and Typically Higher Top Speeds!

There is one 'GOTCHA' you'll need to watch for. When you 'Lap-In' your gear-plate, you transform the gears into a 'set', or 'unit'. DO NOT change out the Idler Gear after completion unless you plan on Re-Lapping. Mark the Idler as to which side is up, as putting it in upside down will undo all your work! Same goes for changing out any of the gears in your new 'set'. It's also a good idea to mark line-up points on your new set by scratching in a few alignment marks where the gears meet. This way you can remove and reinstall the center idler gear to its proper position every time.

Believe it or not, this is a goal. Above is a Thunderjet 500 but lapping works on all metal and most plastic slot car gears.

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What You'll Need.

You'll need a couple of things to try this for the first time.

It only takes a few minutes to do it correctly. The way we do it on a car we're going to campaign for a while takes a little longer - but that's us - and we'll tell you how we do it, too.   First, you'll need an old chassis or a chassis you don't mind getting a little 'gooped-up.' Secondly, you'll need the complete gear-plate (with armature) that you want to lap on that chassis - we think you get the drift. Thirdly, you'll need some polish. Now really any kind of polish will do, however Simichrome is the polish of champions. It's not hard to find if you look in the right places. We sell it - a tube runs about $11 to your door. Or, you can buy it on eBay - just do a search for Simichrome. Why we bet if you type 'Simichrome' into your favorite Internet search engine, you'll find a dozen or so people who'd be glad to ship you a tube!

Now you don't NEED Simichrome, any paste (as opposed to liquid) brass or aluminum polish will work - it's just that Simichrome seems works a little better. Again, we think you've got the drift.

Remove the rear axle and crown from your Lapping chassis, it'll go smoother and rev higher this way. Next put a strong set of magnets in the chassis. The stronger the better - we want it to rev high & strong. Super II's work well for this purpose. Put your Lapping chassis on a piece of new power track, old power track or a power source and let it rev - remember that sound, because it's about to change forever...

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The Messy Part.

Okay, here we go - Before you touch the chassis to power, use a toothpick, your fingers or whatever you are comfortable with and apply polish to the gears. The goal is to get the polish between the teeth and under each gear if possible. Turn 'em as you apply it, you want to get it in the nooks and crannies...

Once you have the gears fully 'saturated', apply some power - slowly at first, adding more polish as the old 'spins off' the gears. As I've alluded to in other sections on this site, think of Lapping In the gears as sort of polishing the teeth. Rev slow at first to get the teeth meshing at a lower speed. What this does is actually wear the teeth down just enough so the the gear(s) next to it are now becoming custom fit to it's neighbor. After a minute or two of run time, rev it higher and keep adding. The polish will turn dark in color, that's your tip off that you need to add more. The grit in the polish is helping all the teeth to get smoother and fit together. Like premature aging of 'the fit'.

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Think of your own teeth...

As the years go by , they slowly wear into what becomes your 'bite'. When you look in the mirror and clench your teeth, that funny shaped tooth on the top just fits its neighbor below a little better each year. Think John Elway, or Mr. Ed - or if you're old enough, Joe E. Brown! These folks all had a helluva set of choppers on 'em - yet as adults, when they clenched their teeth - somehow they all fit together.

The chassis is going to make some noise as you keep applying polish - sort of like it's low on oil. It is. If you fear something is being damaged, slow the chassis down or let it cool. The bottom line is that you want to keep adding more and more polish while continually going at faster and faster speed. You want the polish to get between everything. If you need to add a little oil to it, use the light stuff and use it sparingly - we want the polish to do it's job. 3-in-1, WD-40, Parma, Singer Sewing Machine oil, whatever; then continue with the adding of the polish. Once you've reached a total of 4-5 minutes of run time, you're probably done. You can now begin cleaning - or if you're us, it's time for step two.   We like to do a two-step lapping using Simichrome polishing compound followed by Noxon. Here's why... It's just like polishing aluminum or steel on an old 1:1 car. Recently, we polished the grill on an old Ford, wanting to take all the old anodized finish off. So we began with a coarse sandpaper - 250 grit - to basically removed the finish. We followed it with finer sandpaper - 400 grit - to remove the visible marks the 250 left. Then we followed that with 600 grit to remove the marks the 400 left. Then 700 grit. We followed that with Noxon, which left us with a mirror-like shine.   We also prefer Noxon polish. It comes in a green bottle and it sold in hardware stores and liquor, err Drug stores. However you can also use Brasso or just about any 'fine' liquid (not paste) polish as your optional second step - very easy to find. Again, totally optional. we remove the gear plate from the chassis, clean it lightly with a toothbrush and literally douse the top with the liquid (not paste) polish. Try not to get any on the commutator plate - it won't hurt it per se', but it may interfere with getting good electrical signal from the brushes to the Arm. If you do get some on the commutator plate, just wipe it off. Put the gear plate back into the chassis and run it for a total of a minute or two more, adding more liquid polish if need be. Simple as pie! Probably not Baked Alaska, just a regular pie.

The Bottom Line? Slow speeds and paste equal 'fast wear'. High speeds and liquids equal 'high shine'. Use thicker polish for the 'metal-to-metal' gear teeth. If you think you may want to try and polish things like the idler boss or other gear plate 'plastic-to-metal' matings, use thinner polishes (like Noxon, Brasso, etc.). One of our FRHO racers actually uses DuPont automotive polishing compound to lap JL plastic gear sets - with GREAT success! The end result is a virtually silent (and smooth) JL racer.

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Cleaning Up.

Now here's the hard part. We need to clean our new gear-plate assembly. Long term, the miniscule grit left behind by the polish will do your gear plate in. We need to get it out of there. If you have access to an ultrasonic parts cleaner, use it. we bought one from eBay for under $30. Most 'paste' polishes will 'breakdown' with a mild detergent that contains a 'degreaser'. Zep Orange is my favorite flavor. Dish soap usually won't do it.   You can also use compressed air and 3-in-1 oil (or any 'light' oil. Lots of oil. First clean it the old fashioned way, with a toothbrush. Then run your lapping chassis, adding oil to it excessively - don't let it get too hot! The oil helps to flush the grit out. Run and rev until you are confident the grit is gone. Since all we are doing here is adding oil, you really can hurt anything, you just make a mess. Once you are confident the grit is out, remove the gear-plate and spray with something like brake cleaner or electrical cleaner, something 'thin' that will quickly evaporate. You may repeat the last 2 steps a few times if you feel it's necessary.   Sound like a lot of work? We guess it can be if you don't have an ultrasonic cleaner. However, the time you'll spend doing this translates into many hours of trouble free high performance for us - it will for you too! The first time you do it, you'll question every step. The second time you do it you'll feel like a pro and it'll take half the time.

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Game Over.

Lapping in the gears on a racer it something that almost every rule-set allows for - we'd bet 99.4% (like Ivory Soap) of all cars in the Fray or Shootout have had this done. We'd bet 100% or better of all competitive Drag Cars have had this done. Too bad we're not betting men... The benefit of doing this on gears which utilize a Nylon Idler gear is debatable, you'll have to draw your own conclusions on that one, but we'll mention that our results have been positive so far.

As if this article isn't long enough already, we'll leave you with 'Jerry's final thought'. You WILL run into gear plate that won't 'settle down'. Even after heavy lapping, they'll still make that 'rattle' as they go down the track. This is usually caused by the cluster assembly being too loose... If you are set on using that particular gear plate, try changing out the cluster shaft - it'll usually get rid of the noise. Remember, while we all go to great lengths to maximize performance, these things were toys at the time they were manufactured - tolerances will vary.