Lapping TJet Gears [Archive] - HobbyTalk

: Lapping TJet Gears

09-06-2013, 06:06 PM
What does everone use to lapp the gear train on a Tjet? I use permatex lapping compound. I have seen some people use tooth paste etc.

09-06-2013, 06:19 PM
I have tried Novus #2 and a 50/50 mix of Brasso and Break Free gun oil. Both work pretty well.

09-06-2013, 06:32 PM
someone gave me valve lapping abrasive once (I'm not gonna mention Sgrig's name) without any advice.
I smoothed out a complete set of gears quickly. yes, no teeth left on any of them. so, you want something a tad LESS abrasive than THAT

09-06-2013, 06:39 PM

09-06-2013, 07:51 PM
SemiChrome & Transmission fluid, less of the 1st, more of the 2nd


Bill Hall
09-06-2013, 08:34 PM
Something around 400 and 800 grit and a lubricant that trips your trigger...give er take.

Everyones got their own voodoo formula. I prefer the the squeezens from a ripe banana and some comet; but the banana must be picked in Havana during the full moon or your results may vary.

400 to 800 is roughly around the size of all the crud and corruption the gear train normally eats over time. The idea is to simulate in short fashion, a gear set that has been well broken in over time. Under magnification, my observation has always been that T-jets are dirty nasting tar burning beasties. You can polish it to perfection but everything returns to it's naturally scuffed state after some use. I'm not trying to make light of a top five in the fundamental build process, but the idea is to hone in the optimum running clearance with a minimum amount of fan fare and call it good. Most especially after you've scrambled up gear sets. Extra drama time is better spent on selecting good gears in the first place or better yet, pick up shoe voodoo. Lapping should be a one and done process.

LOL @ Al...I'm sure Jim is just expiditing the process in his own way by using ashtray sand, obviously some stuff cuts quicker than others and for the inexperienced is best to sneak up on the process and learn the intricasys . It's been proven time and time again that there is more than one path. I just snip up some small squares of 600 wet and dry carefully polish the top and bottom gear sides on a running gear set with synthetic motor oil. The freed abrasive mixes with the oil and creates the exact slurry we require. I just use an old Aurora tranny and press the contacts against the screws with the chassis front tilted downwards so the spent slurry drools out neatly to the front of the gear plate. Once I hear that the "no go" point is achieved, I spritz the chassis with electrical parts cleaner, then blow it off, and re-lube.

In most cases there's no need for the insanity lappings. No sloppy prolonged immersions in tubs of secret potions, or slathering the contents tubes of high dollar goo and flinging it on yer shirt. Papering the gear set properly cures a multitude of ills. For the special builds, your chosen rouge goes any where 600 paper cant by applying it with the model murdering specialty application tool part # 0001 (aka der toothpick).

Enough lapping is just right, and too much is called excessive wear. Simply put it's and ear thing and the process itself must be developed to suit yourself and your own bench/needs.

09-06-2013, 10:17 PM
While we're on the subject of reducing friction, has anybody tried filing down the gears? See idler gear in photo below. I saw this idea in an old issue of Model Car Science, the theory being the less gear there is to mesh, the less the friction produced, so I tried it. All I did was lay a file on the workbench, and slide the gear back and forth until it was about half the thickness. I wish I could say it allowed me to run amazingly faster, or the motor ran way cooler, but I don't have any way to objectively test, and didn't see any obvious differences. Has anyone else tried this? (

09-06-2013, 10:45 PM
From the standpoint of less tooth contact friction, your idea is sound. It will, however, make the idler gear less stable due to reducing the contact area at the post. The net effect will at best cancel out. A wobbling or loose idler is a huge power drain. Most rules (if you race under them) do not allow that kind of treatment anyway.

+1 on Simichrome

old blue
09-06-2013, 10:49 PM
I have not tried that, but I did try beveling the teeth of the gear to reduce the friction against the other two gears. I would love to say that it worked great, but in the process I opened up the center hole so badly that now all I have is sloppy beveled gears! Live and learn.

Old Blue

09-06-2013, 11:02 PM
I do it the easy way RT-HO gears, just put them in. No lapping needed.

Slow Ed

09-06-2013, 11:16 PM
I started off using crest tarter control toothpaste
hook a dremel to the axle and spin backwards.

I then decided to make my own cutting oil.
I ground polishing compound (polishing sticks from lowes) into marvel mystery oil. I have a chassis that i put the gear plate on flip it upside down into the mix and lap the gears in.

works very well

what surprise me was the 1st time i did it, the mix was to high and it came shooting up though the brush holes and did not miss a beat

09-06-2013, 11:23 PM
I use an ameter in series and measure the current, that way you can see when the load meaning the amps decrease you are reducing the friction.

09-07-2013, 03:04 PM
I have to agree with Ed,rtho and go!

09-07-2013, 03:28 PM
I like those too, but for those on a budget lapping may be worth a shot.

09-07-2013, 11:00 PM
the rtho gears are great.

but knowing how to lap gears can not hurt

09-08-2013, 05:47 PM
The thickness of the gear has nothing to do with friction. You gain nothing with a thinner gear. Do the math...

The reason for the thinner gears is to reduce the rotating mass. That will give you a performance boost. You get the same effect with drilling holes in the gears.

09-08-2013, 07:11 PM
Snappier acceleration?? Maybe a slight increase in braking since there's less rotating mass?

09-08-2013, 11:46 PM
I only use my dremel and a stainless wire wheel. Small one. Just spin em up and move the brush around to where it needs help. No stuff needed like toothpaste or anything. Works for me.

09-09-2013, 12:44 AM
The thickness of the gear has nothing to do with friction. You gain nothing with a thinner gear. Do the math...

:confused: What math, Mr. Thomas?


On the larger topic, here's a lapping method advocated by Tom Bowman in his article on How to Build a Fray Car ( Here are his photos and text. I'm not saying it's better than the ones listed here, just mentioning it because nobody else has.
Next we are sanding down the idler gear on the foam board. Be careful in this process, since chamfering the gears are not allowed and too much time sanding on a foam board will chamfer the gears. You just want to polish the gears up, again to remove friction.
I'm using a wire brush to remove the roughness on the gear teeth. This does the same thing as using a polish like Simichrome and Brasso on the gear train. It's much faster and less messy too!

His comment on chamfering (beveling the outside edges of the gears) would seem to indicate that some Fray racers believe it helps. But that proves nothing about whether it actually does, of course.

-- D

09-09-2013, 10:52 AM
:confused: What math, Mr. Thomas?D

Friction is a function of the material in question (brass gears) and the pressure between the two contact faces. When you reduce the face of the gear by half, you have increased the pressure between the faces by a factor of 2 because you still have the same horsepower spinning the gears. Half the surface area times twice the resultant pressure equals the same friction you started with.

Any further questions? :wave:

09-09-2013, 05:57 PM
This does the same thing as using a polish like Simichrome and Brasso on the gear train. It's much faster and less messy too!

I used to do this as well.
But my dyno readings give me better numbers when the gears are lapped.

the wire brush will remove spurs, but not shape the gear faces to each other as lapping does, in my experience

09-10-2013, 02:22 PM
Hey guys:
Also for the budget minded slot car racer:

I sell CNC top plate gears for $ 17.00 ea or 3 sets for $ 45.00; when I'm at the Midwest Slot Car Show in Hammond. They're not quite as good as the RTHO offerings, but They're not as expensive either.

Several guys in the Great Lakes series have them, I'll let them comment on the gears worth.

I'll sell them to Hobby Talkers at the same price, PM me if you wish.