Tamiya M03 Mini Rebuild

January 31, 2009 · Posted in How To, Maintenance · by simon

Over the past few weeks, the performance of the mini has been somewhat poor. I felt a rebuild was in order…

I thought about what had changed: lipos, motor, UJs, servo, bits and bobs.

The motor was good, great even last time I used it, so I wasn’t convinced it was that (despite the smoke last night – just comm-drop overexuberance…), the UJs concerned me a little – they were different to my previous ones.

I took the front end suspension off, and there was some binding on the front shafts, the bearings seemed fine, but I think they were a little over-shimmed, a little too tight, so I took all the shims out, now nice and loose.

The gearbox came apart, nothing too untoward. The bearings got cleaned, the gears were in good condition, so they got cleaned too.

The ball diff was a bit notchy, so that was stripped and cleaned. I love the thrust race in the optional ball diff, it’s huge, way bigger than any I’ve seen in a touring car, or even in a 1/10th off roader…

I rebuilt it, and it was nice and smooth once more.

The critical point came when it came to putting it back together, I put the two gearbox halves together and spun the box, and it span really nice. I put some screws in, span it again, and oh, no, it wasn’t as free… Clearly the clamping forces of the screws was causing some binding.

I backed off some screws, and it helped a little but not a lot. I took out everything again, and cleaned out all the bearing seats to make sure everything had maximum ‘room’, and, I cleaned out the hex on the ball diff cover, to make sure the diff was as ‘narrow’ as possible. I also tried the diff in the opposite way around.

Put the gearbox halves back together, and even without screws it felt better, screws went in, and I only had to back them off maybe half a turn or so to get the gearbox running as free as without any screws in it.

What you’ve got to remember here is that a second per lap on the track is gained in tenths of a second, that gummy gearbox might have been worth 2 or 3 tenths…

I put the front suspension back on, and found a couple of binding arms, so freed them off with a drill bit here and there. Nice free suspension arms again.

I put the driveshafts back into the diff output cups and secured everything, and it span okay, but not brilliant, then I tried spinning it at full suspension compression, and bingo, the problem became clear. It was very tight indeed, hardly spinning at all.

The driveshafts were pressing on the rubber o-rings inside the diff output cups, causing binding as the suspension was compressed. I took them out, and the difference was HUGE. The gearbox now spins and spins at any point on the suspension travel.

My suspicions about the UJs possibly causing issue were confirmed. They are obvisouly slightly different in size or tolerance than the non-rebuildable ones I was using previously. O-rings in the drive cups (as per the instruction manual build) were too much, not giving them enough room.

I’m now very pleased, I feel the transmission is the best I can get it, it’s certainly never been as free running before.

It’s important to note here, that none of that tuning was done by spending money, it was just by careful attention to detail, making sure everything was clean, and checking for good operation every step of the way.

Next up, I’ve coca-cola dunked my motors for 2 minutes at 3v. I’m going to put the same motor back in, just to confirm that it was the gearbox build, rather than the motor.

I look forward to the next opportunity to compare the mini against some others…

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