3Racing Out Tuned Ball Differential For F104

As the title suggested this hop-up is designed for the Tamiya F104, but like so many options parts for the Tamiya F cars it can be fitted to the F109.

Almost since day 1 Tamiya cars have had the simple but annoying diff setup of having the thrust washer in the wheel. Changing tyres meant disassembling the diff and then have to reassemble it once the new wheels/tyres had been fitted. If you are like me and consider diff settings very important to your overall setup and car balance (unlike those that simply do them up) then this soon becomes majorly inconvenient.

So when the I discovered the F104 Diff housing set, commonly referred to in these parts as the "Pro" diff, I though my problems had been solved. Unfortunately not, as the pro diff, while making wheel/tyre changing more convenient, actually makes adjusting the diff even more difficult by putting the adjusting nut in behind a little cap which has the stud for the wheel nut on it. I've also found this cap and stud to be a weak area, having broken the stud off on more than one occasion, and Tamiya being Tamiya you have to buy a whole new diff set to get a spare.

3Racing Diff as it comes out of the packet

The 3Racing diff takes the pro diff idea one step further by engineering an adjustment ring and thread behind the spur gear between it and the motor mount. This allows adjustment of the diff without removing the wheel at all and thus far holds the setting very well during the course of a race day. Building it is not that different from the F104 diff with the main differences being the threaded diff joint and the adjustment ring and o-ring and the removal of thrust washers to be replaced with a aluminium spacer. At first this concerned me more than the rest but once built makes for a VERY smooth diff action and very precise adjustment.

Installed on car

Cap in place

Cap removed showing main diff nut - Initial adjustment is still done with this nut

 Simply adjust the diff as per normal before fitting a wheel. Adjustment is then made by inserting a 1.5mm hex driver into one of the holes in the adjustment ring and turning the opposite side wheel. Adjustment done. So far, on the track it has worked very well, holding the setting without changing and being a very consistent diff generally. In my opinion this is a very worthy upgrade and given its less than $15, it is bargain bling as well.

Adjustment ring

A few things to be aware of. Firstly it does move the spur gear closer to the wheel in order to make room for the adjustment ring, though it doesn't change the width of the car or move the wheel etc. It's only a couple of millimeters but I've read that it can be enough to make it hard to mount pinions on some silvercan motors. There has been no issues fitting pinions to the brushless motors I use. Secondly - the first one I bought the diff joint (the long black bit in the picture) wasn't machined straight. This made the diff wobble and wasn't usable. Finally you WILL need the shorter F104 axle or (as I did) cut down the F109 axle.