I’ve read a lot of articles on how to build a TRF gear diff, and some of the are pretty extreme. My way is a lot simpler and doesn’t require any parts to be modified.
This build guide has come about after I had a few difficulties with my diff over its first few races, I’ve got the balance between smoothness and leaks sorted now.
1. Choose your parts. Mine is a TRF418 diff to which I have added TRF silicone o-rings (#42259) and some 0.1mm thick 5x10mm shims (Core RC #CR458, also in Tamiya #51466). The 418 diff includes the metal cross pins (#54311) and lightweight outdrives (#51535) as standard.
2. Make sure the mould lines are well cleaned from the internal gears but DON’T file them down.
3. Assemble the diff according to the instructions but don’t fill with oil yet – take into account that the optional o-rings are a little fatter than the standard black o-rings and you will need to use the fine shims to get the right setting. I start off by shimming so that there is a tiny (<0.1mm) amount of float in the diff outdrive when the pin is inserted, if the outdrive is tight, remove a fine shim.
4. Lubricate the o-rings and gasket with silicone oil and assemble the diff dry. There is no need to over tighten the screws, they just need to be snug enough for the gasket to do its job. The action should not feel too “geary” nor should it have too much backlash. A simple test is to see if there is any float in the outdrives, if there is a little bit it means the gears are not being forced against each other. Adjust shimming to suit – for reference my diff runs with 0.2mm on the long side and 0.1mm on the short side.
5. Fill the diff with your preferred oil. I’m usually using #1000 or #2000 depending on conditions. There are plenty of nooks and crannies in the diff case for the oil to hide in so I always work the diff to help the oil settle. As usual, fill to just above the cross pins. Assemble and install.
This should give a reliable and consistent diff with no leaks from the outdrives. The Tamiya diff does appear to shed oil from the pulley gasket however I do not believe that is a leak, rather the remnants of the oil that seeped out as the diff was screwed together.