T18 Upgrade

For those that haven’t been following my other post in the Intermediate Tech section then this write up explains how I modified a 20mm bearing T18 gearbox to suit the newer and better 23mm bearing.

The gearbox came out of a 1979 I6 J20, being an I6 version it had a shorter input shaft than the ones normally seen in a V8 FSJ. The box was given to me for free so it was a bargain to begin with.

Unfortunately when we pulled it out of the J20 we found excessive wear on both the front and rear main bearings and some damage to the 1st/reverse gears due to the PO not knowing how to change gears without syncros.

We stripped the box completely and went about ordering a rebuild kit for it. It was at this point I found out that the only available 20mm bearing was a cheap and nasty Chinese version. Not happy about this I started asking questions to gearbox rebuilders and the likes of Novak and Advance Adapters to see what their kits/rebuilds used, the answer was simple the same crap Chinese one.

With the help of an experienced machinist we then looked at ways to fit a 23mm bearing in place of the 20mm. It then became obvious after looking at the ghost markings on the 1st motion shaft gear that it did not match the full width of the cluster gear (see pic).


If we shaved 3mm off this face the 23mm bearing would fit and not affect any other setup.
I ordered a 23mm Timken bearing, upon arrival we noted that the bearing inner race had a additional step in it to accept a dust/oil shield. There was approx 2mm of additional material here that wasn’t on the original. That gave us an idea on shaving this surface first and less on the gear face as we knew the gear metal would be extremely hard. We took 1.6mm off the bearing inner race (see pic)


and took a further 1.4mm off the gear face (see pic).


This gave us a total of 3mm.

The box went back together exactly as it came apart.

The only mistake we made was we never took the additional material off the front inner race surface (same as rear) as this meant the circlip that locates the bearing to the shaft did not fit. Luckily there was another narrower circlip in the kit and this was used in it’s place.

I have only just got it back in the Jeep but so far it’s perfect, there are no noises and it’s changing gear better than it did originally.