13spd to 18spd

Switching from a 13 speed to a 18 speed is not a good idea if the truck wasnt built for it. Its gonna cost you a lot more in the long run than outright buying the 18 speed straight. you would have to change the fitting on the slave valve from the top to the center air line which is the main supply and then dismantle the roadranger valve and grind the interlock to allow the splitter to be used on the low side but splitting the 13 on the low side is where the problem begins The 18 speed uses a bearing in the hub of the splitter gear and a thick round washer to retain the gear and bearing to the output shaft of the transmission, the 13 speed doesnt. the splitter on the 13 speed doesnt see use until the trans is on the high side. The splitter on an 18 speed will see use in both the high and low side of the trans which is what the bearing is for. This means the splitter must be able to handle the torque being amplified in the low side of the trans. the splitter gear is tied directly to the output shaft so there is no difference in speed between the gear and shaft on the 18 speed when in high range. The difference in speed between the splitter gear and the output shaft when split on the low side of a 13 will damage your transmission overtime And then you gonna need a new truck. Lol.

 

Easiest would be to swap the whole deal for an 18.

Eaton puts most of their parts books online. Download the one for yours and the one you want to build. If the cases have the same P/N's you've got a good start. The costly part will be you will probably have to change most, if not all the gears in the main box as the "A" and "B" models use different ratios IIRC.

The back box is the main issue. The 13 speed auxillary box isn't designed to be run in high split while in low range. It'll grenade pretty quick. You may be able to convert the guts over to an 18 if the cases are the same.


TL;DR
Just buy the 18 already built. Less headache and cheaper.

 

If I'm not mistaken, there is also the ratio differences. 1st direct is lower than 1st direct in a 13 speed.

 

I know a guy that cut the little tab off the red button many years ago still working today not saying its a great idea

 

Should be able to trade up to the 18 for a fair bit less than trying to rebuild the 13 into an 18 and do it right.

 

I just as soon buy a 18 outright. Particularly around a truck that comes with it.

I love my 13's but I don't dare try to add more to them. And I am not as smart as some of the others talking sense here.