Hello tx_shooter,

And to land!!

That's awesome that you got one of our t/cases, congratulations!! You are gonna love it! It is possible to swap out the input gear however the input is part of the low range reduction system and as a result you can only swap out inputs from the same ratios. In other words, your current 21-spline input is from the stock 2.28:1 reduction gear set while the 23-spline input you just picked up is from our 4.70:1 reduction gear set. They cannot be interchanged.

To make matters a little bit worse, we make two different 4.70:1 gear kits: One from 8600-series material (our XD gears) and another from 4300-series material (our Competition gears). Whenever you have two gears meshing with each other, it is absolutely critical that they both have the same material properties as well as annealing process so that they have highly similar hardness, durability, and tensile and yield strengths. Otherwise the gear that is harder and more durable will rapidly wear into and destroy the gear its meshing with.

So to answer your question: Yes, absolutely this is possible. But we need to find out what gear kit you have ... if it is our Competition gear kit or our XtremeDuty gear kit. Do you know anything about the history of the t/case? Do you know how old it is? Does it have our name with a serial number stamped on the front housing that says "Marlin Crawler 4.70:1 123" where 123 would be the serial number?

At any rate you are going to have to open it up to swap out the input. Take a look at our 4.70:1 Installer (link) to get an idea of how it all comes apart (if you haven't already done this before). Once you've got the gears exposed, replying to this thread with a couple pictures wouldn't be a bad idea either

Regards,
BigMike