22 to 23 spline crawlbox adapter/coupling for Manual trans to 4.0L Marlin Crawler Tacobox

UPDATED 10-13-22

If that ain't a beautiful damn piece of machined steel, I don't know what is.





If you want to put a 23 spline marlin or trail gear crawl box behind a 4.0L V6 2nd or 3rd gen tacoma with a 6 speed RA60F or RC62F manual transmission, then you will need two parts that aren't so easy to come by. A 22 to 23 spline adapter and an oil seal extension housing. I have done the leg work and worked with a a local machine shop to bring you both of these parts. Edit: this was designed for the RA60F, but SHOULD also fit the RC62F. But since I did not test fit the 3rd gens, I cannot guarantee it won't require modifications.

Edit: With permission from G&K Machine, I have uploaded the drawings of both parts.

22 to 23 spline adapter:



The Marlin Taco Box has a male 23 spline input whereas the the RA60F/RC62F
trans has a 22 spline female output. Don't believe anything Marlin says or you read on their website about the spline count. They are wrong and they refuse to admit it. There are 22 splines folks! To deal with this, you need 22 spline male to 23 spline female adapter/coupling that no one makes. And I mean no one. The only company I am aware of that ever made the spline adapter is Inchworm who is no longer in business. There are a handful of Marlin tacoboxes behind manuals, but they were done when Inchworm was around, and even Marlin used to source their adapters from Inchworm when they shipped crawlboxes to manual customers. Last I heard, Marlin isn't even taking orders for 6 speed manuals since they don't have this adapter that you need to install it.

Marlin is supposedly working on designing an adapter, but has made no commitments as to when they will be ready. The last I heard was October 2019 but that has come and gone. I called to speak with them back in July 19 about their plans and asked them if they wanted to work with me on designing an adapter with my local machine shop. They respectfully told me to pound sand. They said they are going to do it their way, on their time, with their machine shop. Good luck with that 19 spline adapter, Marlin.

Well boys and girls, I have some good news! I bit the bullet and did all the hard work to make fantasy a reality. I officially have two prototypes of version 1.0 of the spline adapter, made locally here in Denver by G&K Machine. One of them is currently in my truck, with a Marlin crawler Tacobox behind it and over 1,000 miles on the gears. It's even Moab tested! These things are beefy as hell and I am really pleased with the result. Made from 4140 heat treated steel.

Figuring this out really wasn't that hard. Just expensive. The process was that I dropped my tcase out of my truck and pulled the 22 spline input shaft as well as a 23 spline input from an automatice trans FJ case. This is an identical spline pattern to the crawlbox input. I found a local machine shop that does hobbing and gearing, I explained to them what I needed and they came up with some drawings. I took some measurements with digital caliper, did some maths and made some adjustments to the drawing. While my original design does need some minor modifications as outlined below, I freaking nailed it. Not bad for an accountant.

Version 2.0 will be slightly shorter on the 23 spline side and will have a bevel instead of a hard 90 degree shoulder where it transitions from 22 to 23 splines. See below for details.

The modifications:
In all my infinite wisdom, I failed to take into account the thickness of the adapter where it fits into the trans and the bevel of the trans output. The adapter is so thick that the shoulder prevents it from seating all the way in the trans like I thought it would.



As a result, the adapter was about 1/8th of an inch past the trans housing, wheres as it needs to sit about 1/2 inside the trans housing due to the depth of and design of the crawlbox input. So close!



Fortunately, the transmission output has a slight flair/bevel to it, and the fix was simply to grind down the 90 degree shoulder of the adapter to match.




For good measure, I also took a dremel to the inside of the 23 spline side to allow it to accept the crawl box box input just a little deeper. I thought this was easier and more preferential than attempting to shorten it.

Before:

After:


Installed with oil seal extension housing prior to the oil seal:


These two changes were all I needed and everything fit perfectly.

Want one? Well having custom parts machined in small batches ain't cheap. Pricing provided by the shop:

1 adapter = $1,500 each
2 adapters = $1,050 each
5 adapters = $600 each
10 adapters = $475 each

I am not making money on this guys, but I reserve the right to change my mind and recoup some of my investment later.

Oil seal extension housing:

The other thing you need is an oil seal extension housing. I think Marlin is currently the only one who makes one since they are currently the only ones making crawl boxes for our trucks, but honestly I don't even know if they do at this point.

I was lucky enough that a good friend had an original Inchworm oil seal extension housing for me due to a fiasco during his install that ultimately resulted in him having an extra. Before installing in my truck, I took some measurements and had the machine shop provide me with a quote, but didn't have any made.

I've also included a drawing I made with the measurements if someone wants to work with their own shop. It's a much cheaper part since it's just machined aluminum and there are more shops out there that can make something like this.

1 housing = $298 each
2 housings = $184 each
5 housings = $120 each
10 housings = $82 each

Photo of the extensions housing for manual (left) and auto (right).


This gets installed to the transmission after the spline coupling (not shown) and then receives the oil seal:

 

Reserved

 

Reserved...

 

@Trident904

 

Why you trying to spend my money? Lmao

 

Day late and dollar short it seems

 

Story of my life

 

Put me in as tentative. I can't do $1200. $600 is still wicked 'spensive for that little adapter. $200 is tolerable. I want me a damn crawl box for some low-lows action.

 

I'd be a tentative if it would help knock the prices down. Granted I have one already installed but would take a spare if the price was reasonable

 

Ok so I got a bug up my ass and decided not to wait till winter and pulled the tcase. Going to the machine shop this week to get some quotes. Wish me luck!

 

Tentatively in.

 

Added to the list

 

Dropped everything off at the machine shop during lunch today. Should have pricing by mid-week.

 

so you have to have this adapter piece for a taco box to work in a 6spd manual trans? I thought it was a bolt in deal and had to work on driveshafts...what part am i missing?

https://www.marlincrawler.com/trans...oma-2nd-gen/dual-case-tacobox-40-liter-1gr-fe

 

Correct. You need a spline coupling whether it's a manual or an auto. On the auto, the coupling is 23 spline female to 23 spline female which marlin makes or at least has a source for. Below is a picture of the AT coupling.


Marlin has never made a 22 to 23 spline coupling. The only tacoboxes I am aware of that are sitting behind a manual are using an inchworm coupler which you can no longer get.

You also need an oil seal housing extension that is specific to the manual trans. This goes over said adapter. Once again, here's the one for the auto:


I am not sure whether or not Marlin makes this for the manual currently, but I have a friend who has one for me and it's not as difficult a part to have made as the spline coupling.

Hope this helps. I am by no means an expert...this is all stuff I've learned in recent weeks since getting my hands on a tacobox and trying to make it fit.

 

Hey Brian, Find out how many they need to make to get it under $200 and post up a GB.

 

Seriously steve...what exactly do you think the purpose of this thread is?

 

well put me as tentative depending on price. This has been a loong anticipated project one day and I guess i need this piece if it were to ever happen. What about the oil seal extention? might as well start collecting pieces now

 

Added to the list! You may be able to get the extension housing directly from Marlin or have one made. It's just a piece of aluminum. I do not have it in my hands yet but could probably get you dimensions on it if Marlin does not have it. I honestly think they do.