Dana 300 Shift Rail and Detent Mod Twin Stick


Registered User
After searching the internet for hours I was able to find scattered information about twin sticking a Dana 300. I wasn't able to find much technical specs for how to mod the shift rails with dimensions and what to do about the detent springs. Also, I couldn't find much about preventing the t/c from popping out of gear other then "shim the springs", "increase the detent groove depth", buy "JB Conversions stiffer detent springs", and stuff like that.

I now believe the JB Conversion springs are just the stock weaker Dana 300 spring (110 lb/in) and the rear shift rail is shimmed with a 5/16" ball. I was able to find one obscure thread about the springs being different. I really wanted data.

I could tell visually that the springs were different. The spring for the rear shift rail (inner rail, left rail when viewing t/c from the rear) is a tad longer, a thicker coil wire diameter, and stiffer. The front shift rail (outer rail, right rail when viewing t/c from the rear) is a shorter spring, smaller coil diameter, and not as stiff. I ran some spring tests at work and discovered that the rear shift rail spring rate is 265 lb/in and binds at .695". The free length is 1.010" and coil wire diameter is .075". The front shift rail spring rate is 110 lb/in and binds at .603". The free length is .971" and coil diameter is .061" (see attached Detent Parts pic). The depths of the rear and front shift rail detent spring bores are different. The rear shaft detent spring has a deeper bore and in stock form has the stiffer longer spring (see Front Bearing Retainer pic). However, the deeper bore with the stiffer spring actually deflects the higher rated spring less and the rear rail moves easier than the front. After some measuring and math I was able to determine what to do to equalize the spring force between the two shift rails when doing a twin stick thus preventing the t/c from popping out of gear.

I used 1 #6 machine washer that measured .050-.055" thick under each spring in both shift shaft detent spring bores (see Detent Parts pic). The detent ball only deflects .105" when the rear shift rail moves. The shim causes the spring to sit higher in the bore and the detent ball higher in the shift rail detent groove. This equals app 27.5 lbf down on the spring to move the rail. The front shift rail deflects .250". The front shift rail spring has a resting deflection of .125" due to the shim and then deflects another .125" when the front shift rail is shifted. This equals app 27.5 lbf down on the spring to move the front rail. I also increased the depth of all 3 detent grooves by .020" in both shift rails. The detent ball depth for all the grooves is now .125" deep. This also helps prevent the t/c from jumping out of gear. These two mods will increase the force require to shift each of the twin sticks but prevents the t/c from popping out of gear. I also increased the aft interlock pill groove in the rear shift rail from .420" to .840" to allow front high and prevent binding the t/c in rear low which can happen if you just pull the inter lock pills (see Front Shift Rail mod pic) (see Rear Shift Rail mod pic). I used a .375" mill bit to do all the mods to the shift rails.

To seal up where you removed the aluminum plugs to access the interlock pills I used a 1/4" pipe tap and two allen style pipe plugs. Pipe tap works without drilling the holes bigger. Hope this helps.

Front Bearing Retainer.jpg
Detent Parts.jpg
Front shift rail mod.jpg
Rear shift rail mod.jpg
Last edited:

I Lean

Mbryson's hairdresser
Supporting Vendor
Wow, awesome information! (particularly because I need to reassemble my D300 sometime soon-ish)

One question though--2WD low-range is accessible any time with a twin stick without any shift rail mods or anything else, so I'm not entirely sure why you needed to change the interlock detent. :confused:


Registered User
You are exactly right. I meant to say the rear shift rail interlock pill detent mod will allow you to put the t/c in front high and not allow you to accidentally put the rear in low at the same time binding up the t/c. I fixed the statement for accuracy.


New Member
My apologies for bringing this thread back from the dead .... but i'm hoping someone can offer input. :-\

A friend of mine offered to do some machining on the shift rails of the D300 going in my project Jeep. We were going to try this route instead of the JB spring route. However, I've been stumped by the dimensions provided vs. the dimensions of my shift rails.

Question #1: People will lengthen the groove of the rear shift rail for the interlock pill. I've seen this mod suggested on multiple websites, so this obviously isn't rocket science. :D The common measurement I am seeing is to increase the length of the groove from .420 to .840. Easy enough. My rail measures .375 at the widest point. I am measureing at the top of the groove which be including the length of the floor area in addition to the radius. Do we want to machine the groove to .840 or do we want to lengthen the existing groove to .795 (.375 + .420 longer)?

Question #2: This article/thread suggests taking a mill and making the 3 detent grooves deeper so that they are .125 deep. My rails have no wear on them, yet already measure an average of .175 deep. Leave it alone and possibly shim them up more? That would mean that the spring pressures would be even lower that suggested. Shim both to even thicker amounts? Put a 5/16 ball under each OE spring?

Please guys .. you gotta help me ... my buddy is making all sorts of "shaft" jokes and it's making me nervous. :rofl:

I will continue to search RME, Pirate, and a few other sites for any alternatives or solutions .... but if anyone here at RME can offer up some input, it would be appreciated by me.




New Member
Alright ... I have Question number 2 figured out.

The crowns of each detent do not go all the way to the full rail diameter. Meaning each "top" portion of the detent ramp is shorter than the full thickness (.622) of each shift rail. When measured from the tops ... each groove averages to .112 deep. Now I will machine them the extra .013 to ensure the poppet balls are in each pocket a little deeper.

Still need to figure out Question number one. Any takers?

Hope I'm not boring anyone that is reading this ... but I hope this extra info is usefull to someone in the future that didn't look too carefully the first time.