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.
Eric
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.
Eric
Last edited: