Tips for improving the Dana 300 transfer case Short Cuts

| Jeep Section | Isuszu Section | |

By: Dan Houlton - Feb 2005

Dana 300 quick mods


All the bells and whistles added.

While doing the full rebuild of my Dana 300, I decided to add some improvements while I had it torn down.  Some of the specific things I wanted to address were:

Oil level sight tube

You've probably seen these on Atlas cases.  It's a clear plastic/nylon/poly tube that connects into the top and bottom of the case somewhere.  The tube fills with oil from the case and you can easily tell at glace what the oil level is.  I took it a step further to also facilitate filling the case a little easier and mess free than with the stock fill hole.

The sight tube gives a clear indication of oil level at a glance. I set up another sight tube on my Klune-V (with 1/4" NPT fittings) using the existing holes.

For my case, I decided to drill and tap 2 new holes in the back.  The holes are tapped with 1/8" NPT pipe threads.  You can use common pipe fittings from the hardware store, but I wanted to use the push lock, quick disconnects.  I've used these for many years for things like vacuum lines and water injection and I'm a huge fan of them.

Basically, you push the tube in and it seals.  To pull it back out, you push in, hold the retaining ring down, and it pulls right out.  Same type of fittings an ARB locker uses at the solenoids if you've seen those.  

To fill the case, I'll just pull the tube out of the top fitting, and use a short piece spare tubing on the fill bottle to plug into the top fitting.  Squirt oil in until the level is where you want it in the sight tube.  Very little mess, and no pipe-threaded plugs to pull out and reseal.

I used 5/16" tube and fittings ordered from McMaster.  The tube is Nylon 6 clear tubing.  Nylon 6 is pretty stiff tubing, but it is clear and is oil and chemical safe as well.  In this case super flexibility is not needed.  The push lock fittings are nickel plated brass, 1/8" NPT with a swivel head and sealant already on the threads.  They're quite sturdy and corrosion won't be an issue with them.

Nylon 6 clear tubing - McMaster #8359K15
Fittings (1/8" NPT) - McMaster #51495K219

See Notes below

FWD HI range

Using a twin stick on the D300 gives you a shifter for each output shaft.  A couple of the things this allows is the use of 2wd (RWD) Low for crawling over hi-traction areas like slickrock, and FWD Low for doing front digs.  To prevent the possibility of putting one output into Hi and the other into Low, there are 2 interlock pills in the front output bearing housing.  A side effect of this is that the pills also prevent the use of FWD Hi range.  Some people remove the pills entirely, but you risk blowing the case up.  Getting one output into Hi and the other into Low will bind the t-case internally and something will give.  Either the gears, bearings or the case itself.

This notch in the rear shift rail needs to be extended forward as shown. Notch extended with an angle grinder and a well-used (rounded edge) grinding wheel.

You can, however, do a little bit of grinding on the rear shift rail that will allow FWD Hi range while still preventing the possibility of binding up the case.  But why?  Well, if you're just running the D300 alone, then it probably won't do a whole lot for you.  But, suppose you're running a crawler box with it's reduction engaged and the D300 in Hi.  Normally, you can't just pull the rear output into Neutral to do a front dig.  You need to do some more shifting, putting the D300 into FWD low, maybe disengaging the crawler box and/or picking a gearing that is less than optimal to do the dig.  With this mod, you can just slip the rear output into Neutral and pound away.

To perform the mod, you need to remove the rear shift rail from the case and front output bearing retainer.  That task is beyond the scope of this article, but is done as part of a rebuild of the case.  To allow FWD Hi, the back notch in the rail for the back lockout pill needs to be extended forward.

Another view of the new, extended notch in the rear shift rail.

The notch is about .42" long.  The distance between the two shift indents (Hi and Neutral) next to it is about .42" as well, so the new length of the notch needs to be about .84".  In 4wd HI (levers full back, rails full forward), this notch extension allows the rear shift rail to move back and reach the Neutral position before the pill stops the rail from going any further.

To extend the notch, I used a 4.5" angle grinder with a well-used grinding wheel in it.  The worn wheel has a nice, rounded leading edge instead of the sharp corner of a new wheel, and it worked well for shaping the "ramp" at the new front of the notch.  Just go slow, and stop frequently to check your angle, depth, etc.  You don't need precision in the thousandth's here.  You can easily eyeball it as you go and be plenty accurate.

Check the new length of the notch frequently and stop once you get to about .84" and you're done.  Re-install the rail and you'll now have FWD Hi capability and still be re-assured that you cannot shift one rail into Hi while the other is in Low.

Vent tube replacement

The stock vent tube in the D300 is pressed into the rear output bearing housing.  There typically isn't anything wrong with it, but mine was well-rusted up and after pulling it out for cleaning and painting the housing, it will always be loose if you try to press it back in.  The hole for it is about the right size though for tapping with 1/8" NPT threads.


Push-lock, swivel connector replaces the pressed in vent tube.

Rather than using a new hose barb, I used the same push lock type fittings as for the oil sight tubes, but for 5/32" tube instead of 5/16".  The tube I used is Nylon 11 instead of Nylon 6 as well.  The Nylon 11 is much more flexible than the Nylon 6 and makes it easier to run around bends and corners.  I also have about 50' of it because it's the same size that I've used for all my water injection lines in the past.  (Note: 5/32" and 4mm tube and push lock connectors are interchangeable).

The D300 and Klune vents merge into a single tube that runs up to the engine bay.

Running the vent tube, I looped it around the rear output housing to make an oil trap and then forward.  I used a similar fitting for the vent in my Klune-V as well.  They both use the same gear oil, so after the loops in each vent tube, I "Y'd" them into a single vent line.  In the off chance that they do mix a little bit of oil, it won't matter.

The "Y" connector is another part from McMaster, this one is glass-filled nylon instead of nickel plated brass.  The brass ones are all "T's" instead of a "Y".

See Notes below

Flange head bolts

Another thing I didn't like were the stock hex head bolts for the various housings and cover plates.  They're all plain hex head bolts with no washers used.  All of my housings and plates had over-sized holes ground into them from the hex head of the bolt chewing them up.  The AL housings were especially bad, but even the steel cover plate for the front output shaft had a ring ground into it.

Flange head bolts on the rear output housing. Flange head bolts on the front output, rear cover plate.

During the rebuild, I sanded most of these ground rings out to smooth the surface again, and tossed all the stock bolts.  The new bolts from McMaster have a flanged head and are much easier on the housings.

My D300 also leaked like a sieve.  Of the various places it was leaking from, the bolts were among them.  Oil would leak out the bolt holes around the hex head bolts.  I went to extremes (overboard maybe?) in preventing leaks this time around.  In addition to RTV'ing everything, I also put a ring of RTV under the flange of each bolt head to make sure they seal the oil in.

To replace the stock bolts in the rear output housing, front output housing and front output rear cover plate, you will need:

(13) 3/8" coarse x 1" long (McMaster #92316A624)
(1) 3/8" coarse x 1.75" long (McMaster #92316A630)

To replace the stock pan bolts (these are already flange head bolts so you probably won't need to):

(10) 1/4" coarse x .5" long

And for the single bolt for the idler pin retaining plate, use 1/4" coarse x 1/2" - 3/4" long.  I didn't replace this one though.

One other set you may need are the flat head, torx bolts in the front output bearing retainer.  I had to drill out 3 of them on my case to get them out.  McMaster has replacements with a larger T30 torx head (stock is T27) so they should be a better bolt for removal in the future.  You'll need:

(6) 1/4" coarse x 1" long flat head bolts (McMaster #94414A542).

See Notes below



A few notes to help find these parts on the McMaster website (  Their online catalog is huge, but their search engine works very well for finding what you need.


Related Links:

Trending on 4x4Wire TrailTalk Forums

New Topics on TrailTalk
LED Headlights
by 87Raider - 08/06/22 03:43 AM
The End is Near
by 4x4Wire - 07/19/22 06:07 PM
Bump steer problems with ball joint flip/spacers?
by Colombia - 07/04/22 04:17 AM
Sad day for my wife Gen III
by JAVYPRO - 12/04/21 04:30 PM
OG folks chime in!
by PajEvo - 03/07/21 03:45 PM
suzi the psychic
by - 03/26/10 06:59 AM
Recent Posts on TrailTalk
OG folks chime in!
by MarkG - 08/10/22 04:39 PM
suzi the psychic
by rascott - 08/07/22 05:28 PM
LED Headlights
by 87Raider - 08/06/22 04:23 AM
The End is Near
by rxinhed - 08/02/22 01:23 AM
Sad day for my wife Gen III
by JAVYPRO - 07/15/22 08:24 PM
Bump steer problems with ball joint flip/spacers?
by Colombia - 07/13/22 04:22 AM

4x4Wire Social:

| 4x4Wire on FaceBook

OutdoorWire, 4x4Wire, JeepWire, TrailTalk, MUIRNet-News, and 4x4Voice are all trademarks and publications of OutdoorWire, Inc. and MUIRNet Consulting.
Copyright (c) 1999-2019 OutdoorWire, Inc and MUIRNet Consulting - All Rights Reserved, no part of this publication may be reproduced in any form without express written permission
You may link freely to this site, but no further use is allowed without the express written permission of the owner of this material.
All corporate trademarks are the property of their respective owners.