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Unimog 404 - 6 to 8 speed conversion

Austin

Blame Canada
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With a few modifications the 404’s transmission can gain 2 extra forward gears on the low range side. The mod is nothing new and has been well documented in this popular PDF. It does have a few downsides I wasn’t crazy about. First is that you have to add yet another lever to operate in the shifting process. Second is that with the new lever’s location you lose the option of having a PTO. 404 PTOs are like Unicorns, so most don’t care but I’d like to have one someday.

So this is the stock shifting pattern, low gears on the left and high (road) gears on the right:
Screen Shot 2022-11-23 at 1.05.26 PM.png


The white dot in the pic is the Q-Transverse and is not very smooth to shift across in the 404. The go-to method outlined in the PDF above basically blocks off the low side’s shifting gates on the guide plate and uses the higher gear’s H shaped gates for both the high and low gears. The lever you have to add shifts the gate between the high and low side resulting in a shift pattern like this:
Screen Shot 2022-11-23 at 1.12.26 PM.png


So while going from 4th to 5th, you have to throw an additional lever so you don't end up back in 1st .... which the drivetrain wouldn't like.

So I went another route. The biggest advantages, in my book are no extra levers, it keeps the option for a PTO open and uses a more standard shifting pattern like this:
Screen Shot 2022-11-23 at 1.16.56 PM.png


Still have that Q-Transverse in the middle but I think keeping a standard shifting helps make an already complex transmission easier for somebody to jump in and drive or move.

After you remove the shifting levers and top cover from the transmission it exposes the guide plate. You can pull it right up and off of the transmission housing. Take a picture of the top of the plate so when you reassemble you can be sure to have the shifting forks in the same position.

Here are the top and bottom sides of the guide plate.
unimog_404_guide plate_before.jpg
unimog_404_guide plate_bottom_before.JPG


For reference:
A = Angular Lever – This will be modified.
B= Pivot Bolt – This is an alignment bolt.
C = Latch Lever – No longer be needed.
D = Shifting Slide Plate – This will be modified.
E = Shifting Slide Plate Retainer – No longer needed.

Green numbers 1 – 6 are reference points for top/bottom pics.

Making the Modifications
  1. Remove the Latch Lever (C) by removing the cotter pin & washer. None of these parts will be reused.
  2. Remove the Shifting Slide Plate (D) by sliding it (left if going by pic above) out from under the rails holding it in place.
  3. Remove the Pivot Bolt (B) to pull off the Angular Lever (A). IMPORTANT: Turning the Pivot Bolt adjusts the location of A and should be marked for reference before loosening or removing.
  4. Remove the Shifting Slide Plate Retainer (E) by grinding down the rivets. Once removed, smash the rivets back down to hold in the lower retainers. I was able to get my guide plate into my press to smash these.
You should have something like this:

unimog_404_guide plate_parts_removed.JPG


Guide Plate Modification
In order to add the two gears on the low side you’ll need to cut an additional shifting gate next to the factory 1st to 2nd gate. With that cut, gears 1 & 2 will move to the new gate, and the new 3 & 4 gears will go in their factory spot.

Here is a before and after pic:

unimog_404_shift_patten_before.jpg
unimog_404_shift_patten_after.jpg


I had a bit of a documentation fail here. I don’t have a pic from right after I cut the new gate. That pic from after the mod was from later in the process and shows a modified Shifting Slide Plate sitting over one gate and a few extra pieces to the right of the new gate.

Shifting Slide Plate & Pivot Bolt Modification
With the new gate cut into the guide plate the Shifting Slide Plate will need to slide further to the right to grab those gears. The bolt to the right of plate and the ‘ear’ on the Angular Lever will prevent it from sliding as far as it needs to. To correct this, the areas highlighted in green need to be removed. The single green line in the pic represents a hard edge that needs to be beveled to match the opposite side so the shifter can easily operate at that angle. The dark green area is not for is not for clearance and will be explained later.
unimog_404_shifting_slide_plate_modification.jpg


Cutting the tab off the plate and beveling the edge is a no brainer. Cutting the Angle out of the top of the Shifting Slide Plate and the edge off the Angular Lever can be tedious. It’s something you want to sneak up on. Cut some, reassemble the assembly and test. This process will go much faster if you marked the factory position of the Pivot Bolt before removal. Here is a pic of the Shifting Slide Plate and Angular Lever reinstalled to get an idea how the plate can move to the new gate unrestricted.
unimog_404_shifting_slide_plate_modification_after.jpg


Once you have the Shifting Slide Plate assembly moving freely across all the gates, you’re done modifying the top of the guide plate. Next up is modifying the shifting forks under the guide plate.
 
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Modifying the Shifting Forks
After completing the guide plate mods above, flip it over and you should see this:
unimog_404_guide plate_bottom_plan.JPG


F = It’s hard to see but, for all better purposes, that’s end of the shifter you operate in the cab. You shift it through the gates and it pushes the shifting forks around.

G = This shifting fork engages the stock 1,2,3 & 4 gears (3,4,5,6 as marked above). When shifter crosses the Q-transverse (white dot) the shifting fork held in place by green rivet #4 shifts from low/high gears. This is how they share a fork.

H = This shifting fork crosses over G and engages gears 5 & 6 in stock form. In the modified form it will engage 3,4,7 & 8

If you look to the left, you can see the new gate that was cut in the guide plate. The two new low gears, 3 & 4, will be where they used to be. In order for the fork that engages 1st & 2nd in stock form to be able to grab them in this modified form we need to remove the part of the fork in green and more/less move it over the new gate. Once that’s done, we’ll replace the material and connect it to H. It’s easier than it sounds. The ‘new gears’ are really just the gears 5 & 6 but now they are being activated on the low side.

To get started we’ll need to remove two of the shifting forks.
  1. Drill out green rivets 3, 5 & 6. #5 & #6 have springs, spring retainers, a pin and barrel pins that will be released on the other side. You can remove them first, I laid the plate on a old rag to keep them from flying off.

  2. On the top side of the guide plate there are two bolts to remove to be able to get shifter fork H off the plate. The only reason this one is removed is because you can’t get shifter fork G out with it in place.

  3. Remove the material from shifter plate G. The width of a piece of electrical tape is the perfect width. It needs to be long enough to allow a new shifter fork to move back and forth inside that space.
    unimog_404_shifter fork_pre_cut.JPG
    unimog_404_shifter fork_post_cut.jpg


  4. You’ll need to cut out two new pieces from 5/8” & ½”stock to add to the two shifter forks. I did these in Sketchup, I’m not sure how to export them.
    unimog_404_shifter_1-2_shifter_fork.png
    unimog_404_shifter_3-4_shifter_fork.png


  5. Weld the new piece labeled I to the side of shifter fork G. This will go over the new gate where gears 1 & 2 are. You can’t see in this picture but piece I is welded to G at top and bottom. Shifter fork H is covering the bottom weld.
    unimog_404_shifter_fork_weld.JPG


  6. Weld the piece labeled J onto shifter fork H.
    unimog_404_shifter_fork_weld_complete.JPG


  7. Weld the drilled-out rivets #5 & #6 for assembly of the springs and bearings on the other side.

  8. I had to replace Rivit #3 with a bolt which will not interfere with anything. If you don’t misplace the pin you may not have this problem.

That’s it! No need to touch a single gear or anything in the case. Flip it back over and make sure you can easily cycle through all 8 gears. Check the picture you took earlier to make sure the top of the shifter plate, mainly that the springs are in the same place before setting the plate back on the case for reassembly.

Now you got 2 more gears, in a logical shifting pattern and you can use the PTO. :smokin:


Shifting through all 8 gears ... did I mention shifting through Q was a bitch?

:beer::beer:
 
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So what are the ratios for that? Close enough that you skip gears on the street?
 
So what are the ratios for that? Close enough that you skip gears on the street?
The original 1,2,3,4,5,6 gear ratios are all the same. 1st, 2nd, 3rd & 4th use the same physical gear but in 1st & 2nd the low range gear in engaged. This mod allows 5th & 6th to be engaged from the low side as well. The new 4th on the low side and new 5th (previously 3rd or 1st gear on high side) are very similar in ratios. Makes it nice to have that ratio on both sides of the Q.
 
So now you have a "fast & furious" transmission. :flipoff2:

Seriously, The shift pattern makes more sense, and your not speed shifting it anyways, so the mod seems like a great option,

And also keeping the PTO open for the what if....... in the future.

To date I have 5 trans shifters in my Auto Toyota:homer:
 
Can any gear be reverse? Ex. Can you reverse in 1st and shift to 2nd while moving in reverse.
You can upshift and downshift while in reverse in all Unimogs. And yes, it turns every head within earshot when you rev one out in reverse and then grab another gear. Only the lower range gears (2 stock, 4 with shifter mods) can be used in reverse. All shifts in a Unimog transmission can be made "on the fly" except switching the reverser, that has to be done while stopped.

This is a really cool way to mod the shifter! Much better than the "2x4" conversion, which is actually a copy of a rare factory 8 speed shifter (I've held a real NOS 404 2x4 shifter assembly in my hands.) The biggest improvement from adding the extra two gears is having two higher reverse gears. "Fast" reverse in a stock 404 is really freaking slow. Painfully so if you need to go backwards for any distance, or need wheel speed to get out of slippery mud or snow.
 
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