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RockLobster Build - Part 1 - Dis-assembly


I have embarked on building my own low ratio transfer box. I have previously built two boxes, one for my SJ and one for my Jimny but both of these were 4:1 boxes built using the "Difflock" cut & shut gears. This is a frequently maligned process but the current Jimny one has served nearly 10 years so far so they cannot be too bad, however with 32" tyres it is time for something more low range.

Since I did the cut & shut one, a variety of aftermarket gearsets have appeared that provide a "drop in" solution with pre-cut gears. There are a variety available from cheap Chinese versions through to Japanese manufactured ones made in the original factory.

I went for the Trail-Gear 6.5:1 set as it represents a mid priced solution that has been on sale in the USA for a number of years. This should bring back the gearing quite nicely on the road whilst providing a very low gearing off-road.

Also needed is a Suzuki SJ transfer box, either a 410 or 413 will do (except for the very early 410 versions which are useless). See the section in the Wiki - HERE - which tells how to identify the correct box. Late 413 boxes are particularly good as they have the same larger flanges that the Jimny has, however it is straight forward to drill the old ones if necessary.

If you are doing this from scratch you will also need mounting arms, propshafts and a speedo adaptor to fit it to a Suzuki Jimny. Of course, I already have those parts on my as it has a 4.1:1 Transfer box already fitted.

So the build begins......BigJimny's Transfer box.

So why "Do it yourself" (DIY):

  • Its not that hard, I have built 2 previously,
  • You know what went into the build, the quality of any bearings you changed etc.
If you want then you could:

The instructions from Trail Gear do not assume any special tools and show you how to do it with a hammer, some metal strips and a piece of pipe, however you will see me in the pictures using proper tools.​

SJ413 Transfer box

​Starting point is an SJ Transfer box, any variant EXCEPT the early 410 will do. Here it is already stripped of the flanges and shifter stick (make sure you get both if you are buying one!). Along the left hand edge you can see at the top end the square edge of the PTO moulding and halfway along the bump for the Counter Shaft which help identify it as a suitable box.

Replacement Gears
Trail Gear 6.5:1 Transfer set

The picture on the left shows the Trail Gear 6.5:1 kit (105004-3-KIT). As you can see it contains the new gears, new shafts where appropriate and the gaskets and seals. Although it contains the small set of needle bearings it does not contain the main bearings. Within the instructions they re-use the old bearings but as I am stripping it right down it would be silly not to change all the bearings, again the article HERE lists the bearings. The larger needle bearings are from Suzuki.

Paraffin bath
Cleaned Transfer box

The first step as far as I am concerned is to clean up the transfer box to make working on it more pleasant. I soaked it in a bath of paraffin and then worked on it with a stiff paintbrush and a jet wash jet. IF YOU INTEND TO RE-USE THE BEARINGS I WOULD NOT DO THIS AS IT FORCES MUCK INTO THE BEARINGS.

4WD switch ball bearing
4WD Switch

Now the strip down can start. First remove the 4WD switch along with the small ball bearing hidden in the hole. In particular keep the ball bearing in a safe place (note: it is larger than the other ball bearings you are going to find in this process so it is easily identified later).

Remove Detent spring cover
Clean out Detent cover Allen Key socket

The Transfer box selector levers have grooves in them with ball bearings and springs. This gives the selector positions. The first spring/ball hides under this cover. Clean out the end and remove with an Allen Key 

Catch small ball bearing
Remove first spring

​Carefully remove the spring, keeping for later. Rotate the transfer box over and catch the ball bearing as it falls out!

Prise cover apart
Remove front output cover

Undo the bolts on the front housing and prise the case apart

Remove intermediate shaft retaining bolt
Keep the bolts in correct position

You will find  that the case bolts are of different lengths etc. Keep them in the correct order for ease of replacement.

Undo the retaining bolt and remove the metal tag that are holding the intermediate shaft in place.

Speedo drive removal
Speedo drive housing

Remove the speedo drive housing sleeve. This has often corroded and stuck in position. The manual shows a hopeful picture of it being pulled out with pliers.

As it is inevitably stuck, I cut a groove in it and using a chisel hit it so that it rotates in the housing and breaks the seal. It is then usually possible, with much further fighting, to get it out. However, I used an expanding bearing puller to grip the inside of the housing and then the slide hammer attachment pulled out the drive, easy!

Prise case apart

​With the speedo drive removed you can then split the case. Remove all the bolts and remember where they came from.

With the bolts removed the case should come apart, some hammer work may be needed on the edges to separate it.

Be aware that another small ball bearing may well come out!

Remove all seals

Also the two main shafts have shims fitted. Carefully remove the shims noting how many there are and where they came from (They are different sizes so it is not possible to mix them on the wrong shaft when re-assembling).

Also remove all the seals from the inputs and outputs around the casings.

Box of parts!
Front output case bearing

The front  housing has a bearing in it retained by a C-Clip. Remove the shaft by tapping it out and then, with the C-Clip removed, you can drive the bearing out.

When dis-assembly is complete you end up with a collection of parts!

Cases cleaned up

With the case fully broken down, I would take the opportunity to clean everything up.

Now on to part 2 - Gear replacement.

RockLobster Build - Part 2 - Building the gears
Differential Failure

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