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rebuild box with n13 internals!!

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Posts: 38
Joined: Thu Aug 03, 2006 9:21 am

rebuild box with n13 internals!!

Post by Jayman »

Over the next couple of days ill be doing a more detailed step by step of this conversion.
Ill be adding pix along with the procedures in a step by step format, highlighting any traps for the unwary. There are a couple of little traps worth mentioning.
Ill go as far as disassembly of the N12 box here.

- Drain the oil out of both boxes by removing the drain plug from the bottom of the diff housing.
Give both boxes a good good clean off with a stiff brush and degreaser.
Lay the box on its bellhousing face.

- First job is to remove the speedo drive from its housing. Youll need this later so keep it handy

- Next, the output shaft bearing race retainer plate on the end of the gearcase held on with M6 bolts and note the shims which sit on top of the bearing outer race. These provide the correct preload on the output shaft to prevent shaft endfloat.


- Remove all of the gearcase retaining bolts.
I like to do these in an even "crosshatch" pattern by cracking each bolt first working your way around the housing and then removing them with a speedbrace.

-After this its a simple matter of cracking the case halves apart with a soft mallet.
They can be a little stubborn to get apart as the case sealant and alignment dowels hold it together pretty well.
Try to avoid levering the cases apart with screwdrivers, chisels and other subtle items that can damage the case joint faces.
The output shaft bearing outer race will tend to remain in the gearcase but falls inwards easily enough.

I reccommend new bearings thru the box but if the bearings are in good condition you can re-use them but keep bearings and races together. The rollers and races are made as a matching assemblies and subtle differences in contact between unrelated rollers and races can result in quick failure.

- Once cracked apart, the gearcase will lift easily away from the bellhousing leaving the selectors, gearsets and diff sitting in the bellhousing.
Pay attention to the reverse idler gear and its spacer pointed at in the pix as they both are located on a shaft that tends to remain in the gearcase, NOT the bellhousing and the spacer can fall off.


- Remove the selector fork shaft, just grab it and twist it out noting the spring that lives inside the tubular shaft right down the bottom in the hole in the bellhousing.

The spring will remain sitting in the bellhousing.


After that, just swing each of the selector forks away from the selector mechanism itself, starting with the top one and pull them out, but pay attention to the small selector end bushes. They just sit there and can fall off the ends that engage into the selector mechanism.

Leave the low/2nd and reverse gear selector in place for now but remove the reverse gears endfloat bush.


Next is to remove the selector fork mechanism. Undo the 3 retaining bolts and lift the entire assembly out along with the low gear selector fork.


Now you can grab the entire output shaft and lift it out.


-Fish out the spring from the hole in the bellhousing before it gets lost


The gear selector shaft has detent balls and springs in both forward/backwards and roational directions. You should fish then out or they will piss off for mileswhen you roll the bellhousing over.


- The diff carrier will lift out next


- Then the input shaft plain ball bearing is retained in the bellhousing by a retainer plate.
Undo the 3 M8 bolts and the input shaft with its bearing can be tapped out from the outer (clutch) side of the bellhousing.


Tapered roller bearing components should NEVER be mixed and matched with unrelated bearing parts.
These bearings are made as a matched set of components and subtle variations in dimensions can see massively uneven loads placed on the rollers resulting in short bearing life and even a complete bearing failure.
Dont use the inner race and rollers from one box with the outer race from another.

This is an example of such mixing of bearing parts.
The mainshaft tapered bearings are similar in appearance but have different inner hole sizes.
Additionally, the bearings were made by different manufacturers.
In this case the outer races have been mixed up and altho the pix is not very clear the damage to the rollers at one end and the heavy wear to the bearing outer race is quite apparent.


The rollers have been heavily loaded at one end and the hardening has flaked off.
Probable total bearing failure would have been the end result if this had been let run on too long.

OK, you have a couple of options with bearings.
1: Fit a new set of bearings altogether
2: Reuse existing bearings if they are in good shape

The input shaft bearing is a plain ball bearing on the clutch side and a needle race on the other end. They should be fairly robust and should be Ok to reuse.
Look for any damage on the balls, needles and race contact faces.

The output, or mainshaft bearings are tapered rollers (just like front wheel bearings) with the bearing preload set by the shims removed from the end of the gearcase during initial disassembly.

Just resist the urge to assemble the box with unrelated bearing components mixed from both boxes.

You can remove the inner races and cages from the output shaft without damage using a bearing puller like this:


When you get this bearing off watch out that the spacer under the bearing and the gear cluster retainer clip halves dont fall off and vanish!


Youll need a fairly small and shallow puller to get the inner race and rollers off the diff pinion end of the output shaft tho.
These bearings are reasonably tightly pressed onto their shaft ends.
Dont try to chisel the bearings along the shaft and then lever them off with screwdrivers or other such butcher tools.
You can almost guarantee you'll bruise or chip off pinion teeth levering against them!

Or you can just extract the outer races from the alloy cases. The output shaft race floats quite loosely in its gearcase housing. Just gently drift it inwards from the gearcase. This ones pushed in by hand.

Youll need a special bearing race puller to remove the outer race from the bellhousing end tho.
You could try heating the casing around the race quickly with a flame. The races are not too tight in their recess and may just fall out.
Do NOT lever under the race with screwdrivers and other brutal shit to get them out!
If you dont have an outer race puller, and/or the heating does not work, the take the case to a gearbox builder and ask them to pull them out for you.
Better still, get them to press off the old shaft races as well and press on a new set while theyre at it!

To remove the shifter assembly

Now, the 4.11 diff out of the N13 box will not fit into the N12 diffcase so you have to remove the ring gears and fit the larger N13 diff gear onto the N12 gear carrier
The retaining bolts are 14mm head but are usually pretty tight so youll have to grab the crownwheel pretty hard in a big vise with ally jaw guards sos you dont damage the gearteeth.

Once this is done set it aside for reassembly.

Ok reassembly is prewtty much a revesal of dissassembly so begin by replacing the input shaft oil seal and then installing the input shaft.


Take the crown wheel from your n13 gearbox, and put it on your n12 diff.
You'll find that the n13 diff just won't sit and spin in the n12
bellhousing, which is why we reuse the n12 one, but with the appropriate
gear diameter.

You need to do this so that the final gear on the output shaft of the
n13 gearset matches it's original counterpart, the crown wheel.

> i also know that the gear train needs to be swaped over...but am
> unsure of if it is a straight swap....


Pull everything that is n12 related out of your existing gearbox,
hanging onto the diff.
Keep the bellhousing part of the gearbox, aka the bit that bolts to the
engine, you don't need the cover that goes over all the gears

You then take the n13 input shaft, and put it in the n12 bellhousing,
likewise with the output shaft. Keep the n13 selectors with the n13

This is very important. Additionally, I found I even had to swap the
reverse gear section that bolts through the side of the bellhousing on
into the side of the input shaft.
It's an ass to get out, you'll need a really big headed phillips
screwdriver to get them out.

So take this out of the n13 bellhousing, and put it in your n12
bellhousing, along with the n13 selector.

To remove the selector, it's actually in two pieces, you'll need to
knock out the split pins (2, one inside the other) about half way down
the selector - then you can get each half out.

Basically everything gets swapped over. Oh with one nasty exception -
the speedo drive, you use your original speedo drive BECAUSE the speedo pickup is *after* all
the gear ratios, it's going at wheelspeed, not 'diff speed' hence no
matter what gearset you have in there, the speedo is still spinning at
the same speed as your wheels, so you don't use the n13 one. Another
trap for the unlucky amoungst us.

The dog selector gear has teeth on both sides of it. These are
what crunch when you don't fully engage gears. They are all made the
same, even the one that engages 5th gear. This means one side isn't
used. So, when doing the re-assembly, this gear can be popped off
and swapped end for end giving brand new never used teeth to mesh
with. Just be a little careful as there are 3 locating pins which
are spring loaded, these need to be pushed into place to allow the
dog gear back on the shaft.Image
Before you decide to criticize someone, walk a mile in there shoes.....

then your a mile away, and you have their shoes!!
Posts: 787
Joined: Thu Aug 03, 2006 1:17 pm
Location: Perth, WA

Post by raddavey »

Good pics! I noticed there is a plastic oil spout thingo that is missing. It should be behind the output shaft bearing below where the little yellow plastic oil feed thing sits. Yeah i know exactly what i'm talking about LOL

I"m not surprised its missing because to remove that bearing outter race from the housing it's impossible to do without breaking the little plastic thing.
ACL Pistons, 226/236 cam, T28 Turbo, FMIC, FJ20T Injectors, Wolf V4 ECU, NPC Clutch, VL TB, Custom High Mount Manifold, N13 Internals G/B, 3" SS Exhaust, Walbro 500hp Pump.
13.52@108.95mph (street trim, leaky hg)
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