NC 3.7 to 4.1 Diff Gear Swap!

duggan_nugget · Postby **duggan_nugget** » Sun Mar 13, 2011 1:51 pm

So recently I decided to do a FGR swap on my 2006 NC. In a moment of optimism I have now come to regret I decided I would do the change myself with the help of a mechanic mate.

Fast forward 3 weeks without a car and I'm nearly done!

However, after getting the gears installed correctly (the blue showed good contact, the lash was within spec) I've encountered a problem.

When bolting the housing back into the car the passenger side drive shaft (left side in direction of travel for you US guys) can't go in far enough to form a seal. There's about 6 mil extra to go.

The reason for this is the 4.1 gear set has a substantially thicker ring gear than the original. This made me have to move the carrier towards the passenger side to get everything right. So now the end of the spline on the drive shaft is hitting something in the diff carrier and not moving any further.

I've thought up three solutions.
1. Take it to a real shop. (not ideal, I've already spent a lot of money doing this.)
2. Shorten the spline on the drive shaft by 6 and a bit mil so it will fit.
3. Shave 6 mil off the ring gear and move the carrier with spacers (the handy thing is the spacers for the horizontal movement are about 6 mil each, so I'd just have to move one from one side to the other. Easy, in theory!)

Option 1 requires less of the car to be removed (for the 3rd time!) and doesn't damage the gears, however could be more expensive than option 2.

My only issue with shaving Height off the ring gear is that the bolts that hold it to the carrier will have less length in the ring gear, giving it less torsional strength (I think, not really a mechanical engineer).

Are there any experts here that could help me make a decision?

Any input greatly appreciated!

The gears came out of a 96 NA.

flycasta · Postby **flycasta** » Sun Mar 13, 2011 3:13 pm

What Centre are you using?

The reason I ask is because it may be the centre itself. Most of the crown wheel and pinion sets are interchangable until you get to a 4.77:1

It sounds to me that something has not been done correctly to create the problem you have. Have you used the scorrect side carrier bearings?

Is there side adjustability of the carrier like in the earlier diffs, you wind the side adjuster nuts across to get a better mesh of the gears.

It is possible that the NC crown wheel carrier is made with an offset to suit the 3.63 crown wheel and if the diff centre is like the later RX7 stuff with no adjustment, only spacers to space the crown wheel across, when re using that with another crown wheel you will get the problem you are describing. If that is the case you will just need to get a carrier from an NA8 or NB8 to fix the problem.

Let me know first of all if there is adjusting nuts either side of the carrier.

duggan_nugget · Postby **duggan_nugget** » Sun Mar 13, 2011 3:29 pm

The carrier is positioned with shims/spacers horizontally.

The bearings are the same as the original ones so I don't believe they are the issue.

I'm guessing changing the carrier would mean exchanging all the interior parts of my LSD into it? That sounds like it could be a massive job.

flycasta · Postby **flycasta** » Sun Mar 13, 2011 4:30 pm

Ok sounds like the later RX7 setup which is a pain in the butt and really makes the crown wheel carriers significant for each of the ratios. Effectively that means you need to get a crown wheel carrier for a 4.1:1 ratio. Your driveshaft is hitting the cross inside the lsd and cant be fixed unless as you say you shorten then driveshaft but then you will need to regroove the spline to put the circlip in the correct place.

I am pretty sure that the carrier you have in there will look like an open carrier but has a spring setup inside that will preload the side gears so probably not the best lsd to start with. The only other way that might be possible is to space the side bearing away from the carrier that is creating the problem. If you want to talk about it send me a pm and I will give you my phone no.

duggan_nugget · Postby **duggan_nugget** » Mon Mar 14, 2011 8:55 am

I think I've decided to shorten the driveshaft. Does anyone know any good shops who could do this in QLD? I figure I can just take the spline off the shaft and send it by itself cheaply.

pcmx5 · Postby **pcmx5** » Mon Mar 14, 2011 6:03 pm

duggan_nugget wrote:I think I've decided to shorten the driveshaft. Does anyone know any good shops who could do this in QLD? I figure I can just take the spline off the shaft and send it by itself cheaply.

Have you done the sums on selling what you have and buying a Jap dm 4.1 .

I love what mine has done to my NC and wish you well!

Peter.

duggan_nugget · Postby **duggan_nugget** » Tue Mar 15, 2011 2:43 pm

Actually weighing up now. Realised that if I mod my driveshaft I either have to get it engineered, which would be very expensive and I figure there is no garantee the engineer would agree with what I have done. mx5plus has 4.1 gear sets available for my car for 440. I think this would be thebest option... Though still very expensive.

flycasta · Postby **flycasta** » Tue Mar 15, 2011 6:43 pm

Firstly if 1 of your shafts is too short it must mean the other is now having to reach to far in and shouldn't be clipping in properly?? is that right?

Secondly I cant imagine that Mazda made 2 different thickness crown wheels.

What is the tooth count of the 4.1 ratio you installed?

What was the thickness of the crown wheel?

I will measure it against 1 I have and also compare that to a 3.73 and 3.90

The carriers are normally changed to keep the crown wheel better positioned, not the crown wheels themselves.

duggan_nugget · Postby **duggan_nugget** » Wed Mar 16, 2011 3:15 pm

I thought the same thing, but the other one seems in there pretty tight and it took some force to get it past what I thought was the spring clip.

Before I put them in I counted the teeth and did the equation (ring / pinion or visa versa) and they are definitely 4.1's. The ring is definitely substantially thicker though. At first I thought the smaller diameter pinion (at least to my eyes comparing the old and new) would make up for it but apparently not.

flycasta · Postby **flycasta** » Wed Mar 16, 2011 8:25 pm

Doesn't sound good...... The driveshaft should go in and be fiarly easy to slide in and out just a little, a couple of mm or so. Did you feel it click in when the circlip made it to the recess? If hasn't clicked in properly the shaft may not be all the way in and from what you are saying about the other side I would think it hasn't made it. That can be a problem as it may work its way loose and come out or come most of the way out and then run on only a small amount fo spline and ruin both the driveshaft end and the spline inside the LSD/open carrier.

The other problem is that if you did manage to get he circlip to click in the oilseal may now be squashed so much so that it wont be running on the correct sealing surface and will leak pretty well straight after driving the car.

Can you pull the back cover off again and measure the crown wheel thickness for me and I will be able to tell you if it is close to what I have.

duggan_nugget · Postby **duggan_nugget** » Mon Mar 21, 2011 3:10 pm

Thanks flycasta, but I took it to a shop and it definitely won't work. I've ordered NC gears from mx5plus.com.au and they should be going in this week.

Richard from mx5plus did say that he had both sets on hand (4.1NC and 4.1 NA) and that the NA ones were definitely thicker. I will update when they are installed but for now I think we'll have to assume the NA/NB gears Don't work in our NC cars.

Though a user at miata.net did get a set of NA 3.7's into an NC, he said he had the same issue of the axle not fitting and it leaking oil, however he was able to fix it with thicker oil. I'm assuming his gap was smaller than mine as there was a substantial amount of oil coming from mine.

I'll update when the new gears go in!

Edit: I should have added, the shop I took it to (Diff and Gearbox Industries in Brisbane) said, to get it to work, the carrier would need to be changed to one with a different offset to counter the thickness of the ring gear. I'm not sure if this even exists as the NC 4.1 ring gear is the correct thickness according to Richard at mx5plus.

duggan_nugget · Postby **duggan_nugget** » Wed Mar 30, 2011 10:38 am

OK! So the gears have been installed and working since friday night. An unbelievable difference! The car feels much faster (though the 5 weeks I spent in a Honda Jazz could be accentuating that a bit). As everyone says, it really should have come this way. I can now beat a lightly modded (mufflers, intake and bigger tyres) Aurion to 80km/h instead of being thrashed. The car sits on exactly 3000rpm at 100km/h.

It really is a huge difference. I would reccomend this to anyone not planning to go F.I.

I'd imagine it would be even better with intake, header, midpipe and muffler - which is my next mod!

Thanks for all the help everyone.

Anyone looking for some 4.1 NA/NB gears can PM me!