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mtnickel

Z31T axle swap CAN fit

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Hi guys,

Just thought I'd finally post some of my findings here since this is ultimately the place where I learned everything there is to know about technical aspects of the Z-car. So i present, the making of a z31 adapter. I realize this has definitely been done before and I'm not claiming to be the originator, but I never saw any measurements, or calculations and started from scratch instead. The following may not flow very well as most of it is from the instruction guide I made, but I tried to quickly edit as best I could.

Background

So there’s been a lot of information going back and forth about whether Z31 turbo axles will fit in the S30 chassis. Companion flanges, cage flipping, and then even custom shorter axles to be swapped in…which work, which don’t? Do the axles bind? etc. I’m trying to dispel at least some of the rumors/hearsay.

Here is what I’ve found on my early 260z (I haven’t measured on 240z, or 280z, but I have a suspicion that the pickup points and axle clearances are all the same).

Stock Axles compared to Z31T

I took a measurement of the length of the stock axles while on the car at ride height. I then compared that side by side to stock Z31T axles. It was quickly apparent that it was going to be very close. The driverside looked to have some room, but the passenger side was as long as the axle if not just slightly longer.

Driverside:

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Passenger side (centering lip is very close...hard to tell from pic).

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The Adapter Requirements

Once I saw these measurements, I knew that if Z31T axles were to work, the companion adapter needed to be as far out as possible. Hence we aimed to make the adapter so that it is flush with the stock outboard flange.

Here's how it turned out:

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Test Fitting

With the adapters welded up, we then threw them on the car to check fitment.  

The axles are actually at their tightest point in full droop. Many have stated that when the axles get parallel/straight, they are longest and can bind, thus changing spring rate, wearing diff bearings, etc. They absolutely are not tightest there. I took the springs out, fitted everything up and during testing binding occurred in the lowest 1-2” of travel. Moving the hub upwards resulted in more clearance. I believe this is due to 1 of 2 reasons

  1. The axle and control arm are not parallel. I think closer to full droop, the control arm is pulling the hub closer to the diff at a faster rate than axle is. (in picture, all lines are the same and the distance between the 2 line end points (simulating lower arm and axle) is kept the same. The dotted line is aprox where the strut might be. So at full droop, the strut pulls in faster and leaves more of the axle out to bind against the strut)diagram1.jpg.e0060eefb0af7d466d7f77d970d6330e.jpg
  2. When the axle is parallel, the cage ends are also square, but as they get more angular, the axle may actually get slightly longer. It’s the same reason when the box below couldn’t be removed between 2 fixed walls without it becoming longer first.diagram2.jpg.e2b0c58f73aa1b0773ed876361f4adaf.jpg

 

Comparing the fit on both sides, we find the passenger side is the tighter fitting axle. With the OEMZ31T axles, technically the driverside would be ok without flipping the cage, but you may as well flip the cage since you’re having to do the passenger side anyways. The passenger side OEM axle binds up during the lower 2” of droop. It binds so much so that you can’t even get the axle in when the car is jacked up. Flipping the cage gave enough clearance to both get the axle in and have a little give. Once closer to ride height, the clearance increases further and even at full compression there is lots of clearance.

ROCK AUTO axles: These axles use the tripod design of the 280zx Turbo axle I believe (or maybe NA 300zx as well, don’t quote me on this. All I know is its not the same design as the OEM Turbo Z31 axles. Because it’s a different design, there are actually no cages to flip, so nothing can be done to these axles to make them shorter. Thankfully they fit as is and are just that little bit shorter than the OEM axles. So, Don’t bother popping the grease cap, as you’ll just find nothing to flip.

Cage Flipping Details

The cage flipping saves about 3.5mm of clearance (a little over 1/8”) no more.See the space between the cage and the axle end. This is the total amount of clearance we gain. After the cage is flipped, the axle end becomes the longest point.

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1)    Put your axle in a vise.

2)    Grab the cv end and raise and lower effectively hammering the cage into the grease cap. You don’t pry the cap out with tools, but rather use the internals of the axle to pop it out. Use care here, as the first one I did shot the cap off spraying grease all over my friend. Haha.

3)    Slide the outer cv cage down as far as you can. Mine inverted the seal a bit and stayed down there. You want to have access to the cage and balls.

4)    I taped up the CV to ensure that while I was wrestling to remove it, the balls wouldn’t go flying into the dirt and grime on my floor. In hindsight however, the balls are a fairly tight fit and won't come out without a little persuasion. I found that out when I ended up rebuilding the whole axle as I didn't like the consistency of the grease and realized I shouldn't have mixed so much redline synthetic grease with the stock stuff. Probably for the best.

5)    Take off the C-clip

6)    Slide the entire cage off the axle. It may be a little stuck, so do what you can to pop it off (small plastic mallet, etc). 

7)    Turn the cage assembly around, and reinstall…should look like picture on the right. (I also ground the axle down, but this is not needed and you risk ruining the hardened temper if you don’t do it slow). 

😎    You may at this point repack the unit with grease (compatible) as best you can.

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I ended up grinding the end of the axle a little as I wasn't sure how much room I would need. Not necessary however and perhaps can risk ruining the surface hardening of the splines if you're not careful.

 

Rockauto axles are a lot thinner too:

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Grinding Stub Axle

For the low profile adapter to provide all the clearance we need, you actually need to grind the stock stub axle down a little. 

 

See the nub sticking out from the flange? It would dig into our grease cap if we didn’t grind it off, so grind it down till it’s flush with the flange (use a straightedge to check for high points). There should be plenty of the staked portion of the nut still to keep that nut tight.

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Flange Welding  and Preparation

We ran into an issue after welding up our first set of adapters; the square flange on the Datsun hub is actually not very concentric. That is, the square doesn’t run true to the center of the axle assembly. Therefore if you just slip the adapter onto the square flange as best you can and go from there you may end up with it rotating on a ‘wobble’. I think we had something like 0.040" of runout by centering the adapter on the square portion of the flange.

So it takes a little time and thought to make the flange concentric with the rotational axis. A dial gauge is ideal, but you could get it pretty close if you're good with some calipers:

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The stock centering hole can be pretty rough so it's tough to get it dead nuts. Plus when you tack it can pull the flange to one side. I recommend to shim it with paper or even shim stock if you have it, prior to tacking. Also, if you tack with the 309, it has enough give that you can give it a few whacks with the hammer to shift it a few thou one way or another.

 

We ended up making a few more sets and didn't like all the fiddling, so we made a cool jig that locates the flange on both the stock axle ring and the new flange ring. Plus it bolts down to keep it flat to avoid warping. I could probably rent this thing out to anyone doing the job provided the leave a deposit and pay for shipping and such:

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Welding

The machinist friend who made these with me said if we are going to sell any of these things, people gotta Tig weld them. Whether or not that's essential, I don't know, but I'll take his advice. It at least assures the welder has some experience and will know how to handle welding to cast. (remember the stock companion flange is from a cast piece). He said to use 309 filler.

 

 

After all is said and done, these axles can be used without worry. I'm not sure how deep the MM flanges are or any others for that matter, but I do know its a pretty tight fit even with grinding the stub and flipping the cage. Although more clearance is had when the car rests on it's weight, so success can vary.

 

Here's a few more shots of when we made both the first set and a bigger batch.

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To machine the backside, we cut the profile into those blank set of aluminum jaws.

 

Here's the test fit on the driverside with the cage flipped and the axle slightly ground down. You can see from the amount of clearance, I didn't need to grind the axle down:

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Here's additional shot of one customer who had old school guy MIG weld it. If you can mig like this, pre-heat and avoid cracks, keep the flange centered, and keep it flat (bolted down), then it would probably work too:

 

 

Sorry this is such a jumble of info...hope some find it useful. Happy to answer any questions.

 

Cheers,


Mark

 

 

 

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Edited by mtnickel
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Clarifying one piece of info.... aside from the axles being thinner, the Rock Auto CVs would basically just allow you to weld in the adapters, grind down the stub axle, and bold them right in? 

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2 minutes ago, Zetsaz said:

Clarifying one piece of info.... aside from the axles being thinner, the Rock Auto CVs would basically just allow you to weld in the adapters, grind down the stub axle, and bold them right in? 

Correct. It’s close on passenger side but fits and can be installed even at full droop where it’s tightest. 

 

It would be quite tricky to Center the flanges with the flange in the car. You’d have to remove the stock flange to weld it properly. Ideal to have strut out too so you can rotate it and check for runout before welding. 

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Great information & pictures. One thing I have wondered, if you switch to z31 stub axles can you eliminate the need for adapters & just bolt in the z31 cv's? I have seen some posts that say the z31 stub axles can be installed in s30 struts if the strut is bored out a small amount. I plan on doing a CV swap & going 5 lug when my car is closer to being ready & am weighing my options.

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4 hours ago, caperix said:

Great information & pictures. One thing I have wondered, if you switch to z31 stub axles can you eliminate the need for adapters & just bolt in the z31 cv's? I have seen some posts that say the z31 stub axles can be installed in s30 struts if the strut is bored out a small amount. I plan on doing a CV swap & going 5 lug when my car is closer to being ready & am weighing my options.

No info on that. 

By that point you're custom machining the hub, doing a 5 bolt conversion all around, etc. Also finding the z31t stubs would be just as hard, if not more difficult than finding the axles. If you have some to measure, you could probably figure that out pretty quick though. 

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I purchased these a couple weeks ago and rented the jig. Super easy to do and I had a buddy Tig it all together for me. I'm using the 88 SS diff and axles. After flipping the cages the driverside fit perfect. The passenger side has a bit of bind at full droop. I'm unsure if its slightly longer than the standard z31t axles. Mark has been extremely helpful and I wouldn't hesitate purchasing from him.

 

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We’ll have to figure out whether it’s the SS axles or some other discrepancy maybe between 240 and 280 models. 

 

One thing I overlooked is that my rear struts are sectioned about 1”, so my “full droop” may not be as long as others. But I thought the 260/280z sat higher with a bigger isolator which would somewhat negate that. I’ll see how other regular nonVlsd guys fare. So far no other complaints. 

 

Thanks for the post!

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