mx592

iTrader: (2)

OK so I have done a couple of final drive and diff changes on my G35 and on that car I always reused the original pinion shim and never had any problems (3.6 to 3.9 on OEM VLSD, then back to 3.6 with a Quaife LSD). I knew it was a bit of a gamble at the time, but luckily it worked out. I did have to buy shims to adjust preload and lash on the side bearings because annoyingly the Nissan diff doesn't have screw adjusters like the Miata does (yay!)

Anyway this is the first time I have swapped gears in the Miata and I am going from the stock 4.1 to a 3.9 because I blew my 5 speed and I am swapping in a 6 speed. After reading the service manual and doing some searches it came to my attention that the pinon depth shim can theoretically be calculated without the special Mazda "SST" tool, IF you know the thickness of the old shim and compare the markings on the new pinion and the old pinon. The only numerical marking on the old pinon gear is a "3". The only (alpha)numeric marking on the new pinion is a "K9". Per the service manual, shims are available in .03mm increments. The old shim measures 3.17mm. Neither of the markings, 3 or K9, appear in the table that is printed in the service manual . Also Mazda doesn't state what the "nominal" shim thickness is (the one you are theoretically starting with when you use the SST...). Now assuming the markings correspond to absolute values of 0.03mm and 0.09mm I can see arguments for at least four different thicknesses for the "new" shim:

1. The nominal starting shim is 3.14mm, and the pinion markings 3 and K9 represent +.03mm and +.09mm. So the new shim should be .06mm greater (+0.09 minus +0.03 = +0.06) than the old one, which would equal 3.23mm
2. The nominal starting shim is 3.20mm, and the pinion markings 3 and K9 represent -.03mm and -.09mm . So the new shim should be .06mm less (-0.09 minus -0.03 = -0.06) than the old one, which would equal 3.11mm
3. The nominal starting shim is 3.20mm, but the "K" stands for "+", which means 3 and K9 represent -.03mm and +.09mm. So the new shim should be 0.12mm greater (+0.09 minus -0.03 = +0.12) than the old one, which would equal 3.29mm
4. The nominal starting shim is 3.14mm, but the "K" stands for "-", which means 3 and K9 represent +.03mm and -.09mm. So the new shim should be 0.12mm less (-0.09 minus +0.03 = -0.12) than the old one, which would equal 3.02mm

Does anyone know what the "K" stands for? Does anyone know if the nominal starting shim is 3.14mm or 3.20mm?

If I was a betting man I would bet on case #3 because I think it is the most likely...it gives a reason for why the K is there and it assumes a starting shim thickness of 3.20mm which is a nice even number and is closer to the median of the available shims. Case #4 must be false because they don't even offer a shim that thin. I am going to order all three thicknesses just in case, but I would rather not press the pinion bearing on/off three times to find the correct tooth contact via trial and error if I don't have to...

Mazdated

I can't answer your question but, when I replaced gears in the short pinion 7 inch (exactly as the miata) from my 84 rx7 and did many searches on rx7club.com I found the general rule of thumb (they use) is that the pinions are machined to a tighter tolerance than the housing, so shims are paired to the housing not the pinion. Thus, reusing the shim originally from the housing you are using should get you the proper depth. I never saw a case of somone needing to calc depth.

I did this and checked depth visually with marking compound and it was centered well. Next time I swap on a mazda I'll try the original pinion shim. If it doesn't work I'll mic it and order thicker or thinner ones.

Subbed for more in-depth answer.

bbundy

Always reused the shim that went with the housing and never had a problem myself. it seems ring and pinions set and bearings are all machined to very tight tolerance. May be some variability in the housing machining but if you re-use the shim that came with the housing it always comes out right. I don't even try to measure it. just verify the tooth engagement is fairly flat across the tooth faces by smearing some assembly lube on them to check how it smears out as the teeth mesh. backlash also set by feel. So far I haven't ever had one that I setup fail. done it like 10 times or so.

ctdrftna

iTrader: (1)

Same here, when i did my gear swap i reused the shim, and switched to a solid distance spacer (removed the crush spacer) for preload. I set back lash with the indicator and i matched the carrier bearing preload to the cap distance before i took it apart. Checked the pattern with the special sauce and all worked well

Leafy

iTrader: (1)

Same here. I think the keep all the same shims and crush sleeve recommendation is also a Joe P thing, for your same reasoning. I did have some issues with the solid spacer and shims getting the pre-load correct, I needed like a shim half as much as the smallest shim in the shim pack that came with it to get a proper setup. I ended up just cutting one out of shim stock from work.

Mazdated

Those of you using the solid sleeve. After searching I see that it's stronger than the crush sleeve. But what I can't find is when is the oem sleeve not strong enough? Or is it just baller status.

Just asking because the 7 inch is generally considered "bulletproof edit: within reason", and not "bulletproof with solid sleeve installed"

Insite please?

Madjak

It might be too late but if you lock off the diff by jamming a rag between the housing and the ring you can wiggle the tail shaft flange to get a feel for the backlash and how much movement there is.

I figured its worth learning to set a diff yourself. If you get it wrong the diff will whine and if it does stop using it immediately and try again.

When switching the R & P as with the above posts I found that keeping the shim with the housing worked fine. I replaced all the bearings with new ones... getting the inner race off the pinion shaft is a bit of fun but can be done on a bench with cold chisels and screw drivers with packers.

I ended up reusing the existing crush spacer and just doing up the nylock nut as tight as I could. Measuring the backlash first time with a dial gauge was smack in the middle of the target range.

The only other hard part was setting the preload on the caps on the side bearings. I didn't have the right sized micrometer so I just set some 'c' calipers via a vernier to the correct size then measured across the caps with that. Once again if you feel how tight they are before pulling apart the diff you can get an idea on the torque required. I ended up setting these on the looser side of spec in the hope I gained some power due to less friction.

Leafy

iTrader: (1)

It was easier to deal with than a crush sleeve.

Mazdated

Really? The crush sleeve isn't that hard to deal with. And don't you have to mess with shims on the solid sleeve?

ctdrftna

iTrader: (1)

You can set the preload torque right on the money, and it's just a peice of mind when you dropped it a $600 gear set and $800+ diff

Leafy

iTrader: (1)

Crush sleeve is scary. You **** up and you're out the price is the sleeve and 2 days to get a new one.

bbundy

I just re-use the crush sleeve. Ignore the torque spec on the pinion bolt and just not tighten it to where the pinion bearings are too hard to turn. if you way over tighten it will crush the crush sleeve more and make it so very little torque makes the bearings too tight which is probably not good. It is a nylon lock nut I have never seen one move even with very little torque applied. wouldn't be surprised if it would work Ok without a sleeve at all but the crush sleeve ads some spring rate and added compression stiffness upping the torque level on the nut to get to the right range bearing preload.

mx592

iTrader: (2)

Sweet, looks like I will be reusing the stock shim.

Ordered new bearings and seals, as well as some pinion shims (just in case), two crush sleeves, a new nylock nut, gear marking compound, Amsoil SVG oil and a 7-8" mic (real nice used Mitutoyo I got on eBay). I am going to bead blast the housing and give it a new coat of paint before I put it all back together!

mx592

iTrader: (2)

I don't see how a solid sleeve is a good idea. In order to actually be able to set the preload on the bearings, wouldn't the sleeve have to be shorter than the distance between the inner races, and thus completely pointless? Otherwise you will just be squeezing the bearing races against the solid sleeve and you would never be able to get the bearing preload to increase after a certain point. Isn't the whole point of a crush sleeve to adapt to the distance between the races when the bearings are at the proper preload and in the process provide some constant spring force against the inner races? The bellows shape WILL act like a very stiff spring, but a solid spacer not so much...

The only way I could see it working is if you machine the solid sleeve length equal to your previously-crushed crush sleeve, assemble the unit, measure preload, remove and machine a thousandth off the sleeve, and repeat until you achieve the proper preload. Which sounds like way more of a PITA than a new $12 crush sleeve.

patsmx5

iTrader: (16)

By design, the crush sleeve will crush when too much load is applied. It will bend (but spring back to normal length) under load. A solid spacer is much stiffer, and much stronger, which keeps the gears in full contact even under heavy load, improving wear and strength of the gear set.

Leafy

iTrader: (1)

And you set the preload with shims (different from the shim the op asked about).