Maybe the bushing would be thin enough that the case depth would be deep enough for it to be through hardened easily.

Interesting point, are you saying to swap the bushing to a new pinion for a rebuild? I figured it would need to be replaced with the pinion.

I shot them an email but no response yet.

 

Are there any new developments with this? I'm running a cd009 transmission on a 20b. Before I go to 8.8, I was hoping to find an easier alternative.

 

Some additional details on differentials with the ~8” ring gear that may be suitable. You’ll need to confirm the ratio though as 3.909 was the most popular for some of these and the 3.727 ratio was less so. The only one I know for sure has 3.727 is the USDM B2600 2WD truck models. However some places consider it the same as the B2200 and that one has 3.909. The 4WD models have 4.30. I bought a NOS MPV differential for super cheap taking a chance and it has a 3.909 gear set. 3.727 was very rare for the MPV and likely not easily found.



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I still haven’t done anything on a pinion sleeve yet though, but in needs to get done within the next several months. Someone was in contact with Carlos earlier this year and he supposedly has some, but never heard anything further on it. Sent a message to that person yesterday to see if he ever connected, but not holding my breath.

 

I wonder if Jesse Prather has any input on this...

 

he no longer does rotary stuff and the Miatas FC etc use the smaller R&P gearset.

I pulled that list off an MX5 Miata pdf that somebody has online, but it matches everything else I’ve found and is a nice summary.

it’s all online really, it just takes some time and tenacity to pull it together, but eventually is likely to fade away over time. Hopefully it will help having it here.
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have to get moving on the pinion sleeve solution for this. My idea was to have a hardened sleeve with a flange on the rear about the same OD as the inner front bearing race, press that into bearing bore first, and then the sleeved bearing onto the pinion …

 

i built a big spreadsheet too. there are a lot of questions the miata people have too, to paraphrase, there is something about the Tosen that turns peoples minds into mush

 

I still owe you the information too, always something with a higher priority …

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i had forgotten all about that, the Torsen turned my mind into mush...

 

is it really going to be this simple? 🤔

https://www.mcmaster.com/product/96990A490

I need to double-check the bearing width against the length, but there’s a custom option as well though for tuning the ID, OD, and length as needed

https://www.mcmaster.com/product/8486A901

Rockwell C61 hardness
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finally made some measurements today

4.777 RX8 Pinion gear-end bearing surface OD measured 40.75mm (FD3 is same)

3.727 B2600 Pinion gear-end bearing surface OD measured 30.78mm (-9.97mm)

I’ll need to go refresh myself on press-fit clearance/tolerances before establishing the correct ID/OD dimensions and ordering the spacer bushing. .

For determining the spacer bushing length; from the back face of the pinion gear-head to the far end of the bearing machined mounting surface is ~40mm long. The positioning shim and bearing on the early 3.727 pinion is only ~30mm long from the back side of the pinion gear-head. The larger ID late model bearing is likely longer. So my suggestion is to go ahead and use a 40mm long bushing.

Comparison pics between the early year 3.727 R&P gears and late year 4.777 gears; 3.727 pinion gear is larger OD with thicker teeth, but the ring gear is thinner to compensate for the larger pinion gear OD:



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I ordered a custom-sized bushing from McMaster-Carr over a week ago, but the website states it may take up to 6 weeks for it to ship.

Mike, I also took pics of most of my gearsets, but have two more to do still. I should be emailing that to you in the not too distant future.
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so I received word from McM-C that the bushing is pushed out to June due to the all too common material supply availability issue excuse these days.

I only put it that way because there seem to be others offering custom sizes without issue. It’s not unique material or complex machined forging; it’s roughly a 35mm ID x 40mm OD x 40mm long piece of commonly available high strength heat-treated steel.

Going to try a different approach then.
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Any new news on this?
Now that my HP levels can support it, I'd like to move to longer gearing, especially since I'm running the lower ratio 5th gear in a JDM transmission.

 

I suppose it might be worth a try to see if there’s any interest in some taller rear gear ratios:

11p/41r = 3.727 **Mazda 1980s-90s ratio USDM 929-MPV-B2600i 2WD

11p/40r = 3.636 **never offered in the 8” Mazda P diff that I’m aware of, just a possible option

12p/43r = 3.583 **Mazda 1986 EUDM 929


initial inquiry requires qty. (100) minimum per ratio, so looking for lower volume source to weigh out cost vs qty
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I may be interested in the 3.636 and/or the 3.583. Trying to avoid having to do an 8.8 conversion when I get around to doing my dog ring 4 speed conversion a couple years down the road. (or whenever my OE box has had enough)

Perhaps it may be beneficial to advertise this to the rx-7 facebook group for greater exposure?

Keep us posted!

 

I would be interested in any of the three since they're very close numerically as long as the solution drops into an OEM FD diff and housing.

 

so as stated above, they can be made and would probably sell for somewhere in the $500-$600 USD range

either the 3.727 or 3.626 ratio, leaning toward 3.626

but it’s a sizable investment and I’m not sure there is enough potential volume to make the break-even point?
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Why bother when you can sell your stock diff, PPF, and axles, and then use that money to buy a Ford 8.8, upgraded LSD, and beefier axles for about a break-even? 8.8s usually come with either a 3.55 or 3.73 so if you pick the right one, you don't even need to buy a $200 Ford gear.

Spending $5-600hp on a custom gear ratio for the stock diff doesn't make sense when you'll still be limited to ~400whp by the weak cast iron diff housing breaking, PPF breaking, or axles snapping.

https://motoiq.com/project-fd-rx7-re...-differential/

 

I'm a little sceptical you could sell a stock diff and PPF for enough that an entire 8.8 conversion would only cost you $5-600 out of pocket. It would probably be half that just to get a shop to weld up the subframe with the new bracing.

 

What are you proposing be included in the $500-$600 price?

 

if you think it should be more than a new custom ring & pinion gear pair then this is already dead in the water.

Because that’s about the wholesale cost from Mazda on a new mass produced std r&p. Back in the day the price from Motorsports for the 5.1 was ~$1,400. The 5.1, 5.3, 5.8 ratios being sold on ebay now from Australia are ~$750 shipped to the USA.
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You're right, the "break-even" for the 8.8 swap (with an upgraded LSD) was when you consider buying a $1,200 LSD for the stock diff. So if you only considered this $5-600 custom ring gear and were going to keep the stock 350-400hp capable LSD, then the 8.8 swap would be a break-even if you also kept the stock 8.8" LSD and didn't upgrade it.

 

I've been getting along just fine with over 400@the wheels on track boost, and 500@the wheels on street boost with a stock torsen 4:10 for years. The stock trans though not so much. This would be an interesting option if I were to upgrade to a 370Z trans, you really want a 3.6 or 3.7 with that I think. $5-600 seems reasonable to avoid not needing to do a whole rear end conversion at the same time.

 

The 8.8 is actually pretty easy to do. I thought it was going to be a lot more difficult.

How many track days have you done on that setup and do you have a diff brace?

What is "over 400whp"? 401whp? 450whp? You're probably right on the edge where fatigue will set in at some point and force you to upgrade the diff, while losing nearly $1K on broken diff housings and PPFs.

I personally don't think it's worth the gamble or losing $1K from waiting until it's too late. Or losing $1,600 if you did buy that 3.6 gear before breaking the diff housing.