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Discussion Starter · #1 ·
I've decided the .72 overdrive 5th gear of my T5 is a bit too tall. I could live with that except that decelerating in 5th gear often puts the car in the 2,300-2,500 range that sets up a resonant exhaust noise through the chassis. I've made it a bit better by getting rid of a straight through glass-pack muffler that sat just under the passenger's seat, but I think raising the rpm slightly would take care of the rest of it and improve the driving experience too.

I just had the rear-end rebuilt in my AMC Javelin for about $800. It didn't need new gears, but got new axles, seals and wheel bearings. I can't imagine that the Opel rear diff would be that much different, except perhaps for the cost of the new ring and pinion.

I understand that to go from 3.44 to 3.67 or 3.89 I either need a new carrier or a 4 mm spacer. Are these still available?

Does anyone in the U.S. sell the ring and pinion, or should I just plan on finding a used Manta set (or maybe the whole diff?)

TIA!
 

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What is your tire size ? Lower tires have the same effect as a numerically higher ratio. For example, 185/70 13 tires with a 3,89:1 ratio is about equal to 185/60 13 tires with a 3,67:1 ratio.
 

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Discussion Starter · #4 ·
So I guess I'd need a new carrier if I used the 3.90 Isuzu Impulse ring and pinion too, is that correct?

Are there any other bolt-on options for raising the GT rear axle ratio without a bunch of fabrication?
 

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Discussion Starter · #6 ·
Thanks. Would this be the same spacer ring that I'd use for an Opel 3.89 gear?
 

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Opel Key Master
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Why not just have an .86 overdrive put in the T5? It would be easier, and a lot cheaper. And if you don't have the setup tools for the rear end, you have a lot more work to do or pay someone to set it up, and that is iffy.
 

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Discussion Starter · #9 ·
Why not just have an .86 overdrive put in the T5? It would be easier, and a lot cheaper. And if you don't have the setup tools for the rear end, you have a lot more work to do or pay someone to set it up, and that is iffy.
I did consider that, but it seemed to me that removing the rear diff was a lot easier than removing the T5, and beside the rear end has probably 130k+miles on it so I figured replacing gears and bearings wouldn't hurt. I would have a shop do the gear changes for me in either case but can remove either the rear diff or T5 from the car myself.
 

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Opel Rallier since 1977
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This needs to be explained more:

You don't just remove and then re-install the rear gear carrier. You should pull the whole rear axle (though I am sure it has been done in the car, but I would nto do it that way; there is too much precsion work to do.)
  1. You have to adjust the shims behind the side bearings on the carrier to laterally locate the ring gear exactly for proper R&P gear mesh; if that is not done right, then the R&P wear can be rapid, you can get clunking and gear howling, and so on. As you do this part of the setup, if you miss the shim pack thickness on either side and the gear mesh is not right, then you have to pull the carrier out and changes the shims..... and you have to pull the side bearings off of the gear carrier each time you adjust the shim packs. It takes a certain thin jawed puller and then a press to R&R the side bearings.
  2. There are also pinion shims to adjust the pinion depth in the diff housing, but those are not so hard to change out as the side bearing shims.
  3. AFAIK, the shims are not easy to find. But others may have a good source on those. They might be common to other axles, and a shop might have them.
  4. And finally, there is a proper crush sleeve for the pinion installation; IIRC some substitutes have been made, but that part is hard to find.
  5. It is a very time consuming process compared to other rear axles, and your shop needs to know this and be willing and able to do it properly, and have the right tools.
(FWIW, I've done all this Opel rear gear setup and so when going for a new rally car rear axle, I went with a Toyota pickup rear axle! It beats the stock Opel for strength, cheap gears, and quick rear gear changes. But that is not in your scope of work....!)

Sooooo.... Keith is steering you right... .the OD gear swap in the T5 is easier.... faaaar easier. And I do see your point of having the rear axle rebuilt, but still, be sure you understand what the rear gear change entails.

BTW, the standard Manta gearset is 3.44 except for the Manta Rallye, which is 3.67.
 

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Discussion Starter · #12 ·
Thanks for the details. I've had rear diffs rebuilt before... had no idea that Opels were more difficult than others. Was thinking they were similar in design to Chevy.

Anyway, going from 3.44 to 3.67 requires a new carrier or shims, is that correct?
 

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The diff is just like most other Salisbury type diffs concerning rebuild. The shimming process can be made a little bit easier by enlarging the holes in a pair of old carrier bearings just enough that they slide on/off without a puller.
Changing the 3,44:1 gears to 3,67:1 and numerically higher always requires either a new carrier or the spacer ring.
 

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The shimming process can be made a little bit easier by enlarging the holes in a pair of old carrier bearings just enough that they slide on/off without a puller.
Changing the 3,44:1 gears to 3,67:1 and numerically higher always requires either a new carrier or the spacer ring.
Dang I never thought of that! LOL But, it seems like a problem to try that, as an older set of bearings will have wear in it and so the shim packs would seemingly end up a bit thicker than needed with new bearings. And even new bearings (and races) may have thickness tolerances between each set. So to make this work, it seems like you would want to measure the stacked height of the old bearing+new race and compare to the stacked height of the new bearing+race, and compensate if needed. Sounds like it could be done....

Thanks for the details. I've had rear diffs rebuilt before... had no idea that Opels were more difficult than others. Was thinking they were similar in design to Chevy.

Anyway, going from 3.44 to 3.67 requires a new carrier or shims, is that correct?
Cam, the shimming process is part of put in ANY new R&P set in ANY axle housing, and applies to a lot of different types/brands. It is done to compensate for how the pinon gear, and then the carrier and ring gear, set in your specific axle housing and carrier and all their machining tolerances. The end goal is to set the pinion depth, and also the ring gear's side-to-side setting, to get the right mesh pattern between the gear teeth. The FSM has procedures to make it easier, but you still have some aspects that are not as easy as some US axles, and the ready tools are not common-place in the US. Having to do this in the actual full axle housing makes this a bit more trouble (compared to the 3rd member axle type, where the hole R&P is on a removable cast housing that bolts into the main axle housing).

The different carrier, or the spacer under the ring gear, is a separate matter, and has to be the right parts combination to get the ring gear in the approximately right location. Then the shimming and gear setup process can be done.

Hey, just take an Opel manual and some info to your local axle shop and see what they say... they may be fine with such stuff. But you don't want a surprise, with someone just faking it and telling you it 'went fine'....
 

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Opel Key Master
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What year rear differential do you have? Or what style, early with metal vent, roller bearings press into diff, and axle is a C Clip style? Or later with plastic vent, bearings press onto axles? I have a 3.67 gear set with shims and all, so all they would have to setup is the pinion pre-load. But mine is from an early axle. Also I have some YouTube videos of setting up Opel differentials....no the tools are not available for the borrowing...lol
 

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Discussion Starter · #16 ·
Mine's an early axle with C-clips. Car was made March 1970 as I recall. How much do you want for the gear set, Keith?
 
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