differential Ring and Pinion treatments to increase strength

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Vkgallo
Posts: 20
Joined: Sun Feb 08, 2015 2:43 pm
Your car is a: 1981 Fiat Spider

differential Ring and Pinion treatments to increase strength

Postby Vkgallo » Sun Oct 16, 2016 5:08 pm

Hi, I am in the process of rebuilding my pumpkin for my 70-78 rear axle. I have decided to rebuild and possibly add the limited slip/positraction differential newly available from autoricambi. I currently have a 4.1 gear ratio in my rear and was planning on replacing with a 3.9. I have heard that the gears are generally weak (both OEM and aftermarket) and I suspect positraction would only make it more prone to failure. Have a few questions:

1. Is one ring and pinion brand stronger than another? If so what?

2. Is there any other companies that make a positraction conversion?

2. Is there a company that anyone has used or heard about to heat treat/cyro the gears to make it stronger at a reasonable cost. I did get a quote from a cyro treatment company and they wanted like 800 bucks to treat the ring and pinion and I thought that is crazy.

Thanks-Vince

vandor
Posts: 3997
Joined: Sat May 23, 2009 1:23 pm
Your car is a: 1971 124 Spider
Location: Texas, USA
Contact:

Re: differential Ring and Pinion treatments to increase strength

Postby vandor » Sun Oct 16, 2016 5:40 pm

I checked into cyro treatment ~10 years ago and it seems it was a lot less thna $800. I ended up not doing it tho.

I've had the 3.9 gears in my Spider for ~10 years now, have used it with 3 different types of LSDs. One time the pinion nut came loose and I thought that was the end, but after re-tightening it all was ok. That was 2 years ago and it's still going strong.
Csaba
'71 124 Spider, much modified
'17 124 Abarth, silver
http://italiancarclub.com/csaba/
Co-owner of the best dang Fiat parts place in town

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divace73
Posts: 1380
Joined: Sun Oct 17, 2010 5:59 am
Your car is a: 1980 Fiat 124 Spider Silver
Location: Sydney, Australia

Re: differential Ring and Pinion treatments to increase strength

Postby divace73 » Sun Oct 16, 2016 8:25 pm

I have a 3.9 from a LADA and the torque bias center in mine. I have tracked the car and all was fine. You'll like the diff Autoricambi have.

I think if you drive normally and maintain the diff all will be okay. I do drive mine hard but DO NOT do burnouts or big rev and dumps.

If you want to do doughnuts and burnouts then choose another car
Cheers David
-=1980 silver Fiat 124 Spider=-
If you want to see pics of my car (and other random stuff) >>click here<< OR
see my >>You tube channel<<

djape1977
Posts: 986
Joined: Sat Apr 21, 2012 6:08 pm
Your car is a: 1970 fiat 124bc
Location: Belgrade, Serbia, eastern Europe

Re: differential Ring and Pinion treatments to increase strength

Postby djape1977 » Mon Oct 17, 2016 12:16 pm

have your diff checked, assembled and set up by a specialist and forget strenghtening of gears.
if you plan to upgrade to over 150hp, get a rear axle off fiat 132/argenta. those are good for 400hp easy

Nut124
Posts: 19
Joined: Wed Oct 25, 2017 6:39 pm
Your car is a: 1978 124 Spider 1800

Re: differential Ring and Pinion treatments to increase strength

Postby Nut124 » Mon Dec 04, 2017 6:07 pm

Vkgallo wrote: ...I have heard that the gears are generally weak (both OEM and aftermarket) and I suspect positraction would only make it more prone to failure. Have a few questions:

Thanks-Vince


Vince, what kind of failure reports have you heard?

I have broken two -78 pumpkins in 20 years of driving my 1800 hard. Both times at hard launch.

My observation is that there are two weaknesses:
1. The diff carrier small side bearing journal on the ring gear side shears off. This is made of weak cast iron.
2. The diff carrier bearing outer split journals are too loose and allow the carrier bearing outer race to spin.

In one of my failures, the gears were intact and usable. The carrier bearing outer races had spun big way.

I think item 2 above leads to incorrect carrier preload and results in out of spec gear alignment under load until the inner journal shears off.

I have built a new pumpkin with Lada gears and the Russian LSD. I believe the the LSD solves problem #1 as it is made of steel, not cast iron.

I tried to slightly reduce the split outer journal ID to prevent the race from spinning. We'll see.

Nut124

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