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I know this subject has been hashed through a million times, and I expect to get into some heated arguments in this thread, but I would like to hear from people who have acctually used an FJ80 high pinion diff w/ reverse cut gears in the rear of your Toyota, and what were the results?

I contend that the weakness issue with a high pinnion diff in the rear is another urban ledgend, and if I can formulate my argument into written words, I sure like to prove it.

I sat for hours arguing with a buddy of mine last night. I was arguing the weakness point and he was arguing the opposite. Turns out he was right! The reverse cut gears in a high pinion are theoreticaly no weaker in the rear than normal cut gears in a low pinion in the rear, unless there is some other reason than just being "reverse cut".

I would like to hear from some folks who have tried it before I "try" to go into my explanation.
 

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I have sold several Hp diffs for use in the rear and I have warned people that it's going to be a problem. I usually get a call back that the R&P failed.

Nearly every unit built for the front holds up well for many years, but I am seeing about an 80% failure rate on rear units. If that's is ok with you give it a try.

Marlin tried it twice and broke both times. Now he runs a V6 in the rear.
 

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hopefully someone will chime in later with better info but maybe this will at least give you something to search for.

i was under the impression that they were weak not just because of being reverse cut, but because with that setup you end up putting the power to the coast side of the gear teeth instead of the drive side. thats about as much as i know.
 

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I run a HP diff in the rear of my zuk. The zuk is a trail only vehicle. I have to over-fill the rear diff to make it work; to fill the diff, I pull the elocker motor and fill there.

I've already burned up one set of pinion bearings, and I am more afraid of that than killing the R&P gear.







 

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kjmcdaniel said:
I know this subject has been hashed through a million times, and I expect to get into some heated arguments in this thread, but I would like to hear from people who have acctually used an FJ80 high pinion diff w/ reverse cut gears in the rear of your Toyota, and what were the results?

I contend that the weakness issue with a high pinnion diff in the rear is another urban ledgend, and if I can formulate my argument into written words, I sure like to prove it.

I sat for hours arguing with a buddy of mine last night. I was arguing the weakness point and he was arguing the opposite. Turns out he was right! The reverse cut gears in a high pinion are theoreticaly no weaker in the rear than normal cut gears in a low pinion in the rear, unless there is some other reason than just being "reverse cut".

I would like to hear from some folks who have tried it before I "try" to go into my explanation.

it's pure fact. pyisics(sp) don't lie, it's how the R & P is configuired, 20% weaker in rear, 20% stronger in front, then standard setup. I guess if you like to argue & don't belive in math, we won't convince you:D take it from Marlins shop, they see more R&Ps then we ever will, just ask any diff shop.
 

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kjmcdaniel said:
Chris,

What is the theory as to why so many break in the rear?
Drive side vrs coast side.

The high pinion is reverse cut, so when it's in the front and your goiing forward your on the drive side of the gear. Use it in the rear and your on the coast side.

Same is true with 4Cyl diffs. The reasion we see so many front 4cyl r&p failures is that diff is setup and cut for use in the rear not the front. The 4cyl is running on the coast side of the gear when used in front.
 

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Chris Geiger said:
Drive side vrs coast side.

The high pinion is reverse cut, so when it's in the front and your goiing forward your on the drive side of the gear. Use it in the rear and your on the coast side.

Same is true with 4Cyl diffs. The reasion we see so many front 4cyl r&p failures is that diff is setup and cut for use in the rear not the front. The 4cyl is running on the coast side of the gear when used in front.
Ok stupid question time, one thing I've never understood is why can't someone cut a ring and pinion for a low pinion 4cyl front end that'll drive on the right side of the teeth? Should just be a matter of making a mirror image of a regular R&P right? I don't see why the pinon would have to be raised up to get the gears meshing properly. :confused:
 

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like chris said, going to be runnig on the coast side, which is weaker, The rear takes alot of abuse. So it will not last. The front is the same way unless you run a high pinion. but it dosent see the forces that the rear does, so you shouldnt see much of a problem up front with any type. You cant just cut reverse gears for a standard pinion. The pinion comes throught the bottom on a standard gear, and thru the top on a high pinion, which is why a high pinion diff MUST have reverse cut gears. You would have to flip the 3rd over to get the pinion on the other side and run high pinion gears. which I dont even know for sure is possible, since I am probably missing something.
Like the pinion would have to come out on the other side of the ring gear(left to right) to turn in the proper direction . something like that. ????
 

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IIRC the oil slinger turns the wrong direction too so that it why you go through pinion bearings? MAybe for a trail only rig but I wouldn't do it in anything that sees any miles at all...
 

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Overkiller said:
Ok stupid question time, one thing I've never understood is why can't someone cut a ring and pinion for a low pinion 4cyl front end that'll drive on the right side of the teeth? Should just be a matter of making a mirror image of a regular R&P right? I don't see why the pinon would have to be raised up to get the gears meshing properly. :confused:
Why would anyone spend time and money to do this? The High Pinion gives you more ground clearance and works great.
 

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Tim Florian said:
Why would anyone spend time and money to do this? The High Pinion gives you more ground clearance and works great.
I just asked if it could be done. I don't even run an 8" up front anymore. But lets say for arguements sake I have a truck running an 8" up front with all the bling bells and whistles, 300M birfs, unobtainium axle shafts, driveflanges, ect. and I'm now at the point where my low pinion 4cyl ring and pinion are the weak point. Now I could just swap in a high pinon unit and be done but unfortunately my dying grandfather gave this 4cyl 3rd member on his deathbed and I simply can't part with it. To make matters worse I can't run the highpinion diff due to oil pan clearance and a life long fear of clowns and FJ80s. Now given those set of circumstances and my huge trust fund is it theoretically possible to get a set of gears for a 4cyl 8" that'll run on the drive side when mounted in the front axle and going forward?
 

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The diff would have to be run upside down, so the pinion is ontop and not on the bottom. They only make V6 reverse rotation gears. so it would have to be a V6 unit upside down.

It does run on the drive side now, just only in reverse. Still not sure if that is all that is needed. I can't seem to visualize it.
 

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Chris Geiger said:
Drive side vrs coast side.

The high pinion is reverse cut, so when it's in the front and your goiing forward your on the drive side of the gear. Use it in the rear and your on the coast side.

Same is true with 4Cyl diffs. The reasion we see so many front 4cyl r&p failures is that diff is setup and cut for use in the rear not the front. The 4cyl is running on the coast side of the gear when used in front.
What about factory solid axle trucks? Is the front 3rd actually a rear 3rd?
What about running a V6 diff in the front - when moving foward, isn't the pinion turning on the coast side of the ring too?
 

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ease of assembly from the factory. yes they are actually strongest in the rear. THere is no difference in 4 and 6 cylinder diff applications, unless high pinion. (thus the pinion hits the ring high in the diff).
 
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