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Gear Ratios??

B

bcshaner

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Hello
I am new to this the world of rearends, lol. So my question may seem simple to you but is confusing to me. I have a 89 Blazer, full size, and it has a 10 bolt rear end diff and a 10 bolt front end diff. The rear end diff failed again for the second time. Both times the carrier end supports that hold the bearings, the driver side one broke off and that side of the carrier was no longer supported. After doing a little research I am finding out that this is a common problem with these "light duty" rear ends. My ring gear is stamped with 13 40 and this seems to mean 13 teeth on the pinion and 40 teeth on the ring rear. This appears to give me a ration of 3.076923. Most people i talk to are calling this a 308 ratio?? I guess they are rounding up. It was recommended to me to upgrade to a 12 or 14 bolt rearend so it does not fail again. I purchased a 12 bolt rearend complete and was told it was a 308. When I got it home and took it apart to drain oil before install I noticed these gears are stamped 14 43. This gives me a ratio of 3.0714285. Even though the seller was calling this a 308 it does not have the same ratio as the 10 bolt 13 40. So my question is this: Is it OK to have these two different ratios on a 4x4 or are they close enough? Does GM make a 12 bolt 13 40 that I should use instead? Are these two rear ends both essentially 308's? Hope this has an easy answer and I am just being too picky.
Thanks, BCshaner :confused:
 
Those ratios are close enough you will not notice any difference....

Look for a cheap 3/4 ton with the gears you want and go that route. Prolly hard to find a 3.08 14 bolt....

That 12b isn't much stronger if any than the 10b....
 
I agree with SUBFAN. Now's a better time than ever to upgrade. It may cost a little bit more after buying different wheels, but the end result is well worth it.

It sounds as if either the housing or an axleshaft is bent, causing the carrier to break twice in the same location.
 
Dry Pavement & Wrong Ratio

Perhaps these ratios are close but they are not the same. If I were to engage the front end and run on dry pavement on the hiway for a long distance the front wheels would turn 181 revolutions for every 182 revolutions of the rear tires with my 31 inch tires. This means an extra revolution on the rear tires every 1/3 mile. Over a 50 mile trip this is 150 extra revolutions. Surely this will cause some damage or at the very least excessive tire wear. Why do they have the 14 43 instead of a 13 40 that would be exactly the right ratio? Is it because they needed to add 3 extra teeth to the ring gear because of its larger diameter than the ring rear in the 10b rearend? Does the extra tooth on the pinion gear have a purpose. I am wondering why they did not just make a 12 bolt 13 40 with bigger teeth and keep the exact ratio. Did any Blazers or Burbs ever come from the factory with the 10b 13 40 on the front and the 12b 14 43 on the rear? Has anyone ever heard of anyone else ever doing this? Is it a common practice or am I doing something unusual that will only result in more trouble down the road. I have no desire to change the front end gears since they have never given me any trouble. I do not want the 14b rear diff because it is way too expensive and it requires mounting mods involving cutting and rewelding spring mounts. :doah:
Thanks Again, BCShaner
 
First off, you should never run your truck on dry pavement for long distances. There are variables such as you are calculating, and then there are discrepancies between the diameter of each tire, left to right, and front to rear. You can't wear them out equally. The difference in tire diameter is enough to cause damage to a vehicle driving down the highway in 4WD, much less a difference in gear ratio's.

That is why the 203 t-case had its own differential.

Take no offense, but you're thinking about this way too hard. Some Dodge trucks came from the factory with a 4.09 front and a 4.11 rear. It was close enough to run, but far enough away for people to question.

Just be sure that whatever you put in there is somewhere around a 3.08 ratio. Unless it's custom built or a Dana axle, our trucks have only the following available gear ratios: 2.73, 3.08, 3.42, 3.73, 4.10, 4.56, 4.88, and 5.13.
 
Hey BC guy. Those ratio's are close enough. Don't drive in four wheel drive on dry pavement. As for your 10 bolt there is something wrong if you've had that problem twice. Most guys don't trust 10 bolts but I run a 10 bolt front and rear with a Detroit in the rear and a lock Right in the front on 35" tires. I have broken axles front and rear but I bought all new Yukon axles for fairly cheap through Randy's Ring and Pinion and have had no problems since. I do however cary an extra rear axle on the trail.
Signed,
Fellow BC guy
 
No, the 241 doesn't. The only t-case offered in our trucks with a differential in it was the New Process 203. GM used it in its trucks during the 70's. It was a large, cast iron beast. It was the predecessor to the NP208 and NP241 in that it was chain driven, but the actual reduction portion was a small range box mated to the front of the case, whereas the 205, 208, and 241 are all in one single case.
 
Thanks

Thanks to everyone who took the time to respond. You have thouroughly answered my questions. I can go ahead and install this new rearend now and not worry that it is the wrong one.
Thanks, BCShaner :wink1:
 
A 14b out of a 3/4 ton would be a direct swap....

Seriously, check the price differences on the 14b and 8 lub conversion versus just redoing that rear end. With a bit of searching, it might surprise you how close the cost is. Keep into account the probability that you might have to replace that 12b again, vs normal maintenance on the 14b....
 
Locking Diff

Well I almost got the new rearend in tonight after work but the U bolts were giving me a lot of grief. I removed the dif cover to remove the pinion pin so I could remove my rear axles and just put my original brake backing plate right on the new axle with all the brake hardware intact and I discovered that this dif is a locking dif. Is this a good or a bad thing? There seems to be a lot of small moving parts in there that were not in the simpler 10b standard rear end. How do these GM locking difs stand up to normal use? Are they the ones that I heard can "grenade"? Do I need to know anything special to make sure it works right? When does it lock up?

BCShaner :rolleyes:
 
It pretty much locks up when it wants to and blows up when it wants. That's why we call it the "Gov-bomb".

To ensure it doesn't blow up on you, remove the counterweights and springs. It will now be just a plain-jane open diff.
 
The way the "Gov-Lock" works is that when one tire spins it flings a counterweight outwards and then engages (or locks) the other side. This means that it always engages under a shock load (bad idea IMO)...

12 bolt diff's are about the same strength as the 10 bolt you just replaced. In some places the 12 bolt is weaker than a 10 bolt (pinion most notably) Add the Gov-lock to the mix and I think you'll be doing this job again sometime soon.

Regarding ratio discreptancies. Common factory discreptancies include 2.73/2.76, 3.07/3.08, 4.10/4.11, 4.55/4.56/4.57

Within 1-2% is quite fine and commonly found.

Regarding 14 bolt swap, a 14 bolt from a 3/4 ton 4x4 is pretty much a direct swap. Spring perches are in the right spot, and so are shock mounts. They are commonly sold with matching 3/4 ton front ends so ratios match, and that is a direct swap as well. If you have new tires, all you'll need is a set of inexpensive 15" 8 lug rims. If you need tires anyways you could go with some 16" rims.

Lastly, where abouts in BC are you?

Rene
 
bshaner,


It's a little late for this info (since you've already got a replacement) but the general rule of thumb is that your TIRE SIZE dictates the axle you need to use for best strength and longevity.

I don't recall seeing a tire size listed, so I'll assume you're trying to accomodate 35" tires (or less)......at least I hope you are with a ratio of 3.08!!!

As tire diameters get in the 36"+ range, you really need to be moving toward a 14BFF or other 1-Ton rated axle. There's a LOT of rotating mass and a 10-bolt really isn't going to hold up to the extra load. Everyone says 10-bolts are crap, but remember that the factory never intended for them to hold up to the massive tires people keep bolting to them!!! :wink1:
 
the front wheels would turn 181 revolutions for every 182 revolutions of the rear tires with my 31 inch tires.
Click to expand...


Looks like 31's to me.

Rene
 
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