How strong/weak are the 1985 K10 axles?

[99-LS1-SS]

I'm looking to get an 1985 K10 truck and I'm hard on stuff. How weak are the axles and rearends on those trucks?

[Firebirdjones]

Should be an 8.5" 10 bolt.

If so, they use the same 1.625" pinion that the passenger car 12 bolts had (truck 12 bolts are smaller at 1.4") So the pinion is stout.

The ring gear diameter isn't much smaller than the 12 bolt (8.5 vs 8.875) so that's hardly an issue.

The axles are 28 spline. GM went 30 spline on the 8.5's starting around the 1988 model year, and that's an easy upgrade if you install a 30 spline center section.

Overall the 8.5" 10 bolts are fairly decent pieces (better than a truck 12 bolt because of the larger pinion)

Dave Henninger showed everyone the abuse they could take in his 8 second 160 mph camaro several years ago when they are built properly.

[99-LS1-SS]

So, if I'm reading what you wrote correctly, the pinion tends to be a weak(er) point on these trucks if they have the 1.4" pinion?

[Pyrotechnic]

Stock for stock 73-87 rear ends, the 12 bolt has stronger axles and a larger ring gear but a smaller pinion shaft. The 10 bolt has a larger pinion shaft, but a smaller ring gear and smaller axles.

Keep in mind, the Ford 9 inch has a pinion shaft even smaller than the truck 12 bolt! A lot of things play into how strong a given shaft will be, such as metallurgy, length, support bearing size, quantity and placement. Judging on diameter alone would be like judging rear end strength based on how many bolts hold the cover on. More/Larger does not always mean better. For a long time people were saying how weak the 10 bolts were compared to the 12 bolt, and now people are saying the 12 bolt is weak because it has a smaller pinion shaft. I think they are both good rear ends and people have gone fast with either one.

I think the bigger concern, especially on an offroad truck with big tires is axle breakage. Both the 10 and 12 bolt are C clip rears, so if you break an axle, it and the tire will leave the vehicle. If I had to choose stock rear ends for this application, I'd take the 12 bolt just for it's larger axle size.

My recommendation? Neither. Get a late model 14 bolt 6 lug and weld on perches and shock mounts. Lots of strength with stock parts without getting into the aftermarket.

[Firebirdjones]

The 14 bolt is a nice piece, larger pinion dia and larger ring gear. Still a 30 spline axle though, just slightly larger diameter. The outside retention in the full float is nice.

It's not the axles that bother me in the 10 and 12 bolts, because they are easily upgraded with aftermarket parts that are nearly unbreakable, and C-clip eliminator kits are easily adapted. It's that tiny truck 12 bolt pinion I have broken twice, and there is nothing I can do about the size of it since the housing is made around it. That's where the 8.5" 10 bolt shines.

Never understood why GM would make the 1.625 pinion for the passenger car 12 bolt and all the 8.5" 10 bolts and then make a truck 12 bolt with a 1.438" pinion, something we all know is destined (in most cases) to be a work horse from day one.

The reason the smaller dia pinion in the 9" is so successfull is that the Ford uses an extra pinion bearing support on the end of the teeth to reduce flex, which is needed anyway due to the hypoid offset of that rear, which also in turn is what give it it's strength.

[99-LS1-SS]

Who makes good axles and c-clip eliminator kits?

[Pyrotechnic]

14 bolt SF 6-lug & 14 bolt SF 8-lug (No different except for the lug count)
Ring Gear: 9.5"
OE Spline Count: 33
Shaft Diameter: 1.37"
OE Yield Torque: 6,828.4 lbs-ft

14 bolt FF 8-lug (The regular 1-ton rearend, not the newer 11.5" behemoth)
Ring Gear: 10.5"
OE Spline Count: 30
Shaft Diameter: 1.50"
OE Yield Torque: 8,966.2 lbs-ft

The 14 bolt SF 6 lug that would be a good candidate for the swap uses 33 spline axles. Still a C clip rear but the large axles and 3/4 ton rating offer some peace of mind against breakage. If a used one could be had in serviceable condition and the entire swap could be done for a reasonable cost I'd say it's a pretty good upgrade without getting into the aftermarket.

Yes, the tiny Ford pinion shaft is strong because it's well supported with many bearings. Digging even deeper, the alloy that was used to make the gears will play a part in strength. Who knows if the earlier truck 12 bolt gears are made of something different than the later 10 bolt. Age is also something to consider too. Any rear from these trucks is anywhere from 39 to 25 years old, with many owners, possible tampering with the rear, and really no way to know what they've been though. Just so many things that make it tough to judge just how strong a piece of metal is.

I believe the Truck 12 bolt actually came first and the Car 12 came later. Why they decided to keep both designs, I am not sure as I wish they would have just used the Car 12 on trucks also. GM used to be not so corporate minded, with each division getting their own V8 and even their own axle designs. I have a feeling the truck division had their own rear, and Chevy/Buick/Pontiac/Olds agreed on the Car 12 as the axle for the shared car platform. This is just my speculation though. Later we see the 8.5 10 bolt come in as the one size fits all rear.

If you are really hardcore though, racers have successfully installed pro gears for a Car 12 into a Truck 12 with use of a different bearing. The potential is definitely there for the Truck 12 housing if needed.

I think it's just one of those things where your mileage may vary. Some have killed the pinion shaft in the Truck 12. Some have trashed 10 bolt axles. Either way, if you're racing on stock parts, you're gambling. Guess that's the fun (or headace) of racing (or wheeling) on a budget though. Run what makes sense from a cost standpoint for you, and see if it lasts.

[Firebirdjones]

Originally Posted by Pyrotechnic

14 bolt SF 6-lug & 14 bolt SF 8-lug (No different except for the lug count)
Ring Gear: 9.5"
OE Spline Count: 33
Shaft Diameter: 1.37"
OE Yield Torque: 6,828.4 lbs-ft

14 bolt FF 8-lug (The regular 1-ton rearend, not the newer 11.5" behemoth)
Ring Gear: 10.5"
OE Spline Count: 30
Shaft Diameter: 1.50"
OE Yield Torque: 8,966.2 lbs-ft

The 14 bolt SF 6 lug that would be a good candidate for the swap uses 33 spline axles. Still a C clip rear but the large axles and 3/4 ton rating offer some peace of mind against breakage. If a used one could be had in serviceable condition and the entire swap could be done for a reasonable cost I'd say it's a pretty good upgrade without getting into the aftermarket.

Yes, the tiny Ford pinion shaft is strong because it's well supported with many bearings. Digging even deeper, the alloy that was used to make the gears will play a part in strength. Who knows if the earlier truck 12 bolt gears are made of something different than the later 10 bolt. Age is also something to consider too. Any rear from these trucks is anywhere from 39 to 25 years old, with many owners, possible tampering with the rear, and really no way to know what they've been though. Just so many things that make it tough to judge just how strong a piece of metal is.

I believe the Truck 12 bolt actually came first and the Car 12 came later. Why they decided to keep both designs, I am not sure as I wish they would have just used the Car 12 on trucks also. GM used to be not so corporate minded, with each division getting their own V8 and even their own axle designs. I have a feeling the truck division had their own rear, and Chevy/Buick/Pontiac/Olds agreed on the Car 12 as the axle for the shared car platform. This is just my speculation though. Later we see the 8.5 10 bolt come in as the one size fits all rear.

If you are really hardcore though, racers have successfully installed pro gears for a Car 12 into a Truck 12 with use of a different bearing. The potential is definitely there for the Truck 12 housing if needed.

I think it's just one of those things where your mileage may vary. Some have killed the pinion shaft in the Truck 12. Some have trashed 10 bolt axles. Either way, if you're racing on stock parts, you're gambling. Guess that's the fun (or headace) of racing (or wheeling) on a budget though. Run what makes sense from a cost standpoint for you, and see if it lasts.

I agree, and currently looking for a 6 lug 14 bolt for a swap as I have snapped the 12 bolt pinion enough. Although like you mentioned, the first 2 were GM gear sets and probably of marginal strength, and this current set is Richmond made of stronger material. I can only cross my fingers though and don't care to tempt fate in a vehicle that we travel hundreds of miles from home in if you know what I mean.
So to bullet proof it and some piece of mind, I'm going for the 14. Just haven't found one that suites me.

[99-LS1-SS]

Originally Posted by Firebirdjones

I agree, and currently looking for a 6 lug 14 bolt for a swap as I have snapped the 12 bolt pinion enough. Although like you mentioned, the first 2 were GM gear sets and probably of marginal strength, and this current set is Richmond made of stronger material. I can only cross my fingers though and don't care to tempt fate in a vehicle that we travel hundreds of miles from home in if you know what I mean.
So to bullet proof it and some piece of mind, I'm going for the 14. Just haven't found one that suites me.

Where does someone start to look if they are trying to find a 6 lug 14 bolt set of axles?

[Firebirdjones]

I've been scoping a few of the junk yards and Pick-A-Part for 3/4 ton trucks from about late 80's to mid 90's. No luck as of yet.

From what I understand different platforms had different widths. I'm not worried about spring perches and shock mounts as those can easily be moved, but the overall housing width has to be the same as my 72 blazer. I'm told not all of them are which makes the search more difficult.

Then there is the fact that I want mine to have 3.73's, and prefer a posi/locker so I'm looking for G80 codes in trucks etc.. It also has to be the 6 lug because I don't want to get into switching wheels around = even more money. So it's all making my search more elusive

Just have to keep digging I guess.

[Firebirdjones]

Originally Posted by 99-LS1-SS

Who makes good axles and c-clip eliminator kits?

I prefer to use Strange products, but there are others to shop from that offer good parts and similar prices like Moser, and Currie as well.

[INSIDIOUS '86]

Originally Posted by 99-LS1-SS

Who makes good axles and c-clip eliminator kits?

Moser makes good drag ones, strange make good street and drag capable ones
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[Pyrotechnic]

I believe the 6 lug 14 bolt came under 88-98 light duty 2500 trucks. Look for 2500 badges on the doors and 6 lugs. They MAY have come under some Z71 trucks too, being that they were 6 lug, but I am not sure. Width wise these are a great choice because 88-98 axles were only a touch wider than 73-87, which is fairly close to 67-72.

1999+ trucks used the 6 lug 14 bolt as well, but I am not sure if later trucks got wider or not. They came under SS trucks, Vortec High Output, Vortec Max, basically any half ton that has a 6.0 or 6.2. Tahoes/Suburbans/Avalanches can have them too but you've got more mounts/pads to grind off.

73-87 trucks did use the 14 bolt SF, but they are all 8 lug. Perfect width and will even bolt right in if you can work out the bolt pattern. May be a way to get a late model axle to slide in, or weld and redrill the pattern.