how weak is the 12 bolt rear?

[Vee]

ok, I just bought this K5 and it has the 12 bolt rear. I have a line on a 14 bolt rear but it's 8 lug. money is tight right now and I don't really have the cash to lock the 14 bolt, buy all new wheels and tires, plus convert the front to 8 lug. I already have a locker for the 12 bolt and found an extra axle shaft to carry as a spare. alloy shafts are under $200 a pair if I want em. I have a 4x4 book that has a section on axle shaft strength........
these are yield torque ratings using engineering formulas and are approximates

12 bolt rear stock shaft 6,044 lbs-ft, 4340 alloy shaft 9,924lbs-ft
14 bolt FF stock shaft 8,966lbs-ft

so according to "engineering formulas" strength of materials and shaft diameter, the alloy 12 bolt axle can handle more torque than the stock 14 bolt FF shaft.

what do you guys think? open 14 bolt and carry 2 spare tires (an 8 & 6 lug) or run the locked 12 bolt w/ alloys. anyone break a 12 bolt w/alloys?

I know the 12 bolt is a c-clip, but I've changed both a dana 35 and dana 44 rear on my old TJ, and realistically they both take about the same amount of time as long as you have the right tools and know what you're doing ahead of time. so the c-clip is not really a contributing factor for me

 

[Hogback Fabrication]

ok, I just bought this K5 and it has the 12 bolt rear. I have a line on a 14 bolt rear but it's 8 lug. money is tight right now and I don't really have the cash to lock the 14 bolt, buy all new wheels and tires, plus convert the front to 8 lug. I already have a locker for the 12 bolt and found an extra axle shaft to carry as a spare.

I don't think you told us what size tire you're running and how heavy the rig is, but spares are always a great idea.

alloy shafts are under $200 a pair if I want em.

Which 4340 shafts are under $200/pair? Remember that most aftermarket 12 bolt shafts are only drilled for the car 5 lug pattern.

I have a 4x4 book that has a section on axle shaft strength........
these are yield torque ratings using engineering formulas and are approximates

12 bolt rear stock shaft 6,044 lbs-ft, 4340 alloy shaft 9,924lbs-ft
14 bolt FF stock shaft 8,966lbs-ft

so according to "engineering formulas" strength of materials and shaft diameter, the alloy 12 bolt axle can handle more torque than the stock 14 bolt FF shaft.

Well, I'll go ahead and promise that those are nowhere close to the real world.

FF 14 bolt shafts are astoundingly strong, substantially stronger than 35 spline Spicer (Dana 60 and 70) shafts (FF 14 bolt shafts are closest to 37 spline Dana). Its likely the 14 bolt shaft's strength comes from a good alloy, good taper, the fact that it's a full floating axle, etc.

A FF 14 bolt is really considered unbreakable until you hit the 47-49"+ tire range.

4340 12 (or 10) bolt shafts, IME, are maybe good to 37" tires or so.

All things equal (taper, shaft composition, etc), FF 14 bolt shafts are almost 140% stronger in torsion than 12 bolt shafts. Its likely higher than that, I've heard that FF 14 bolt shafts are made of a higher grade steel than most (with a better induction heat treat) plus its a full floating axle. Not to mention their supposedly ideal taper (which is well past my knowledge to analyze) even though they neck down a bit more than some shafts.

Are your numbers coming from a chart or did you do some kind of calculations?

what do you guys think? open 14 bolt and carry 2 spare tires (an 8 & 6 lug) or run the locked 12 bolt w/ alloys. anyone break a 12 bolt w/alloys?

Its been covered many times here, search

I know the 12 bolt is a c-clip, but I've changed both a dana 35 and dana 44 rear on my old TJ, and realistically they both take about the same amount of time as long as you have the right tools and know what you're doing ahead of time. so the c-clip is not really a contributing factor for me

Well, the c-clip thing isn't so bad until a wheel bearing goes bad and destroys the shaft (not covered under warranty) or you break a shaft and don't have any more spares and the truck is your ride home.


I'm not telling you that 12 bolts are bad or that a FF 14 bolt is the only answer. It really all boils down to the fact that you haven't told us anything about your truck (weight, tire size, intended use, etc.).

 

[Vee]

I don't think you told us what size tire you're running and how heavy the rig is,

35x12.50 MT/R's, not sure about the weight the K5 is stock, top is on, 350 motor, TH350 trans, NP208 T-case, 10/12 bolts, plus maybe 200lbs in the back for toolbox & spare tire. I plan on just doing moderate trails, some rocks, steep hills, etc

Which 4340 shafts are under $200/pair? Remember that most aftermarket 12 bolt shafts are only drilled for the car 5 lug pattern.

looks like it's 1541H not 4340 this changes everything............ I think it might be too much weight on the rear

Are your numbers coming from a chart or did you do some kind of calculations?

chart

 

[lq9blzr]

My 12 bolt held up for about 3 years, 400 hp small block, lock right and 4.88s. on 37" mtrs. I sold it when I got my 1tons. Its a good cheap rear end. Yes I had it totally rebuilt, no I t never went on the hammers or trophy truck racing. I did beat the **** out of it in the dunes and some local (moderate) trails. only issue i had was never should have put the bling chrome cover on, they bleed quite easily. LOL