Hi folks I have 69 GS (export model) with 12 bolt posi, changed the gears from 3.08 to 3.73 and added a TA cover. It works flawless with my T56 Magnum, but now I have a new engine with 700+ torque. I drove around and in the first and second gear, the car spin the tires quickly (street tire). The question is, is the rear end safe because the tires can’t transmit enough power to the ground to break it? Or are my thoughts wrong and I should get a stronger rear end? Cheers Robert
As long as tires (traction) are the limiting factor, it will hold up. Should you install slicks or sticky DOT tires, and the car hooks, then there may be some concern.
The stock axle shafts are the current weakest link.They were never intended to handle that kind of power. They are tapered before the splined section,and that section is thin.That is where the axle breaks,and since it is a c-clip,that is how the shaft comes out when it breaks.Anything like a stick-shift,trans-brake,slicks,or anything else that adds more additional blunt force to the rear,is how things get stressed and broke back there.The next concern would be the posi unit.If it is the original unit,from 1969,then it likely has the 18/10 side gear/spider gear combination.This had a history of breaking the spider gears.The newer Eatons,and similer posi units now have a 17/10 combination,to give a beefier spider gear. If you decide to upgrade your existing rear,get a set of 33-spline shafts. You can either purchase a complete 33-spline unit,or you can get a 33-spline side gear/spider gear kit,to install into your existing unit.I actually just shipped a package like that to a customer in Ohio yesterday,that is in a very similar situation.If you use your existing posi unit,you will need to hone the bearing hub bores to .480" to allow the thicker 33-spline shaft to slide through.
More opions and info Old parts - You have to also remember all the heat up and cool down cycles on the parts. No matter how well designed parts are ...they temper and part get brittle and wear.... NOTHING you can do about that. Need for 33 spline axles may not be necessary BUT after market axles are a MUST. If you can afford a true trac unit they seem to hold really well these days as the cases are not just cast...They are machined steel component case. big difference in case rigidity- Also the true trac will have more touch contact points tot he side gears which drive the axle shaft ...again more strength. If you are still worried about c-clip and axle shaft breakage...You can also use Disc brake rear conversion to help as a aid in that worry. If the axle does break it can be at least held in by the caliper which holds the rotor which is attached to the axle flange end. This is a way that you can deal with that issues and not cut up your original housing end and spend bigger money on bolt in race axles. etc. Just more thoughts and options you can chew on. Jim J D
Thanks for the advice's . Your input give me a new sight of safe/risk and now I'm considering to buy a new rear end. I'm looking for a direct replacement rear end with no c-clips. I saw Moser rear ends and Mark Williams but I don't now what I should take: Spool, eaton, true trac... I'm open also for new sources... Robert
I've built a lot of the Moser 12-bolts. I just order their housing and axle shafts.I assemble them with the parts that I want. Their housings have strength improvements,as well as changes in some of the geometry,to improve traction. You can order it with the #7900 ends,and you can run the large SET20 tapered roller bearing,and it has the same flange pattern for your existing GM brakes.No need for Ford brakes. They cast the upper control arm ears in a higher location,to improve instant center,and they move the shock mount in,behind the lower control arm,like a G-body,to take some of the angle off the shock.This helps improve 60fts and traction as well. The Moser 9" housings are also made with the same geometry as their 12-bolts.
yEP i BUILD ALL TYPES OF mOSER 12 BOLTS FROM STOCK C-LIP ENDS FOR STOCK REPLACEMENTS TO THE 7900 LARGE FORD BEARING ENDS THAT STILL CAN USE STOCK gm 9.5 X 2 DRUM SYSTEM The upper ears can be a plus or a minus though depending on the ride you like. Sometimes the pinion angle change can cause a slight drive line vibration in some cases of engine swapped cars that the drive line is not to stock trueness WHICH tons of 50 year muscles have issues with as many many people have been messing with these cars for years. I build the Moser rears so much I have one of every gm type in stock all the time. I am getting done with the stock GM core rears we fine are all bent pitted etc. We do not even seak them out anymore. Said but it is a waste of time and the amount they want for these plain jane 12 bolt casting is crazy 600-1000 for a open rear core.
It's become a no-brainer. For what people want for a 12-bolt Chevy core,you can just buy a new housing and shafts that are far superior anyway.
Thank for chime in Jim. I was on your ebay store and your bang for bucks ratio is great and I know you make great rear ends. But I did notice one problem, the axle length, I need exact the same length. My wheels are now exactly in the middle of the wheel well. Are all bolt in axle shorter? Distance plates is not allowed in my situation.
???? - I do not understand where you got your info from and or on? (web site ebay other?) 1964-1966 rears are narrower 54.25 or 60" over all 1967-1972 rears are 1/2" wider on each side 55.25 or 61" over all. Which do you have? We have them both and we also can make custom any width your want...??? With in reason of the lower control arm mount width stock factory. Of course... Jim Also your terms are confusing as you use length when you probably mean width...length would be the pinion snout distance to the driveshaft. Width would reference the wheel to wheel mounting total measurement. So if you have a true 1969 Buick skylark it takes the wider 55.25 axle tube flange to axle tube flange and or 61" drum to drum width. You really should measure the rear you have under the car to be sure you know exactly which rear is mounted under your car this way you'll get the correct rear. JIm Mitschke J D
Hey Rob- Just to expand on the topic a bit... (not as much for you as for others who will search this topic in the future) There are other factors at play. The logic that the tires will break free and save the rearend whenever too much power is applied is partially true (in some circumstances) but dangerous... For instance if you have a harsh 1-2 shift, before the tires break free for the "chirp" there will be an instantaneous "hit" ("shock") where all of that torque (700 ft-lbs PLUS the additional "torque boost" from the transfer of rotational inertia when reducing engine speed which is what gives the "tire chirp"), but the tires haven't broken free... So for that one brief moment the tires are NOT acting as a "weak link in the chain". Even if you don't break stuff the first time, you're highly fatiguing the parts so that they will break in the future. You are smart to be thinking about this! -Bob
I know the c clip axles are weaker but they say the rest of the rear us stronger than the 8.5. I am running an 8.5, stock 28 spline axles, no girdle, in a 3850 pound car, leaving on a trans brake at 3500 rpm, with 9 inch slicks, car goes 1.50, 1.49 60' times. Never dynoed the motor for power, but we carry both front tire about 25ft and run 10.90s. We are building a new motor shooting for low 10s upper 9s. I only plan to change axles and add girdle. Now I am running a spool so I don't have the internals of the carrier to worry about. I would imagine the larger 12 bolt which should be bigger and stronger should hold up to more. Now I would if it was me install c clip eliminators, just to be safe. They are cheap not too hard to install and might just save your sheet metal or your life. I have seen way to clips break themselves or any other countless thing to set them free.
The 8.5" 10-bolt,even in stock form,is hard to beat,and there are plenty of parts available for those. The stock 28-spline axle are good in a 3900lb car,going 11:70's or slower. After that,you start taking chances with the shafts,and they will take more,but from a builder's standpoint,I will not build one for anything quicker without upgrading to aftermarket 30-spline shafts.They are really hard to beat at this point.
c-clip axle is not weaker than a bolt in axle... THAT is not the problem. The problem is WHEN and IF it breaks it is not retained from the bearing axle flange area. There is nothing wrong with c-clip axles at all. WE always tell people in the 12 bolts to just upgrade to NEW alloy axles and that will usually solve most issues as they will not break and you will not have to deal with a situation. Same with the 28 spline 8.2 Chevy etc. Just buy good parts and on the street most rears will do the job. I have built plenty of 7.5 that have held even at the track under ls Camaros...Always shaking my head cause i know these guys have the money to upgrade even more BUT they do not... Good luck on all the spring driving guys. Jim J D
Like I said,it's the factory 12-bolt shafts. They taper down before the spline,and that is where they break. That area is thinner than a factory 8.5" 10-bolt shaft,thus the reason you can get more out of the 8.5 shaft. It's another reason why we don't mess with the factory 12-bolts anymore. People want too much money for a core,and all we use is the bare housing and caps. That's why a new Moser 12-bolt housing makes more sense. The guys that are restoring an original SS Chevelle can use the factory stuff.
Wow great informations especially the example with the break free moment on the axle. I would say its the same principle with an impact hammer on a wheel nut when the tire is in the air. With a torque wrench I just turn the complete wheel, but the impact hammer uses the hold momentum of the tire weight an can in short impacts loosen the nut. Now to my case: Need a 12 Bolt Moser with stock size like mine, Im sure I have the 68-72 (side to side drum case bolt pattern side = approx. 61.3) 3.73 gears No C-Clips 1350 Yoke Enough spline to be sure it never fails 700ft ( sometime I will go to strip, so it should hold this also) Studs size 7/18 or 1/2 just want to be on the safe side Last question is Eaton or true trac posi ?
That's pretty much the exact rear that I have here. An Eaton clutch unit,or a Tru-Trac will work. Mine has a tuned Eaton in it. None of the new clutch style posi units come with good clutches. I put the good knurled steel ones in,and tune it properly,eliminating the spring pack.This allows the clutches to relax or disengage while cornering,and will also reduce or eliminate clutch chatter,since you don't have springs pushing on them all the time.You get a unit that works better & lasts longer.
