yesterday my 76 vette 4 speed l48 quit moving in the parking lot. Had it trucked to the shop where they diagnosed rear end issue..left rear short axle was partially out and shiny and positraction would not spin either tire.

Opened it up and the short axle had fractured not from external force or gear tooth interference or lack of oil but the metal just went. It looked to us like someone had been in the differential before (5th owner) and I suspect this is not an oem quality or better part. Looking at the break without axle break experience and no high forces on it like drag racing, huge engine, etc. but some metalurgy it looks to me like the metal was possibly incorrectly heat treated and it crystalized over time until it finally just plain failed.

Differential appears in good shape along with bearings. We discussed it and ordered two new hardened short axles. If one was poor quality without knowning who did what in there previously, I had them replace both. Parts due in Friday. I'll delay my race date from 4/13 to 5/5 in the Touring Lights at VIR. Better to find out now than then.

Anyone want to take a whack at the cause from your experience? The circlip was in good shape, by the way though off when they opened it up.

Here's a link with 15 photos, two are of the transmission codes while it was up there. click it and a slideshow starts. you can touch escape to stop it then page individually through the photos.

http://albums.phanfare.com/slideshow...g&a_id=5070502
lance pearson

 

Seeing that twist, its been hammered on a few times in the past.

 

Just from the way that the splines were 'shifted' it would appear that a pretty good shock load was placed on that yoke at some point...probably more than once (!). After the initial damage was done it was pretty well weakened and it was just a matter of time before it finally let go. Interesting presentation BTW...

 

Hi Lance,
Like the phrase in the song..... "took me by surprise"!
I guess a LOT better now than at VIR.
Regards,
Alan

 

Ouch.. .. I had a customer do that by running in reverse and then dumping the clutch in 1st gear. He "said" was getting the front wheels to come off the ground.

 

thanks for the input...this is beyond my experience. Alan..not wild about the labor but it could have been worse and best to find out now as you said. The last p.o. did not drive it hard...his wife drove it...but there were others before us. The engine is only a stock L48 and even with the new true duals it probably isn't more than a maybe 200 hp and I don't run the rpms way up then dump the clutch in first gear to spin the tires but maybe somebody did it before like you guys said. That differential is the same one that handled 300-400 hp or slightly more sae gross h.p. engines in late 60's and early 70's so it was either abuse of what power it has or substandard parts. I took no chances given the size of the labor bill once opened up so went ahead and bought two new ones to match the set for strength. I'm personally past any real ability or desire to do that kind of heavy wrenching myself. the minor stuff I do but at least I have a good shop with skills to do the heavy lifting.

Thanks again for the views and support. You perhaps have some idea how much of an adventure this is for me to be getting ready to drive on the road track. The bumps on the road help make it a better, richer experience for me too.

Quite a community here, all in all. I have a Suzuki hybrid bike as well but that forum is nowhere nearly as talented or experienced or worldwide as this group of people is.

 

Chit happens. That should definitly not happen with a low HP motor like that. I have abused mine and taken one out that was twisted but it did not break. I seem to check mine about every year as it is out for one reason or another so I open the case to check things out. Did they do anything else while it was opened up?

 

The mechanic who is skilled, the shop foreman who races, a friend with a '65 and myself were in the shop over the differential discussing things and we explored what to do while the labor was in it and it was open, spring off, etc. the rear braces and bushings had already been done and the spring is in good shape and is right for the car the way i use it and new shocks so we went over that and focused on the differential. What seemed to make sense as they had turned it and made sure the positraction carrier gears were good, we all felt the pin bearings in each side socket and concluded that from my point of view since I'm still not sure it wasn't partly substandard heat treated metal maybe combined with abuse that the thing to do was spend the $ to replace both sides pre emptively. It will get washed thoroughly as well just in case there is a piece of one of the broken splines hiding in there and then new seals on both sides. We found a weld spot on the back cover where it had been welded then ground down and that's how we knew someone, maybe even the factory, had been in there before. Other than if it were home in my garage I'd probably clean the exterior of the case up and make it a non natural color to clean the appearance up. We also went through all of the u joints which I think are newer as they all have zirk grease fittings on them and they were all functioning and tight to all the mechanics there. When they get the parts they will also lube the front end and the u joints and redo the tech inspection after 4/7 so that it qualifies again for the 5/5 vir track date. None of us could see anything else that needs doing. When you look inside the heavy case you tend to see more "brute" engineering than maybe the fine quality finish you'd see in a Bugatti Veyron but it is still substantial. one other thing is this car for at least a decade has run 255 x 60 x r15 wheels which is a lot more traction than stock as a consideration and minor cause. I like them for traction and once new I don't think I'll have any problems with this function. the car rolls freely in neutral and there is no restriction from the wheels that could have locked it and caused this. I absolutely would have felt and heard that and there was nothing...it happened when I was in first and had clutched in to stop and wait for cross parking lot traffic and then gently engaged or tried to the clutch. No noise at all. No bangs, no grinding, nothing, just no more power to the wheels apparently via the positraction unit.

I especially checked the two cages of needle bearings on either side because that's what the side yoke axles roll on when hooked up and both were smooth. May not be sophisticated but I did it with eyes open on both then with eyes closed just concentrating on feel then lubed it with oil and did it again. Smooth as a baby's butt.

Lance

 

races is the correct word not cages but you get the idea...at least I think it's the correct word for the side roller bearings assemblies.

 

Lance, apart from all the other explanations as to how that happened the yoke could have had an inclusion or some other internal defect that eventually led to that failure...I don't know to what extent GM inspected those parts at the source but I'm sure it was some kind of random sampling at best (If at all). I also know of an individual who owns a McLaren M6GT with a Hewland transaxle that somebody had put Corvette side yokes into...on of those failed but in his case he was pushing some 500 HP through 16 inch wide tires (!). If you're concerned about a repeat failure you can always have the replacement parts inspected (Rockwell/Zyglo/Magnaflux) after you get them just to make sure that they are what their maker claims they are...from your posts you sound like a pretty smart guy so I'm sure you won't have too much difficulty getting it sorted out. Best of luck!

 

The part was damaged from a shock loading condition. But, that part looks (to me) to be thru-hardened so that it was too brittle. That part is supposed to be "case" hardened at the outer surface of the spline (for wear) and have a lesser level of hardening in the core of the part, to absorb stress/shock better. If that part had been properly hardened, it would have to have one helluva lot of torque applied to it to make that thing snap. It might be possible to do that with an L-48 (light the tires and smoke 'em but get wheel hop and come to an abrupt stop), but unlikely, I think.

 

You can buy either regular side yokes of OEM type quality as measured by rockwell hardness with automotive quality steel in them or "hardened" side yokes which are drop forged and twice the rockwell hardness of the originals and aerospace quality steel. That's what we've ordered to put in. I'm no expert but forging in effect smashes the metal completely in the opposite direction totally changing the internal structure of it and either making it more dense but much stronger. Many aircraft components in aluminum and steel are forged to create much stronger physical characteristis.

You are right about the apparent brittleness. Steel without excessive heat normally doesn't age in that way internally and I could not see any obvious large inclusions but if it were improperly heat treated combined with someone abusing it over time it could result in what you saw. I looked at the metal with a 30 power portable scope yesterday at home and it looks almost crystaline to me which is not normal for that kind of steel if heat treated properly.

it will remain a mystery and I'm focused on the fix. I'm sure a 500+ sae net hp engine could maybe do this but that sure isn't the case with a mile 1976 L48 engine. I really suspect the original steel quality in the short yoke plus some former abuse. One of the risks of buying a used sports car...you don't always know who's done what to it and I go through each thing and say: how can I make it better? This is another opportunity.

I appreciate everyone's input because even crystallized if that's true it still took some excessive force along the way to start that and the moment it broke, it did not do so with massive force, noise or anything else so it may have been partially gone for a while. who knew with no external signals? I sure didn't.

Lance

 

Lance, I live in Chester as well. Curious as to who is doing the work.

 

Actually, Lance, drop-forging doesn't do anything to the internal structure of a forged part. It does create smooth flow lines on the surface of the part, making it much less prone to cracking from stress. Hot forging is better than cold forging, in that respect.

Even the 'core hardened' part you described was heat treated. Steel is relatively soft until it is hardened in a furnace. There are vastly different furnace heating, furnace atmosphere, and quench strategies to achieve whatever hardness characteristics you want; but steel must be furnace treated to have any significant increase in hardness. The elevated temps in the furnace cause the change to the internal structure...and the [quick] quenching prevents the steel from reverting back to the softer condition.

I cannot imagine any manufacturer not heat treating for wear resistance (case hardening) on the splines....they would wear out in no time, if they weren't. But, different chemistry steel and different core hardening strategies would provide much better shock and torsional force resistance on the "better" parts.

 

I use Express Auto on Jeff Davis Highway just below Willis Road just N. of Chester. Junior who owns the business has a drag car and a dirt car and both are vette engines. Todd in the office raced at Southside Speedway and Harry in the shop is doing the work. All are knowledgable to talk to. You can drop the car off and Pops will take you home and come get you. They have done good work on my Vette and know it well. They also do the track's required tech inspections.

Lance

 

71 vette I am not as knowledgable on steel metalurgy but worked for many years in a huge aluminum plant including a big cast house, rolling mills, machining centers, etc. and am a fairly competent nearly half assed metalurgist in Aluminum by association. We used to send huge aircraft alloy rolling ingots in aluminum out to forge them which was mechanically making the length the width and so on in enormous presses with the specific intention of increasing strength then we brought them back, rolled them, made product, heat treated and machined them. It dramatically increased many physical properties depending on the alloying agents above what you could get just heat treating. This was almost totally for the aircraft industry or the space program. We machined the barrel panels for years from 2219 plate we made for the 32 big guppy barrel panel panels for the space shuttle as one example.

Steel is different techniques but there are some commonalities. Thanks for the explanation. All I know is that if you want them to wear better and be stronger forging seems to help.

Lance

 

Thanks for the info on the shop. While I do most of my own work, it never hurts to know about a good reputable place when something big
comes along. Good luck on your track day.

 

Your posi unit appears to be an Eaton. While you have it out, you should have somebody that knows about eaton posi units look at it. They are non-rebuildable (throw away) items

They are a cone shaped metal idea and they ware away. Did you see how the yoke ends actually press against the central posi pin when pushed all the way in and the C-clip only allows the yokes to travel out only a small amount. Well as the eaton style cone wares it allows travel of the yoke farther out.

An Eaton is considered higher performance that a stock Vette posi. I've used them before. They are superior because the eaton cases don't break where the stock posi cases elongate the central cross pin hole and then it eventually breaks the posi case.

 

I'll look up the differential from the numbers. While up I did look up the trannie codes and it's a borg warner 4 speed with aluminum case as the manual said it would be. The codes on the side of the differential identified it as a 3:36-1 which was also stock with the four speed. The interesting thing on the tranny case was that it came from the Borg Warner plant that built four speeds for Pontiacs. I'm sure many of these were put in several models. It works quite well.

thanks for the suggestion on the differential. You learn a ton here by people seeing and commenting. Some you ignore but most is just excellent like your comment. I'm always surprised at how small the circlip is and I knew that it didn't need much space between it and the other parts in there from other posts.

Lance

 

I called the shop and asked them to clean the outter case and the half shafts and splash a spray of vht black or silver paint on them while they were out too. I could have done it at home but it's so easy when it's out and on a bench.