308 diff: How much slop is OK ?

Sean started a thread currently a thread running here in technical that is just a few lines down, asking this and many other questions:

"Are Ferrari differential internals common to anything else?"

The cord and spring scale is a very accurate way to measure preload on the carrier, but that has nothing to do with your issue. The differential side gear your refering to runs inside the carrier.

 

By the time you get the output end (which you are tugging on), there is a pretty good stack up of play and backlash within the assembly --- so, significant play is normal. There are specs provided for maximum allowable bearing thrust and lateral / radial play in the WSM for the later cars. Unfortunately, I don't have a copy of the manual for the earlier '78 MY, and I know Ferrari went through a series of progressions on the diffs (and their specs) throughout the 308 range ---- so, I wouldn't want to quote you any potentially non-applicable values.

Perhaps someone with the WSM for the '78 can provide the data? It's also possible that the specs differ between the US and Euro models --- which is yours?

 

Yeah, it's a bit nebulous. The "uniform movement" is just their way of saying that there shouldn't be any "sticky" spots in the rotation. This is similar to when you are checking a wheel bearing --- as you spin it by hand, you don't want to feel any rotational points where the resistance sharply increases, which would be indicative of a deformed or flat spot on one of the roller bearings. The pulley torsional check will tell you whether or not the bearings have the right amount of preload, but not necessarily if they are worn out. Ideally, there really shouldn't be much noticeable radial play in those bearings when you pushing on that output shaft by hand. The question is "are the bearings loose, or are they worn out"?

I would suggest checking the preload per the manual's pulley procedure first. If it is too low, increase it to spec. Then try to wiggle the shaft again ---- if you still have radial slop (or if the preload did not require adjustment), then I suspect your bearings may have excessive wear. Unfortunately, the only way to check the condition of the bearings is to disassemble the unit, pull out the bearings, and visually inspect them.

This is more my intuition talking rather than direct personal experience with that particular differential, so hopefully one of the "pros" here will chime in and be able to tell you how much radial play should exist with good bearings in place and properly preloaded.

BTW, my apologies as I made a wrong statement before ---- the WSM does not list specs for amount of thrust or radial play. Like yourself, I was remembering info from other cars and their shop manuals which did furnish the specs ---- I guess I'm starting to get a bit senile .

 

Measure the actual "end float" ... the vertical slop is a bogus number and is exaggerated by the end float and .003-.004" clearance on the o.d. of the output shaft.

The only number I've been able to get out of a Ferrari guy is "it's a lot". I posted in my other thread that the Porsche race diff guy sets up .015-.020" end float on these output shafts.

It must be that the "professionals" that have "clients" with too much money probably send this kind of work to a "differential professional" so they know it's done right. The more you pay, the better it is right? ... idiots ... sorry ... it's early in the morning and I feel like kicking someone in the teeth.


cheers

 

Correct me if I am wrong here, but the yellow arrow in the picture above is pointing at the side gear/stub shaft, the part with splines that the drive flange attaches to. It was my understanding at the beginning of this thread, that the play/slop being seen was directly related to that part.

The round part surrounding it, the part it physically protrudes from, is a different part of the differential called the carrier. The carrier is supported by the large roller bearing seen in the picture.

To the original poster, are you saying the entire assembly, stub shaft AND the carrier are moving? We need to know exactly what we are witnessing here before we can determine the problem.

The carrier bearings, the large roller seen in the picture, are set up in such a way as to provide zero play in the carrier. In fact, they are squeezed slightly together, just enough to cause a drag, or "preload" in the bearings, and we measure that load by measuring the steady torque required to turn it. The string and spring scale is a very efficient and honestly scientific way to measure the drag. The crown gear or layshaft/pinion gear, would need to be removed to test preload of the carrier bearings.

Backlash is the clearance between the pinion gear and crown gear, but on a 308 there is no way to adjust it. The only adjustment to the gears is drive alignment right to left. One would use a bluing agent to check gear tooth contact patch and bring the pressure point within the centers of the gear teeth by shimming the carrier bearings right or left, then tighten up the bearings with additional shimming to set preload.

 

WAIT A MINUTE GUYS --- there's something screwy here...Thanks to Paul for calling my attention to the actual photo ----

HAMMERSON ---

It looks like (actually, I'm positive) you've got the lateral cover removed from the diff case in that photo. The cover must be installed and the fasteners fully secured in order to check play on the output shaft and correctly measure preload of the bearings. The cover squeezes against the bearing race and (along with the preload spacer inbetween) serves as the bearing retainer and sets the preload. So, with the cover removed, you will have all kinds of slop in the shaft ---- with the cover installed, you should have almost no slop.

I've not worked too much with the 308 diff., but I've worked with a lot of other ZF diffs and they're all pretty much the same design. If you think about it logically, no radial play makes sense --- When you put the drive flange back on that shaft, it spins against the oil seal (which is housed in the lateral cover) --- if you had too much radial play, you wouldn't get a good seal, and the diff would leak oil.

So, to put it simply ---- You've got slop now on the shaft because you've removed the component (lateral cover) that prevents the slop!!!

 

SEAN ---

Try switching to de-caf, brother !!


Cheers

 

Will do ... feeling much better now LOL

 

Hey, good eye. I didnt catch that either until you mentioned it.

 

I'm having trouble following what you've stated a bit...

But, here would be my recommendation ----

With the lateral cover, seal, and shim installed, and only the drive flange removed, check the preload. If the preload is okay, and the shaft turns smoothly, the only other thing I'd worry about is whether the flange oil seal was previously leaking --- if it was not leaking before you disassembled things, I would say the bearings are fine.

If it was leaking, then replace the seal. If the leak stops, you will know you only had a bad seal. If it still leaks, you probably have bad bearings, and you should pull the bearings out of the carrier and inspect them.

If the preload was incorrect when you checked, I would suggest taking things apart and looking for any problems. It's unlikely that the bearings could ever get evenly worn and can be re-used by simply putting in a different shim and getting back the correct preload. Roller bearings nearly always get damaged rather than slowy wear down like a tire or a clutch disc.

Without a spec for radial play, it's hard to say what passes for allowable slop. But, I can assure you that with any ZF differential, if you have proper bearing preload, and the oil seal doesn't leak when driving the car ---- the bearings and slop are fine.

 

On the 308 gearbox if the output shaft is installed you'll have to remove the crown gear from the diff to check bearing preload. With used bearings WSM says keep it on the low side too FWIW.

Cheers,

Sean