Adjusting crownwheel and pinion mesh. Any experiences?

[Elanman99]

I have a supposedly rebuilt 3.55 diff to put in my S4 but I want make sure its OK before installing it. I was told it had been fitted with new bearings and was set up ready to run.

I have removed the nosepeice from the casing mainly because I thought I would check the backlash as it felt greater than I remember when handing diff units previously.

I read through the diff rebuild section of then manual and without even measuring the tooth backlash I could tell it was significantly greater then the recommended 0.005" in fact it was nearer 0.030"! I then made up a 'Cap spread' gauge, fitted it (set it zero) and then loosened the bearing retainers to remove the preload. I found that there had been about 0.002" spread instead of the recommended 0.005"

I have now adjusted the backlash and preload (of the diff cage) in accordance with the manual but I am nervous about using the diff because of how it feels when I rotate it by hand. Before I adjusted it, the gears meshed completely smoothly and the transition from one pinion tooth to the next was imperceptible (albeit with backlash). Now when I rotate the input flange I can hear and feel the gears meshing.

I am now wondering if the pinion is not in the correct position axially. Having found that the backlash and preload is not as it should have been I am wondering whether whoever 'rebuilt' the diff did not check that too.

If anyone has experience of working on the diff themselves I would appreciate whatever guidance or advice you could give. I'm going to Le Mans (Classic) in it and would love to have the higher ratio diff fitted for then, I've only put up with the 3.7 diff for 40 odd years and 200,000 miles but in this 50th anniversary year I thought I would celebrate.

Ian

[JJDraper]

Although I haven't worked on the diff myself, farming out the work, I have fallen victim to a similar problem with a (previously) respected company which replaced the pinion bearing without checking and resetting mesh.. Consequence is a noisy diff with whine on light load at 50mph. I can almost play tunes on it! On enquiry he said "bring it back and I will reset it, but we don't check the mesh after only fitting new bearings. They normally don't need it...".

Oh yes, I'll just whip out the diff at the side of the road and let you do the job properly this time.... I'm looking for a new 'specialist'. The problem seems to lie with the younger mechs who are used to spot-on tolerances. It is my understanding (backed up by the workshop manual) that the mesh should be checked everytime the diff is disturbed.

Anyone got any recommendations for a specialist in the UK?

Jeremy

[DavidLB]

Try David Bruzas he did an excellent job on my mk14 Elite diff a few years ago

[RogerFrench]

If you fancy having a go yourself, of want to check, then clean both crown wheel and pinion and blue the teeth lightly. Rotate the pinion and look at the mesh. You want to get an elongated oval more or less in the middle of the teeth, or where there's the largest wear pattern on a used set. You may find to begin with that you've used too much blue; wipe some off and try again.
To adjust, move the pinion backwards to get the contact patch to move forwards, and vice-versa.
Then adjust side-to-side depth of mesh once you've got the fore and aft worked out.

[Elanman99]

Roger

After I posted my question I looked in a manual I have for a Plus 2, the diff pages are updated compared with the manual I bought in 1968 when I got my car. The later one includes a page showing the tooth contact patterns and what needs to be adjusted to correct errors. The earlier manual just showed what correctly adjusted gears should look like. Having now gained some experience with the cage bearing adjustments I have no trepidation in rebuilding the whole diff from scratch.

As a matter of interest, the old manual does not even mention Engineers Blue, just red lead and yellow ochre, neither of which I would even know where to get!

Ian

[RogerFrench]

You can (or could, but I haven't tried for many years) get both from an artist supply shop.

Prompted by your reply I looked at my Elan manual, which describes the process as you have written. Where it differs from What I learned, though, is that it talks of inspection of the crown wheel, whereas I always did it on the pinion. Chacun a son gout.

[promotor]

The mesh should ALWAYS be checked and whenever i build diffs for customers (I build diffs for a company when they are too busy to do them themselves) i check the mesh BEFORE a diff is disassembled just to see where it's at and to give a good idea of whether it is likely to work out well when it has new bearings etc.

If the teeth are almost "grinding" then the mesh isn't correct as you've guessed!!

If the contact patch is out after the backlash/cap spread are set then you need shims to adjust it (together with the correct tools)!!

The cap spread you mention is about where diffs end up after they've had plenty of use - not freshly set up!

I would suggest getting some engineers blue on the crown wheel and pinion to see where you're at to see if the mesh is OK.

Some diffs get rebuilt and they're still noisy - sometimes you just can't get away from it as crown wheel and pinions have seen so many miles they can't be expected to be like factory new even when you have them re-set!!

[Elanman99]

I applied engineers blue today but I'm finding it hard to interpret the results.

I cleaned off as much oil as I could and applied the blue very sparingly to a few of the pinion teeth. I learned nothing useful after a couple of rotations so I changed tack and used an electric drill to rotate the shaft in both directions until the blue was evenly distributed.

I dont have a consistent pattern of blue on all teeth, I can see areas on some teeth where it appears the blue has been wiped away (because of very accurate mateing?) and other places where the blue is very dense. I loaded the output shafts whilst under power (in both directions) but once the gears are being rotated to inspect I guess the blue gets disturbed which makes it hard to tell what it all means.

Further to my first post, Ive revised my impression of smoothness when rotating the input pinion by hand and it is actually quite smooth so maybe the diff is OK. I really would like to get it checked though before I install it.

Ian

[promotor]

When you turn rotate the pinion flange by hand do you get an initial "stiction/resistance" to that turning followed by no stiction if you continuously rotate the pinion flange?

[Elanman99]

[quote="promotor"]When you turn rotate the pinion flange by hand do you get an initial "stiction/resistance" to that turning followed by no stiction if you continuously rotate the pinion flange?[/quote]

Yes, it is just as you describe. Obviously there is a constant drag caused by bearing preload. My electric drill struggled at full speed (its two speed but stuck in high) but the diff noise was not audible over the drill.

Another query. When I removed both bearing adjusting 'cup' nuts (to deburr the holes) the outer race of the pinion side bearing came out. Even though the main cap bolts are dead tight the bearing outer just slips in to its housing. I doubt it will rotate in use because of the preload but is the cap meant to crush the race diametrically normally? The bearing does not show any signs of wear and the roller tracks look normal.

Ian (in Cheshire)

[promotor]

Okay - it sounds like it's possible the pinion pre-load is correct although it is impossible for me to be able to say it definitely is!

The bearings cups should be gripped by the bearing caps - it's kind of like when you put main bearings in a cylinder block they have slight resistance to being put in. The side adjusters are there to get the right setting and do help to keep things in place but the fact the bearing race just falls out isn't quite right. Think about the movement of that bearing race "up and down" while in use as opposed to one that is nipped in place.

When the caps are tightened to their proper torque you shouldn't be able to move the side adjusters at all. However, in some cases you can get them to move but you have to use a bit of leverage to do it!

That's why you have the locking tabs as things do move on occasion. But those locking tabs aren't there as the a method of holding the cap spread on, more as a back up in my opinion.

[Elanman99]

I presumed the outer races should be tight but since the adjustment procedure begins before the caps are tightened and the capspread gauge measure over the caps its readings wont make sense until the bolts are tight. I can see that moving the race position if the cap is tight would need a lot of torque, and then it would probably move in tiny jerks.

I can see that the race has revolved in its housing but its still a good fit, I could always add a little Loctite to stop it revolving.

Does the preload decrease over time as the bearings 'wear'. I actually measured nearly 0.003" before I started so I suppose that is not a lot less than the 5 to 7 thou value.

Ian

[promotor]

When setting the cap spread the bolts for the caps are bottomed but not torqued. However, once you have set the cap spread to the desired amount and then torque the bolts the cap spread shouldn't change at all - this is of course down to getting the bolts bottomed the right amount and the bearing races sitting snugly while the setting is done.

Are you asking about preload or cap spread ? I presume you're talking about cap spread as preload isn't measured in thou's but pound inches. The cap spread does come off slightly through use.

[Elanman99]

[quote="promotor"]When setting the cap spread the bolts for the caps are bottomed but not torqued. However, once you have set the cap spread to the desired amount and then torque the bolts the cap spread shouldn't change at all.

Are you asking about preload or cap spread ? I presume you're talking about cap spread as preload isn't measured in thou's but pound inches. The cap spread does come off slightly through use.[/quote]

I missed the 9-11 lbs/in until I just looked in the manual, I just took the 5-7 thou spread as applying the correct force axially.

When you check the tooth meshing, how much blue do you apply?

Do you have (or have you seen) the pinion setting tool? Is it much more than a spacer of a precise length or something I could make?

Sorry to bother you with all these questions, I have a deep distrust of dealers and 'experts' and prefer to make my own mess!


Ian

[promotor]

The pinion height setting tool is a very accurate and finely machined piece of kit - i had one made from the original tools and it cost a lot of money.

You could have a machine shop make one but you need to know the dimensions of the proper tool.

As for the blue - plenty! The more you put on the more definite the marks become. You don't have to worry about wiping it off as it'll stay in-situ without any problems.

Questions are fine - i like to make my own mess as i never trust anyone else either. I only pay people to do work when i don't have the tools myself to do it! I know how to do them as i was taught by a Ford trained mechanic who did his apprenticeship on all old fords diffs etc etc.