G50 - How Much Lash Is OK ?

Peter Delaney

GT40s Supporter
I am wondering if any of our experts can give me an opinion on a couple of issues with my G50 :

a) How much lash is "normal" in a G50. With the car jacked up, the wheels at ride height & in gear, I can move one wheel about 20mm (at the tyre surface) before the other wheel moves. Using my 40 year old trig, this seems to work out at about 3.6 degrees of free movement. Does anyone know if this is within normal tolerance ?

The reason I ask is that I can get quite a "clunk" on changing gears (if I don't get it just right), & can induce a repetitive clunking (hunting) on a very light throttle at low cruising speeds (light load).

b) The LH drive flange can be moved in & out about 0.5mm (there is no sideways free play on the RHS). Again, does anyone know if this is "normal", or whether I have a major bearing problem on this side ?

Perhaps this could explain why I have some pronounced clunking from the back of the car when I run down my very rough driveway in neutral ? I have replaced all the spherical joints in the rear end with high-quality teflon units, but that only fixed some of the clunking problems.

Any help would be greatly appreciated.

Kind Regards,

Peter D.
 

CliffBeer

CURRENTLY BANNED
Peter, are you sure this clunking noise isn't coming from the axles? Do you have CVs or ? A repetitive clunk on light throttle sounds like a worn CV to me.....been there....
 

Peter Delaney

GT40s Supporter
Thanks Cliff - but I doubt that it is the drive shafts. Mine are the old-style sliding splines with virtually brand-new GM uni's - all as tight as.

I reckon that the repetitive clunking is definitely related the the lash/play in the G50 - it is not speed related as you would expect from a crook uni. If you lightly blip the throttle on & off, you can get the clunk each time in any gear at any speed. It is just more noticeable when on a very light throttle at low cruising speeds in 4th or 5th when it goes into some sort of "hunting" mode - a bit like when you run a car almost to a stop & forget to release the clutch.

The whole problem completely disappears when under anything beyond very light load, or a fully trailing throttle - as long as there is constant torque in one direction or the other, it is ok.

Hence my suspicions are directed at my G50 (I would be suprised if the people paying big bucks for a Porsche would be happy with this sort of problem as "standard"). So I am thinking that MY G50 has a problem, rather than that all G50's have this problem ?

Thanks & Kind Regards,

Peter D.
 
I don't konow if this helps, but the last time I had the Porsche jacked up to change the rear brakes, I noticed that I could rotate one wheel roughly by the same ammount as you describe, before the other wheel moved.
I do not get any clunking or other nasty issues, so I would hazard a guess that your G/Box is OK.
 

Russ Noble

GT40s Supporter
Lifetime Supporter
Peter, I would say get rid of that play in the drive flange, whatever is causing it, even if it doesn't cure your noise. I could envisage the flange rattling under no load as the wheels and drive shaft move up and down on rough surfaces, moving the flange in and out. Possibly the same could happen under very light accel/decel. If your drive way is down hill or you have a slippery fitted, as a double check before you pull it to bits, why not remove the LS driveshaft and see if the noise goes away? Just a thought.

And a Happy New Year to you and all your Ozzie mates.

Cheers
 
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Was helping a buddy with pads and rotors today on his 993 and with a 17 inch wheel we had about 10-12 mm at the rim and the output spline in/out movement was unmeasurable given what we had laying around but I'd say it was negligible.

Is the through bolt in the output shaft tight? Splines in good order? These gearboxes were always noisy (which was part of the reason Porsche used the dual mass flywheels) but that sounds like possible too much slop/noise.

Best,

Chuck
 

Alex Hirsbrunner

Lifetime Supporter
Hi Peter,

I have no transaxle experience, but I have set up a lot of Ford rear ends, so just as another point of reference, take this for what it is worth. For a standard 9" (Ford) straight axle rear end, the Ford service manual states if you are looking to resolve a clunk, and have less than 1 inch of rotational slop measured twelve inches from the center of the axle on the tire (while the pinion flange is clamped to prevent movement), you need to look elsewhere for a solution (i.e. it is not a gear lash or spline fit problem).

The manual also states that you will get a clunk if your axle end play is excessive. In an an integrated carrier axle for late model mustangs the spec is between .005" and .040" end play with the tight end being greatly desired.

Also, any car that demonstrates end play exceeding .045" (for axles retained with "C" clips) will not pass my club's tech inspection. This is really a concern for how much impact the c-clips have to withstand - so that an axle doesn't part company with the car. This is probably not a safety issue with the Porsche transaxle as I doubt it is retained in such a low tech fashion.

Finally, I would note that it is really difficult to estimate end play without a dial indicator mounted to the axle housing. It tends to feel like a lot more play than is actually measured.

Regards,

Al
 
Peter, While you should try to eliminate any obvious wear/play, I feel you may be chasing ghosts. In your setup with two conventional Ujoints where you do not have equal angles at the inner/outer ujoints of each Halfshaft ( due to toe-in/neg camber etc ) the change in rotational speed caused by the ujoint will be different for the inner/outer. As the tyre/rpm at these speeds is fixed this causes the crownwheel etc to switch from coast to drive mode while being operated under light load conditions which gives the clunking sound you hear.

Jac Mac
 

Peter Delaney

GT40s Supporter
Thanks for the input guys - much appreciated.

Mark, I am beginning to think that a certain amount of overall lash is normal - whether I have too much is the question. If you are getting much the same as me, but without all the clunking, it is starting to point at the LH output shaft & its in/out free play.

Russ, I agree - that output shaft end-play has to go, regardless - it can only lead to bigger problems. Even though the drive shaft / splines / uni's are almost new, there will always be a bit of binding on the driveshaft splines under load - given this & the output shaft end-play, that will result in a lot of in & out "percussion hits" on the output shaft as the susp moves up & down under load ==> side plate fracture ?

Chuck - As per Mark's comments, it looks like some degree of lash is normal in these boxes, so my first task will be to get rid of the LH output end-play. I'll start by looking at the "through-bolt" - if things are flopping about, it will be obvious, but if not, I have no idea from there on (no manual & no idea of how things are put together inside). Either way, I will be a bit nervous about just tightening the bolt (if there are tapered bearings in there, I could really stuff things up). So, like so many other mechanical things on this car, it looks like more dollars for an "expert" !

Mind you, the whole g'box was supposed to have been fully rebuilt & checked by Albins (the "experts" who make all the gear sets for Autohaus, etc) & it came back with new synchros & all else checked as ok) ! When I got it back the 1st time, there was no 2nd gear (synchros fitted incorrectly), then when it came back again, I had all gears, but a huge whine in the new higher ratio 5th ! Needless to say, Albins are now not on my list of "experts" !!!

It was interesting to note your comment about these boxes being inherently noisy - I had (& still have) a pronounced "rattling" noise when it is idling in neutral under 700rpm (noise stops when you push in the clutch). I took the car out to a guy who preps GT3's for a living, & he reckons that is not unusual with lighter flywheels. To prove it, he started up an almost new GT3 - same noise, but twice as loud as mine ! His opinion was that the lighter flywheel allowed a geater degree of angular velocity variation between cylinder firings, thus taking up & letting go the lash. As soon as some revs &/or load were applied, it disappeared - same as mine ! I have the problem if idle is less than 700rpm, but it goes if idle is up at 900 rpm.

Thanks again to you all - and have a great New Year !

Kind Regards,

Peter D.
 

Peter Delaney

GT40s Supporter
Thanks Guys,

Alex, again it sounds like the total lash might be within the bounds of "normality". If 1" at the tyre is ok with the pinion locked on a standard diff, I must be ok - I can't lock the pinion itself, just the input shaft to whole g'box - so I will be seeing the lash in the diff, together with all the lash through the g'box. As this is about 20mm (.75"), then it seems ok.

So the end-play appears to be a more likely culprit. With the drive shaft disconnected from the output flange, I measure a LH output flange in/out end-play of 0.45mm (0.018") to 0.5mm (0.02") (min / max over 360 deg rotation) with a digital vernier caliper set. Not enough to be dangerous in a fixed setup with c-clips, but certainly enough to generate a distinct clunking noise, given the push/pull exerted by splined drive-shafts under load ?

Jac Mac, that is a very good point. My recollection from Mech Eng some 40 years ago is that the cyclic variations in angular velocity in a double uni setup is totally cancelled out, IF, the input & output shafts are perfectly parallel, AND the uni joints are set up in the correct phase (which, naturally, I can't remember) !

So, I may have 2 problems :

- At normal ride height, I have 1deg of neg camber - so not quite parallel at light load cruising.

- My uni's are phased as per the attached pic (I just retained what I assumed to be the DRB factory setup - but the previous owner may have flipped one end of each 90 deg - who knows ?).

Thanks for the help guys - & Happy New Year !

Kind Regards,

Peter D.
 

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Peter Delaney

GT40s Supporter
Chuck, I dropped the driveshaft today to check the through-bolt in the centre of the output shaft flange - it is as tight as. The bolt & flange move as one fixed unit - in & out about 0.5mm, and left/right as I take up the lash.

So it appears to be something internal that is causing the end-play - perhaps a circlip or shim left out by "legends" at Albins ?

Kind Regards & Happy New Year !!

Peter D.
 
Peter,

I had a similar issue, when I bought my G50 (fresh rebuild). It ended up being an axle shaft bearing that wasn't fully pressed into place. The shop that did the installation paid $400 in shipping, and had it back to me in less than a week.

BTW, my gearbox is pretty quiet, but then my exhaust is pretty loud. ; )

Since you are going to be tearing the gearbox apart AGAIN anyway, why not get rid of the old, open diff, and get an LSD replacement?

Albins should owe you that much by now, considering your past problems with them!



Bill
 
On the Porsches the in & out movement of the driveshaft flange can be alarming at first, but it's normal. There is no endplay at the wheel carrier. I have a stock 6-speed out of a 993 now, it has about 1-2mm of axle flange lateral play on both sides, with no axles connected, and this is typical of these. This is with the open diff. With an LSD it may be less.

If the tapered diff bearings are not fully home, you will be able to see radial slop on the axle flanges with a pry bar.

Backlash is measured with the trans out and on the stand, with the input & output shafts locked together. Otherwise you are only measuring the lash of the diff, and gearsets, not the ring & pinion. The amount of the lash is a small number, e.g. 0.16mm. Some people use gear marking compound...

If you can do the same tests you did already to another car in person, that might clear some of this up easily.
 

Peter Delaney

GT40s Supporter
Jac Mac - thanks - I had a feeling it was correct. So that rules out rotational speed variations as a source of clunking under light load.

Bill, I remember you having dramas with the g'box - all part of the tantalising process of getting a 40 on the road - 5 steps forward 4, backwards !! Did you have the exact same problem of in/out end-play on the side that had the shaft bearing mis-fit ?

Jmracecar - Thanks for the info. I am a bit surprised that Porsche would have such lax tolerances. I have no idea about the types of drive-shafts used by Porsche, but I would be most surprised if there was absolutely no force required to shrink/extend the drive-shaft length as the susp went up & down (CV's or Uni's). Either way, the drive shaft needs to change length as the susp works, & any resistance to this would have to bang the flange end-play in & out. I reckon that I need about a pound or two of effort to get the drive-shaft splines to move under no load (probably way more under load), so that "reluctance" would be easily enough to bang the g'box output shaft/flange in & out.

As far as I can tell, I have no radial slop at all - just the end-play on the LHS - the RHS is perfectly tight every which-way.

I realise that with the g'box still in the car, I can only measure the total backlash of all components (still seems a lot to me). When you say 0.16mm, can you tell me where & how this is measured ?


I think I may have found a "real expert" on Porsche/Audi gearboxes - I hope to get him to do test drive / diagnosis in the next week or two, so I'll let everyone know what the outcome is.

In the meantime, HAPPY NEW YEAR, & thanks for all your help.

Kind Regards,

Peter D.
 
Peter, If you have a combination of camber & toe there may be enough to cause the velocity difference. Also you mention that your splines in the halfshafts are fairly stiff to move, may pay to check and see if they have sufficient lube , its not uncommon for the spline to be 'dry' at one end and cause problems.

Jac Mac
 

Peter Delaney

GT40s Supporter
Jac Mac - thanks. I must say that I am a bit dubious about the uni offset / angular velocity issue being the problem - mine is set up as per the standard DRB specs :

- Camber = 1 deg neg
- Toe-In = 1/16" at tyre
- Caster = 4 deg

The camber & toe-in offsets are so small, & I am guessing that of the 50+ DRB's on the road, most of them will have been set up close to this - no-one else seems to have the same problem !

Re the half-shafts - I have stripped them out, fully lubed them, then when re-assembled, filled them again with the grease-gun (& got heaps of grease spun around everywhere for my trouble !!).

So my gut feeling is that the problems are internal - arguably too much lash, & definitely an end-play problem with the LH output shaft.

I'll let you know what the latest "expert" has to say !

Thanks & Kind Regards ( & HAPPY NEW YEAR)

Peter D.
 
Peter,

I had some in/out play (Hmm), due to the bearing issue, but there was very little lash, when I rotated the axle flanges. It was a brand new differential though.




Bill
 
Backlash of the ring & pinion is measured using special Porsche/VW tools.
I don't know how to explain it except to say that you can't do it in the car. I can scan & send you the manual pages, but I don't know what you can do with that. I'd check the rest first.
 

Peter Delaney

GT40s Supporter
Bill, mine also has what I would have called "very little lash" when rotating the drive flanges - but, when the shafts & wheels are connected, that translates into quite a bit of movement at the tyre tread !

Jmracecar - thanks for the offer of the dwgs, but I think that I will wimp out & put it the hands of an "expert" ! Re the backlash - I realise that I can only measure the "total lash" of the whole setup whilst it is in the car, not just the diff lash - it just surprised me how much ! I get fairly similar lash in each gear, so I am guessing that there is not a specific problem with any one gear - just general slop spread over the gear sets & the diff (more likely the diff, as I have a brand new 5th gear set & it seems to have just as much lash there as with the other old gears). We'll see what the "expert" has to say !

It never hurts to have some knowledge when dealing with experts - & for that, I thank everyone who has helped.

Thanks & Kind Regards,

Peter D.
 
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