Greetings of the season to all members.
I am in the process of overhauling my IRS. I have had the differential overhauled by a specialist. The cage, wishbones, driveshafts etc. have all been shot blasted and powder coated in gloss black and they look stunning. I have fitted new brake discs, (rotors) and checking for runout. I seem to have .006 on one side and slightly less on the other. Is this outside normal tolerances? I have replaced all the old shims with new ones of the same thickness and I thought I had wire brushed the diff flanges clear of the paint that had been applied to the diff. Perhaps I will have to go back and do it again? Any advice will be gratefully received. Kind regards, Danny.

 

We love photos of IRS rebuilds. Please post some.
I didn't check all that technical stuff when I did mine (including the diff), just took it apart, put in new stuff and put it all back exactly how it came apart. Done 60,000 km since and lots of track days.

Someone else will be along soon with good advice.

 

I will post before and after photos very soon, thanks for your reply. Regards, Danny.

 

Likewise with all that I have done over the years.

CLEAN, and recleaned, bolted up and never had any issues.

Only thing that slipped a few times, is I forget the count of shims between the rotor and flange, so had to do the rotor/caliper centralisation from scratch. Not hard, just time consuming.

The tolerances at manufacture of rotors is very tight indeed, so if you have runout of a few thou, there be grit or something similar in that area. You did mention paint removal, so maybe there is a blob or 2 still there??

 

Brake run out should be at a maximum of .002. The first step in taking run out readings is to do a run out of the hub it is mounted on the max there is also .002, once that is established as correct then mount the rotor and do a run out on both sides of the rotor. If one side is good and the other is bad then you can have the rotor resurfaced. If you get a positive reading on one side and negative on the opposite side of the positive reading then the rotor is warped and not usable.
Make sure everything is clean and tight. I usually coat the mating surfacing with a high temp compound similar to "Never-Sieze" it keeps them from being difficult to service next time.

 

Very many thanks for the replies. Today was spent removing both calipers and discs, took the electric drill powered wire brush to the diff flanges to remove all the very slight traces of paint. Looking very closely at the mating surfaces of the new discs, I noticed that that there were burrs on the inside where large holes were drilled. These holes were not on on the originals and I believe they are for caliper bolt access through the disc, although not on my car. My bolts are on the other side! These burrs were causing the inside shims to raise up slightly. I filed them down with no trouble. I cleaned and cleaned again the mating sufaces, replaced the cleaned shims and mounted the discs. To my dismay, there was still a considerable run out on the edge of the discs. I suspected that the new discs were rubbish so I temporarily fitted an original disc. There was no improvement. Back to square one! I then swapped the offside and nearside discs over and then tried turning them 180 degrees and finally have arrived at a much better run out measurement. Its not perfect, .004 inch and .003 on the other side. I do not see how I can improve on this because I experimented on part shimming one side of the disc by fitting a half shim. The result was a huge runout on the edge of the disc, obviously because it is magnified. I think to achieve a correction of 3 thou at the edge of the disc, I would need a very skinny shim of .0005 approx. I have concluded now that the runout I have now must have been there all along and I wasn't aware of it or any noticeable vibrations. Unless I have anymore thoughts, I am going to leave it and see how she runs! All the best wishes for 2018, Danny.

 

Just an idea, when I was toolmaking I would need to adjust a piece by a ‘nats ****’ we would use cigarette papers they just happen to be <>.0005”

 

Yeah, were I in Danny's shoes I'd call it "close enough". But, I just might have placed each rotor on a true flat sheet to see if there was a wobble or rock or highs in which a feeler guage might slide in. I've my doubts as to whether the average skim would get them any closer.


And, the same comment for most new ones.


There is another engineering term for tight measure. RCH. For good manners reasons, I'll not use the full term.


Carl