Hi All,
I am hoping you can help me out with my diff re-build. I am new to Corvettes and I am not a mechanic, however I am a mechanical fitter machinist, so I do have a level of competency.

I have pulled down my diff (1977), it was some months back now and am now in the process of rebuildng. I have done a Posi clutch replacement and done a tune, I am happy with how it has come out.
I have replaced all bearings (Timken) and seals are new - gears are being reused as they seem fine. Car is a mlld 350 SBC, Auto trans - just a Sunday cruiser. Non purist car, non numbers matching RHD conversion. Not a high horse power car. Sorry to all the purists.

My issue is clearance/interference on the posi carrier in the housing. From memory it was tight to get it out (pre-load), but now with new bearings I literally cant get it back in the housing. Using digital verniers, I have found I have 0.015" interference. I have two pre-load shims 0.240"and 0.246".

What should this fit measure?
How do I move this unit side to side to set the gear mesh? I have pinion shims, but nothing other than the two preload shims.

One other thing. I have found heaps of youtube content on posi centers, and chev diffs, but no content on this particular application in the C2 C3 diff housing. Is there one?

Thanks in advance to all that respond.

Jeff.
Australia.

 

There's a shim kit to adjust side to side. Add or subtract , measure backlash repeat if necessary

 

The stock shims are cast and setup for the original dial in. If you have a complete master kit then then you should have stackable steel steels and with access to a surface grinder you can preload the bearings and dial in the lash. If you don't have them, you either buy them or make what you need.

 

Get a set of supershims from Tom’s Differentials. These will allow you adjust the thickness of each side in order to achieve a proper pattern (assuming your pinion location does not need to change) and have zero gaps on each side. Once you are satisfied with the pattern, then add an additional 0.005” shim to both sides

 

Thanks Gary,
How much preload should I be aiming for?

 

Thank you. Makes sense. Some after set up, I am seeking 0.010" interference?

 

There is a mathmatical formula for figuring preload based on the gear ratio but I keep total 8-10 over pinion preload. Additional shims depends on the setup preload you're working with. Typically 005 per side is good but I have used more or less, all comes down to how you have it set during the dial in process.

 

Technically, it is not “interference”. The .010” is needed to preload the bearings.

 

OK Thanks for all the above info. I have sourced a set of shims and worked out a easy set up. Next problem.
I cant get the crown to run true. How far out should I expect it to run? I feel it is way too far out at 0.030".
After mucking around a bit, I put the posi in and found it is running true (at the male spigot where the crown gear mounts) at under 0.001" both radially and axially, so happy with it. The pinion is in and preloaded, I don't have a way to check it for concentricity.
How I checked the crown. I marked four different spots on the crown in different colour paint. Then marked every bolt hole 1 - 10. Starting at the same paint mark (pink as zero) I measured the four spots. Then I moved the crown one bolt hole (pink paint to bolt number 2, then pink paint to bolt number 3 etc).
Didn't take long to see a pattern and I had the same results for each location. I only did three, then jumper to bolt number 5 and found the same result. I m happy to continue if it will give me a different reading, but I feel i have proven to myself the crown is not concentric.

This tells me the crown is not concentric to the centre hole. And when turning the pinion, I have tight and loose spots...not even worth trying to get a pattern at this point.
So.
Am I measuring this correctly?
What is an acceptable limit on this?
Am I missing something?
Will a new crown and pinion run true (very expensive in Australia).
Is there better quality brands of gear sets...brands to avoid?
All bearings are new Timkin. I was just hoping to replace weeping seals not replace every component.

Please note, the numbers below and the dial indicator are metric - divide by 25.4 to convert to imperial if you feel the need.

Thanks in advance.


Where I took my measurements from.

 

not sure on how you are measuring others will know..I don't remember measuring that as set up.
My comment is if tuned and dialed axle end play in i would remove the springs... my thread if helpful with advice i got.
https://www.corvetteforum.com/forums...ay-advice.html

 

First, get the GM Service Manual on your car for guidance.

Understand that the sum of the appropriate interference in the fitment of the center section is based on a resultant resistance of rotation of that carrier in the housing, this as torqued in place, this often termed as being a "preload" value, not a particular dimensional interference sum. This sum can vary depending on the the componentry involved (of particular consideration: new vs. used bearings) and then intentions of ideal. And since one is generally trying to cram more in than that which will readily fit, the practice is to utilize a tool known as a "housing-spreader" to ease this task; now, many don't have such equipment, so each to his own often adopt techniques of insertion to get around this deficit, many quite successfully.

The O.E.M. factory "shims", regardless of the manufactures' vocabulary, might be better identified as "spacers" due to the greater mass, particularly thicknesses, than that typically envisioned for "shims", and are engineered that way for a reason; this as we have witnessed repeatedly that when the universal shim kits utilizing the "stack-a-shim" process that these multitude of thin shims often move, this leading to them often becoming displaced or damaged and the preload factor is lost and the relationship in the positioning of the ring & pinion is lost.

In your "run-out" measurement, in referencing the photo of the placement of the dial indicator, this is not a critical dimension, that of the periphery of the ring gear, and is not of concern. But do measure the mounting faces of both the carrier and of the ring gear, that which butt when the gear is mounted to the carrier , for excessive run-out that may be present. But if such were the case this will often be exhibited in the back-lash measurement and in the pattern check inspection also.

Scott.