The Stovebolt Site Forums The Shop Area Engine Shop checking for ring gear runout and lash

Reading Patrick's instructions # 63 & 64 has me a little confused.

#63 checking ring gear runout. According to their tiny picture are we simply checking to see how flat the ring gear is sitting? Looks like the dial indicator is making contact with the back of the ring?

#64's picture is worse. It says to move the dial indicator for lash check. I cannot tell what the indicator is touching? Can someone explain what lash means? It says ideal lash should fall between .008-.012"

The remaining instructions regarding tooth contact, adding or removing shims is pretty well understood. What is the best paint to use for checking this?

Are you also looking at a shop manual? I saw some drawings in there.

I was really looking at Patrick's instruction but just now reviewed the shop manual which talks about lash. It is still sort of unclear to me as to just what lash is but my first guess is that it is the play between the ring gear and pinion. Meaning if I held the pinion really still, I would expect that the ring gear would have just a little play movement. Is this lash?

After reading the manual, it seems that you can take up or increase this play up by moving the ring gear left or right via the carrier bearing adjusting nuts.

Does it matter what part of a ring tooth the indicator is touching? The pic in the manual is showing somewhere near the outside edge of the gear tooth.

Tony M, Don't you see how much fun you're missing by receiving 3:55 gears already installed??

Happy New Year.

PS-Just came back from Harbor Freight who sold me a dial indicator and magnetic base for half the price I would have paid via well known truck gear suppliers.

I would have never attempted that job by myself. I can do engines but will not touch transmissions or rear ends. I got very lucky indeed.

And if I get in any trouble installing mine I will have an expert nearby. You will be an expert by then.

Happy New Year to all.

The run-out should be the measure of how much left and right the ring gear can move in the bearings. The dial indicator should be against the back for this. Lash (or backlash) should be the amount the ring can rotate withouth the pinion moving.

Standard disclaimers apply.

lash or back lash is the amount of free play between ring and pinion hold the pinion still, set the ring against one side of the pinion , set dial ind. to zero. now move the ring till it makes cotact against other side of pinion that is your lash. you need it for heat expansion
Iwork on acrs for a living and dont do rear end to many ways to screw up, find a drive line shop and ask if you can watch while they set it up, every time you move one yhing it changes a angle to something else

highflyer,
Does it matter where the stem of the dial indicator is touching one of the gears? I assume it doesn't as long as it clearly measures the rotating travel of the gear ring, with the pinion being held still.

Tony M,
I purposely put myself in this situation to force myself to see it to the end. Yes, If I can do this, I can help you on your project.

Cmyna

Beware true amateur advice! However, Warren is probably correct re lash. Measure how much one ring tooth moves with the pinion still. The leg of the indicator should be tangential to the outside section of a tooth (ie perpendicular to a line drawn from the center of the ring gear to the perimeter, and parallel to the plane of the gear).

For runout, my shop manual says the bearings should be pre-loaded, so I don't believe there should be any run-out. This comes from a 1954 shop manual. I'll try to scan the images and send them along.

Addendum

The dial indicator should be near the outer edge of the gear. Too close to the inner edge and the measure will be screwed up: good lash will show too tight, loose lash will show okay.

"If I held the pinion really still, I would expect that the ring gear would have just a little play movement. Is this lash?" That's correct. By adjusting the carrier bearing adjusting nuts, you are either moving the ring gear into the pinion gear(less backlash) or away from the pinion gear(more backlash).

"Does it matter what part of a ring tooth the indicator is touching? The pic in the manual is showing somewhere near the outside edge of the gear tooth." The outside part of the ring gear is the best place to set the dial indicator.

Runout is taken on the back of the ring gear. You're checking to see that there isn't any wooble in the ring gear. There should be a spec somewhere.

Do the instructions have the specs for the bearing preloads. This is very important. I don't want to scare you, but if the preloads are not correct, you could ruin bearings and be doing it again.

http://www.thedirtforum.com/differential.htm

The more I think about backlash, the more I believe the importance of just where exactly are you measuring. The further away from center during measuring, the bigger, the movement. The closer to the center you are when measuring the less you would see in movement.

Preloading bearings are not mentioned very much in the instructions. Need to look into this more carefully.

Patrick's instructions mentions nothing about preloading bearings whereas the shop manual says:

Install and tighten cap screws until lock washers just flatten out. Loosen right adj nut and tighten left adj nut while turning ring gear. Continue tightening left adj nut until all lash is removed, then back off left adj nut one notch to a locking position.

Tighten right hand nut for force left bearing firmly into contact with left adj nut. Then loosen right adj nut and again tighten snugly against the bearing.

Tighten right adj nut a minimum of 1 additional notch to a maximum of two additional notches further to a locking position. This operation preloads the diff bearings.

Then the instructions goes on measuring backlash. Shop manual says ideal lash is .005-.008, whereas Patricks say ideal is .008-.012, no tighter.

Too many chiefs, not enough indians....Hmmmm :confused:

You guys are scaring me!

Craig, the looser specs on the Patricks gear set are looser because the gear teeth are a larger pitch (hence the ability to do a 3.55 ratio). GO with the looser lash settings. The lash must be measured in the location specified or else the measurement is meaningless. .001" of lash 1 inch from the center of rotation is the same as .005" of lash 5 inches from the center of rotation. (remember geometry in the 8th grade . . . similar triangles apply!)

As for Run-out - there is radial run out and axial run out.

For Radial run-out, place the dial indicator on the outer machined surface of the ring gear to measure displacement perpendicular to the axle. Slowly rotate the axle and watch the dial. It should remain relatively close to the same point. I would expect to see +/- .001 or so inches.

Radial run out is something you cant do anything about. The ring gear is piloted on the carrier and shouldn't have any slop. If it does . . . you are screwed.

For Axial run-out, place the dial indicator on whatever exposed machined surface on the back of the ring gear you can to measure displacement parallel to the axle. Slowly rotate the axle and watch the dial. It should remain relatively close to the same point. I would expect to see +/- .001 or so inches.

Again, what are you supposed to do if it is out of spec.? That would indicate a problem with the machining of the ring gear.

I have never set up an AD differential. I have done a lot of 10 bolts (because I screwed the first few up and had to redo them) :p

This will be a great learning experience for you (and Tony M).

Ken,
I have not read anywhere as to specifically where the indicator should be touching the tooth except for being at the far outside edge.

Meanwhile I decided to save some more money and just finished making a set of carrier bearing adjusting wrenches out of 5/16" steel rod, heating up before being bent:

http://community.webshots.com/photo/105853469/108165948giWfka

Thank god I won't have to deal with any of this. I got my 3.55 already installed in a spare rear end. I am going to swap the whole thing. Thanks to Joker.

Tony M dodges a big bullet.....Don't worry there's always another one around the corner.