Putting 3.73 gears in the 12-bolt. The pattern seems pretty good with a 31/1000 pinion shim (and 8/1000 backlash). At least, it seems to match the patterns in the references I have.

Got an old Hot Rod that says 3.73 gears often use 25/1000 to 26/1000 pinion shims. I know you can't tell much w/o the pattern, but I want to ask you experts, should I reconsider my 31/1000 shim given this Hot Rod info? What ranges have you seen?

Use the shim that gives you the best pattern, don't worry about what the magazine says. The pattern is always the last word in setting up a ring & pinion set.

Can I get an Amen

I would only add - the "pattern" AND the backlash fiqures. Good driveside contact patterns with to much backlash still mean a songbird rearend.

.008" is perfect.

I would definately IGNORE any information that a magazine gives you conserning a pinion depth shim 'guesstimations'......because that is exactly what it is.......a GUESS.

I will say that there is an acceptable range for most gear manufacturers. Out of the past 300 or so 12 bolts I have done, the range I have seen is between 0.025" to 0.047". These numbers include factory OE sets and aftermarket.

Most of the Richmonds have set up with around 0.032"......standard- thick or thin.

Most of the US Gears have set up with around 0.036".....as well as Precisions sets (they are off of the same cutting machine).

Auburns....around 0.037"....

In reality, these numbers are just a close GUESS. They will get you in the ballpark, but there is more to this than picking a number. Way too many variables to give you the "right" shim thickness.

The best thing is to take a look at the set-up compound (paint) and make your adjustments from that and that alone.

Also, there is no 'perfect' backlash measurement either. But there is a rule of thumb: There should be 0.001" of lash for every inch of ring gear diameter. So, 8.75" would equal 0.00875" and up. Remember that I mentioned it to be a rule of thumb......I have set up street diffs with 0.005" on the low side and all the way up to 0.012-0.015" on the high side. I really would not let a diff go out with more than 0.012"....personally.

Some considerations for lash:
Gear oil viscosity- (thinner oil, can use less lash...thicker oil, can use more lash)
Weight of vehicle and power applied- Cars that make A-LOT of power, can use less lash to combat tooth deflection.

In So Cal I run most shipped diffs with 140wt oil and set for about 0.010" lash (where the vehicle will most likely not 'see' freezing temperatures. In a colder climate I ship with 90wt oil and set for 0.008" lash.....as a guide line.

For point of reference, I set up Top Fuel diff's with ZERO lash.....for a few reasons. But lash is the "end" adjustment to where a gear set either has text book contact or you need to make a depth change.....

[ 03-01-2003, 04:15 PM: Message edited by: chicane67 ]

Could you type a little slower Chicane - I'm tryin' to get this all down....

"The best thing is to take a look at the set-up compound (paint) and make your adjustments from that and that alone.'

'Also, there is no 'perfect' backlash measurement either. But there is a rule of thumb: There should be 0.001" of lash for every inch of ring gear diameter. So, 8.75" would equal 0.00875" and up.'

'In a colder climate I ship with 90wt oil and set for 0.008" lash.....as a guide line."

'Zackly! I couldn't agree with you more. We don't run much 140W up here in the No. part of the state because we do get low 30's and some freezin' weather even here near sea level. (hey - don't you ever go up to Big bear?) But your info on patterns, clearances and lube are right on. graemlins/thumbsup.gif
I think your set-up numbers are great starting points for the different gear sets out there. I have noticed that some manufactures tend to take the same relative shim packs but I never thought to record them and look at the numbers. If you don't mind I'm going to 'borrow' you numbers for my driveline log book.

Thanks for bringing a lot of real world experience into this discussion.

Yeah I do go to Big Bear often (Dad retired years back and lives up there) but the truck doesnt skip a beat in the colder weather......

If you need more 'numbers' I have information on every gear manufacturer who makes products for Ford, GM and corperate GM, Chrysler, AMC, DANA, Borg Warner, Eaton Corp., Toyota, Nissan, Mercedes, BMW and a few others.....

I started this due to that over the years there have been some manufacturing 'issues' due to dull cutters and machine tolerences changing the gear sets. Keeping a book can help locate 'when' these problems start. Gotta keep those manufacturers in line ya know.....cuz if I cant install their product, neither of us make any money.

I too have been compiling a book on differentials and transmissions to include bearing numbers and any interchange part number, shims, seals....etc.

The funny thing is that I know SO many guys that are doing this but have you noticed that none of us are done !?!!

Whoa - small world - Parents retired and we built them a house in Big Bear! Sister and bro-in-law live and work up there to now.

Don't think I'll ever get enough data to do a book myself - just keep them to see what I did anymore. Used to be a line mechanic at a large So. Cal. Chevy dealer (damm - where is that master tech plaque anyway - sucks getting old) and did nearly all of the manual trans, rear end and Corvette drive line work. Keeping logs and numbers helped me spot issues and deal with them quickly. Don't miss it at all really - but it's fun to get dirty on the weekends. I'm down to maybe a dozen rearends (chevy and 9") a year now, mainly for friends and a race shop I'm associated with.
Would love to get any 9" numbers you can spare - specially Richmond and Eaton, tend to work with these the most lately.

Thank you for the feedback. I hope to learn from your wisdom in future posts.