I have a set of 3.73 gears on a 3 series Eaton Posi. The gears have the following markings, that look like they were hand written with an engraver:

On the outer edge of the ring, it says- "BL .008 259P"
On the end of the pinion, it says- "259P Cd 2.827"

As evidenced by a couple of my other threads, I am getting ready to do a differential set up at home, in science-fair project mode. I will be going to a thick 4.10 gearset on the same 3 series posi. Part of the hassle of doing it at home will be the iterative process of shimming and pressing the bearing on the pinion.

What do the numbers on my existing gears represent? If it's currently set up correctly, can the markings on the existing gears be used to calculate a pretty darn close starting point for the first pass at the new pinion set up? Even if it's a 4.10 vs the old 3.73?

I assume the BL is backlash. Do all gearsets have these markings?

I have a set of 3.73 gears on a 3 series Eaton Posi. The gears have the following markings, that look like they were hand written with an engraver:

On the outer edge of the ring, it says- "BL .008 259P"
On the end of the pinion, it says- "259P Cd 2.827"

As evidenced by a couple of my other threads, I am getting ready to do a differential set up at home, in science-fair project mode. I will be going to a thick 4.10 gearset on the same 3 series posi. Part of the hassle of doing it at home will be the iterative process of shimming and pressing the bearing on the pinion.

What do the numbers on my existing gears represent? If it's currently set up correctly, can the markings on the existing gears be used to calculate a pretty darn close starting point for the first pass at the new pinion set up? Even if it's a 4.10 vs the old 3.73?

I assume the BL is backlash. Do all gearsets have these markings?

Not sure about the numbers. As to using the current set up / shim sizes as an estimation of the shim sizes on another set of gears. I kinda doubt it. All this shimming is necessary because of the tolerances in the individual components. .001 here and a -.002 there and pretty soon you need shims.

Just because one pinion shim worked with a given set of gears there is no saying that the same shim will be even close with another set of gears. Suppose the first pinion had a critical dimension that was +.005 from specification. Now ya get another R&P set with that same dimension at -.006. from standard! The shim that put the original pinion right where ya wanted it is not gonna get the job done.

I can't say if these numbers I created outta whole cloth are representative of what you could really see. The point is that the new gears are unlikely to have the same +/- tolerance variation as the old gears. The degree of difference will determine how close you get with the old shims.

Also remember you are gonna use new bearings. Same issue comes into play...just one more +/- variation that has to be accounted for by the shims.

Yeah, tolerance build-ups are going to be a factor! I just want to start somewhat close, not 1/4" off :clonk:

Lacing and truing bicycle wheels is pretty tricky, but easy if you have done a bunch (like me). On the old movie "Breaking Away", in one scene the guy is truing his wheel by turning the spoke wrench about 10 time. From experience, I can say that he taco'ed that wheel for the sake of trying to show that he works on his bike, 1/8 turn makes a big difference. Same thing applies here, I don't want to start out so far off that the patterns readings are garbage.

Richmond gear DO have the backlash recommendation stamped or vibrapeened on them. Most other brands don't have that since they expect you to simply use the car manufactures' backlash spec. The other figure might be the recommended pinion depth, but I don't know if they would be able to determine that since every rear end housing will likely vary in dimensions. You can always call Richmond gear to inquire. Richmond also sells a $6 instructional video to help you set up their gears. Can't hurt.

The mark on the pinion gear is the pinion depth from the center line of the differential bearing bore to the face of the pinion head. There are tools that are made to check this dimentiobn, but they are usually off by about .003. I don't use this tool becasue it isn't accurate enough and it's just a waste of time for me to set it up. You can start with the pinion shim that is in it right now and work from there. Backlash on a 12 bolt is almost always .007 to .009.