Hello everyone, I have an issue and have not been able to find an answer to my problem. I acquired an 8.5 10 bolt 3.42 ratio out of a GN to replace a 7.5 inch 2.14 ratio in my 85 Cutlass Salon and while going thru a few replacement items, axle seals bearings etc. I decided to put in some new side gears and spider gears and wouldn't you know the side gears are smaller and the pinion gears are bigger than the ones I took out. It all fits together fine (believe it or not). I don't know if there are different versions of these that will interchange or not. The axle code on the axle tube is 3TJ 131. The code charts I see say it's a posi but in reality is an open rear end. It didn't appear to ever be opened up, maybe it was stamped wrong in the beginning. Has anyone ever come across pinion gear and side gear issues like this?
I'd bet on it being robbed at some point. I believe ALL GNs came with G80 posi. Not sure about the spider gears. If it wasn't a posi, I'd swap in a TrueTrac diff.
ALL 10 bolt 8.5 'S' spring 410409 and 410408 side gears and pinion gears are the same. Posi to posi and open to open. EVEN the 726 and 725 NON c-clip side gears can fit into the c-clip 410409 and 410408 cases. So I do not know WHAT you picked up as far as parts go...THERE ARE NO NEW parts for the 8.5 GM 'S' spring spider gears So if you bought 8.5 spider gears in the last 20 years THEY ARE NOT s spring posi spider gears. Please send pictures of all your parts i will look them over and will see what you have. Jim JD Race
Hi Jim, Thanks for your help. I have attached a picture. Both the old and the new gears are for 28 spline axles. There is a stamp on the diff case of 1252981. The new side gears have 15 teeth while the old side gears have 16 teeth. The new pinion gears have 11 teeth while the old pinion gears have 10 teeth. It fits together nicely and seems to fit and run smoothly either way. The OE. gears are on the left while the new gears from USA Standard Gear (ZIKGM8.5-S-28) Spider Gear Set for GM 28-Spline 8.5 Differential Are on the right.
Oh never mind I see they are in the same shot... lol. Sorry. THAT looks like a EATON yukon 8.5 gear set. Do the side gears hold the original 8.5 square tab clutches?
if the gears are for NON posi - yes they will fit and function correct. Just make sure you can fit the thrust washers in properly too. Or likely the new gears will have that stuff with it.
Yes this is a non posi. The code stamped the axle tube says its a posi but it is not. The new gears and thrust washers have been installed and seem to mesh and function nicely. I was thinking it was some upgraded items as the pinion gears are bigger but the side gears are smaller, more heavy duty looking. I did not know that there are different combinations that would fit in the same carrier. The reason why I ordered new gears is because the old pinion and side gears had some bad teeth on them (see pictures).
Yep, It does happen in the aftermarket deal...They try to make things better - The metal part is what really should be concentrated on. The number of teeth ?? well it helps but for what your doing and being an open rear...NOT as much of a concern. I think your going to be fine. JIm JDRace
Thank you so much. I'm going to run with the new gears and see how it works. I can always swap out the old parts as long as it doesn't grenade.!
