differential spider and side gears

[bb80]

Hi all,
I'm going to replace oil in differential and I hear little bit knocking when shifting between R and D (on AT) and suspicion is side/spider gears. So I'm going to replace them in one go with oil, but not sure if I need 28 spline or 31 spline. lmr, summing racing and some others are saying 31, but few other sources saying it is 28 (what confuses me most is that chatgpt is trying to convince me that it is 28 and it is common mistake of lmr and similar). I would like to find it out before opening the differential as part delivery time could take up to 2 weeks.
How to find which one is for my diff? I have mustang 2013 3.7 V6 convertible base, on diff i see marking S201C121140471J0.
Thanks for any hints.

 

[DieHarder]

In my experience a little bit of sound shifting between R/D is normal though mine usually makes it going (D-R) or (P-R). I wouldn't go looking to introduce new problems unless it's obvious they need replacing. (I.E. Don't fix it if it ain't broke). How many miles?

 

[bb80]

I have ~85000 miles. I was assuming that changing onlý spider+side gear is no problem ( no need to adjust pattern/ backlash). What kind of problems can it introduce?

 

[skwerl]

IMO do not replace the side gears. You can open it up to change the oil and see how much wear is on the gears. It will cost you 3 quarts of gear lube and a tube of sealant to glue the rear cover back on.