I was getting tired of the “tall” gearing (even with the 3.62 M3 ring and pinion) so I decided to pull it all apart for a gear swap. The 3.62 is the shortest ratio that seems to be “readily available” in production cars - it seems that the 3.91 and 4.10 ratios in 210mm size are “Motorsport only”. 4.10 was my first choice, but seeing the price tag made my head spin. Luckily, the 3.91 was offered for a few years in the E34 M5 (which is still absurdly rare). I got in contact with a guy who had his turbo E30 (with a 210mm rear swap) set up for drag racing that wanted a taller ratio, so we worked out a deal to swap ratios.
The older style 210mm ring gears use M12 bolts, while my E39 M5 center LSD carrier has thru holes for a 14mm. I had the option of ordering bolt sleeves through a BMW Diff shop but decided to get some experience in machining.
I ordered up some Tool Steel rod with a .5781" OD which gave me a nice press fit into the carrier, but had no thru-hole.
It took me a while on the lathe but I got a rhythm going and banged out a qty of 10. The thru hole was relatively large in comparison to the OD so I had to step up the drill size 3 times for each part. Very repetitive but fun.
Threw on the ring gear and tapped it down using a rubber mallet. It’s a very tight fit as you get to the bottom of the carrier flange and there is no room for rotation so you have to get it aligned with the thru holes correctly the first time. I was able to get it right on the first try. Like they always do, ARP included washers with their hardware and I wasn’t sure whether or not to use them since the OEM bolts are flanged and don’t use washers. After measuring the depth of the threaded ring gear bolt hole I found that I still had tons of depth to go even without a washer and decided to leave them out. Some red Loctite and 110 ft-lb torque gets me this:
I also replaced the front and rear pinion bearings and races while I was in there. Bought a HF 12 ton press and was able to bang it all out myself. The pinion probably didn’t need this following step but I wanted to use our basement freezer, damnit.
lol and i quote Bruce: “at least if I make something for you I know you’ll use it” (referring to you). he gave me a few pointers fo sho, good dude
I went to a much shorter tire as well (25.7" compared to the old 27.25" BFG) so that also shortens the gearing which hopefully will be a nice kick in the pants. However knowing how quickly I get used to it I will probably think about a turbo sometime in the next few years.
Definitely cruise when I get it back on the road. You need to come out to Frostys
I found some old pictures from a few months back that I took after I got the fenders rolled in the rear. I use this build thread as a record of sorts and i realized that I never took pictures of the exterior after the M bumper conversion (front and rear). I am not a fan of the stance movement (quite the opposite - stick your stretched tires up your ass) but I do like things to look proportional to the body. Up front: 225/45/18 with a 12mm spacer, and 235/50/18 rear with 10mm spacer. Those rear tires are since gone, replaced with 245/40/18. I never really plan out tire size when replacing- just whatever is readily available in something close to my size, and cheap (with the way I go through tires it makes sense for me).
lol. ironically I changed the rear tire size not long after the pics were taken. They’re much shorter overall and almost look stretched, but seem to fit the overall style well still. (235/50/18 old, 245/40/18 new)
