Savage CEN diff Mod- (Jesus Built My Differential)

[Finnster]

With all the Fluxes coming out, as well as BL conversions, I'd thought I'd do a quick how-to on the Cen diff mod for Savages. Nitro motors break the stockers easily enough, so well see how the new Flux diff hold up BL power!

The diff mod has several advantages, the biggest of all being a huge and rugged diff. Not only are the teeth really, really meaty in the Cen pinion and ring gears, but the spider gears are also larger and a stronger steel than the Savage diffs. Also, the diff uses 8x16mm bearings throughout, so more reliable bearings as well.

The Flux does use a stronger ring gear than other Savages, but still shares the diff cup w/ 10x16mm bearings and the weaker spiders. Also, given the huge pinion gear on the Flux's motor, a drop in drive ratio would be welcome.

The Cen diff does have fewer teeth, to gives a final drive ratio of 2.89 vs the savages' 3.31. You will need to adjust gearing of spur/pinion to give same FDR.
EG: 2.89/3.31*(std pinion)= new pinion size needed
2.89/3.31*(20T)= ~17.5 -> 17T or 18T pinion needed. You can play with ratios easier w/ BrianG's Calc.

Cen Diff parts, and comparison to Savage Pinion, Ring Gear, and Diff Output Cups of a Sav XL:



Notice just how much larger the teeth are:


The Mod is fairly straight forward. The Cen diff is a std 1/8th size. The Savage output cups do need to replace the Cen output cups. The holes are centered properly, so the only real work need is to increase the thru hole for the drive pins from 2.5mm to 3.0mm.
I've seen others use a 1/8" (0.1250") drill bit to do this. I used a 3mm (0.1181") as I didn't want any slop. In the end I had to do a bit of filing to the hole to get the thru pin through easily, as well as the spider bevel gear to go on smoothly. In the end I would prolly just go w/ a 1/8" bit for simplicity. Also, the Savage output cups are not as hard as I expected, so a standard bit cut through it fine.
Be sure to take your time and get the hole as straight as possible. File any edges so the spider gear slides on easily.




Reassemble the diff with the new Savage cups as well as a rubber shielded 8x16mm bearing. Don't use the unshielded bearing Cen uses. Fill w/ your favorite oils.
Note: Not pictured but later installed are Cen Cross Pins. These are optional and not required, but are one more bit for strength, as well as less oil needed to fill.)





One Diff to Rule Them All:





Not quite done! Remember that the Cen ring gear is smaller than the Savage's. Therefore the drive pinion will have to reach further in the diff case to mesh with the diff. You will need several 8mm shims to do this. I used a total of 2.3mm worth of shims to get the proper mesh. YMMV.




Time to reinstall the drive pinion and diff, check mesh, then seal it up and go!




Notes:

• Some directions say you will have to heat the diff to get the 3mm screws out. I didn't have to, but the hex screws were very tight and I have high torque hex tools. Be very careful.
• You can use either Front or Rear diffs. Ebay is easiest place to get whole assemblies. Look for Genesis diff assemblies. (The Matrix TR and the Nemesis use largely the same parts as well.) The Matrix and Nem lists diff part #'s for the diff body. I'm not sure what differences are, as they seem the same. I used the cross pins in the alu diff case too.
• One diff I had came with hex screws and grease, another with oil and phillips head screws (these were loose.) Otherwise they were the same. WTH?
• The Cen Drive Pinion is held by a 5mm setscrew, rather than a 5mm drivepin like stock. Easiest way to get the pinion back in is to just pull the stocker straight out the diff case, leaving the drive cup on the center dogbone. Then just push the new pinion straight thru onto the drive cup, making sure to align the flat spot on the pinion for the set screw. Discard drive pin.
• I used hot glue to plug the hole in the Cen pinion. Not sure this is necessary. as the drive cup is closed.
• The Cen gears are very, very hard. I easily drilled thru the 4 Savage output cups with the same bit w/ little wear. I needed to enlarge a hole on the spider gear brace for the cross pin to align properly, and it was a major PITA. It took forever and it dulled the bit. The cen gears are really really hard.

If you manage to destroy these diffs.... Jesus I don't know what you are doing. At least the parts are cheap!