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-   -   Savage CEN diff Mod- (Jesus Built My Differential) (https://www.rc-monster.com/forum/showthread.php?t=18933)

Finnster 02.21.2009 06:33 PM

Savage CEN diff Mod- (Jesus Built My Differential)
 
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:
http://i7.photobucket.com/albums/y25...ndiffparts.jpg


Notice just how much larger the teeth are:
http://i7.photobucket.com/albums/y25...f/IMG_6189.jpg

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.

http://i7.photobucket.com/albums/y25...f/IMG_6199.jpg


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.)

http://i7.photobucket.com/albums/y25...f/IMG_6202.jpg



One Diff to Rule Them All:

http://i7.photobucket.com/albums/y25...f/IMG_6205.jpg



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.
http://i7.photobucket.com/albums/y25...f/IMG_6213.jpg



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

http://i7.photobucket.com/albums/y25...f/IMG_6206.jpg


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!

Metallover 02.21.2009 06:38 PM

NICE! I want to know how this holds up long term. If it came out of a Genesis I would imagine a savage would have no business stripping them.

lincpimp 02.21.2009 07:06 PM

Very nice detailed writeup! I will be doing this on my flm savvy as I plan to run cen wheels/tires and they are heavy!

I am wondering if you think the x diff cases are strong enough? I looked into getting the flm cases, with towers and the lst shock kit. Not too impressed with the "small bore" x shocks.... If the pastic cases the x comes with are good I will keep them, as I really do not want to spend another 120 bucks if I do not have to. Wondering if they flex enough to cause mesh issues? With the larger teeth on the cen diffs the flex may not be a problem...?

brushlessboy16 02.21.2009 07:10 PM

Great How-to, i vote for a sticky :)

are you dyslexic?
(reading the title, "jesus built by differnetial"

traxxasbasher13 02.21.2009 08:43 PM

Make it a sticky! Great how-to!

Finnster 02.21.2009 09:52 PM

As far as cases go.. I do know the flux had upgraded cases. I would try those first before the flm ones, unless you really wanted them. I think you lose the quick removal capability too (all iirc)

As far as the title....dammit...
Did you read that thread where I proposed greater edit functions? Ya, this is why lol
Its supposed to say Jesus Built My Differential

My other alternate title was "or How I Learned to Stop Worrying and Love My Differential," but it didn't fit. :)

I better go PM Brian... 0_o

lincpimp 02.21.2009 10:45 PM

I will give the stock X cases a try first. Just ordered the flm towers and lst shock adapters from Mike...

I had a look on ebay, seems like some of the diffs have different cases. The GST look to have plastic cases, while the nemisis have al cases. Are the diffs inside any different? Do they both have the al cups? And I also looked for the crosspins for the spider gears...are they really that expensive!!!???

azjc 02.21.2009 10:59 PM

Quote:

Originally Posted by Finnster (Post 263878)
As far as cases go.. I do know the flux had upgraded cases. I would try those first before the flm ones, unless you really wanted them. I think you lose the quick removal capability too (all iirc)

As far as the title....dammit...
Did you read that thread where I proposed greater edit functions? Ya, this is why lol
Its supposed to say Jesus Built My Differential

My other alternate title was "or How I Learned to Stop Worrying and Love My Differential," but it didn't fit. :)

I better go PM Brian... 0_o

when I first saw the title "Jesus built my differential" I did think of the Ministry song with the lead singer from The Butthole Surfers singing in the song...that brings back the early 90s....

Metallover 02.22.2009 02:04 AM

I happened to find an ad on RCU with a bunch of GST diff parts if anyone is thinking about doing this.

http://www.rcuniverse.com/market/item.cfm?itemId=486907

Finnster 02.22.2009 08:50 AM

Quote:

Originally Posted by Metallover (Post 263952)
I happened to find an ad on RCU with a bunch of GST diff parts if anyone is thinking about doing this.

http://www.rcuniverse.com/market/item.cfm?itemId=486907

Unfortunately those are all the parts you toss out from the mod. :wink:

doo540 02.22.2009 11:17 AM

I have also done this mod, for my brushless XL (neu 1515 1y 4s) So far the only thing I have found to be a problem is this. I used the drive cup that came stock with my cen gen diffs. After getting the pinion shimmed correctly it left very little of the pinion shaft sticking out of the savage bulk heads. It was causing the set screw in the drive cup to not fully seat on the flat on the pinion shaft. This caused my setscrew to come loose. So to fix that I grounf off a small amount of the end of the drive cup so that more of the drive cup fits on the shaft. Cant test this mod yet as there is about 10 inches of snow on the ground again. Global warming I sure.

Finnster 02.22.2009 11:32 AM

Any reason you used the Cen drive cup? I used the oem Savage drive cup and there was room for the whole setscrew to contact the pinion.


FWIW: The F Cen diffs come with diff cups. The rears come w/ cvds. I had only rears.

doo540 02.22.2009 03:26 PM

I liked the way the Cen cups look alot better then the oem Savage cups. I figured I paid for them might as well use them. haha I think there are going to be alot of unhappy Flux owners out there when their diffs start breaking. Did you get this mod from RCbasher? http://www.rcbasher.com/savcendiffs.html
Or did they steal it from you? Either way it works, I just hope it works all summer long. I plan on doing some 6s neu 1515 1.5d testing this spring just to see how high and far a Savage can jump. My neighbors have a pool.........

93firebird 02.22.2009 08:08 PM

Owning a Cen genesis and another Cen product, I'm not quite sure why you chose their diffs for the project. Just look on RCU and you will see all the problems ppl have with them.

fastbaja5b 02.22.2009 08:48 PM

Silly q, where can I get these cen parts online? Already hearing stories of Flux diffs giving up!


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