Official Erevo 1/8 scale center diff thread

[sjcrss]

Well , I decided to collect some of the info from MC, and my self and anyone else who has contributed to this CD, so it would be easy to find for everyone, I'll be copying & pasting in the info....
The birth of the 8th scale Revo CD - 08.31.2009, 10:30 PM

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(Mistercrash)
I experimented with a few options and ideas posted on the net about these diffs.
Two of them I found to be very interesting, a third one was made by a guy in France but it was almost a carbon copy of someone else's. The very first was made by CowboyRay some years ago for the nitro Revo 2.5. This was before Traxxas even came out with their own CD for the Revo. It was made with a Thunder Tiger S3 center differential, the TT spur gear was replace with a machined aluminum cap on which a custom plastic gear was secured with 4 M3 screws. I got my hand on a well used CowboyRay diff and inspected it carefully. The diff is smooth and the cap is very well machined, a thing of beauty. I noticed quite a bit of slop on the output shaft that went through the aluminum cap. Might be that a stronger and harder metal is needed there as the aluminum of the cap wore out and allowed the output shaft to wobble a bit too much for my taste.
The other one was made by scjrss and with his coaching; I made three of his CDs. He came up with a clever solution to secure the Traxxas CD output gear to an Ofna Hyper 7 CD spur gear. The Hyper 7 spur gear had its teeth grinded off to transform it into a cap and 4 holes were drilled and tapped in it matching the holes of the Traxxas CD output gear. The Traxxas output gear could then be secured to the Hyper 7 spur gear (cap) with 4 M3 screws of the right length. A brilliant idea and it works. But the part I did not like is threading the 4 holes in the Hyper 7 spur gear. It’s hardened steel that is probably as hard as the steel from the tap. It can be done but really, it’s a PITA. I noticed some seepage of diff lube using those CDs. It seeps from between the threads made in the spur gear and the screws. Not a lot but enough to annoy me.
Then there’s this guy that came up with a different way. I would like so much to remember where I saw the thread, I have searched and searched and can’t find it or remember where I saw it. If you know about it, please post his name here as this is HIS idea. He made four holes in the 8th scale CD spur gear and counter sunk from the inside. That way he was able to secure four aluminum inner threaded posts with M3 flathead screws and with some minor modifications, the Traxxas output gear could slide on the posts and secured to the posts with M3 screws. This post idea is the base of the CD I did last. I used a Losi LST XXL diff, just a regular diff, not a center diff. And instead of using four posts I used two. For the output gear, I used a different Traxxas gear since the Traxxas CD output gear is not sold separately. Once I was done with this last CD from a Losi diff, I decided to take the time to fit this new Traxxas gear to the CowboyRay CD and the sjcrss CD.
It is good to be as precise as possible when modifying the parts to make these CDs, but it doesn’t have to be down to a thousandth of an inch, you’ll see from the pics that the parts I modified are not perfect. It doesn’t matter because the thing works anyway. We are not touching any internal components of the diff and as long as the output gear is centered on the diff, all is fine.

So lets begin with the Losi CD, first you need a few parts, here is where I found the parts I needed. I give you the links to where I got the parts but you might have other sources to get parts from.

The Losi LST XXL diff, I found and bought it on ebay, I am posting a link but it might not work for a long time, just do a search on ebay for ‘’Losi LST XXL diff’’ and a lot will pop up.

http://cgi.ebay.com/NEW-LOSI-LST-XXL...d=p3286.c0.m14

The output shafts, made from the Traxxas 3.3 Revo CD output shafts. Part numbers TRA5415 and TRA5416. The shaft measurements can be seen on the pic.

http://www.amainhobbies.com/product_...put-Shaft-Revo
http://www.amainhobbies.com/product_...t-Gear-Revo-33

The Traxxas gear, part number TRA3985X. I think they were output gears for the old E-Maxx tranny.

http://www.amainhobbies.com/product_...r-33T-2-E-Maxx

For the aluminum posts, I had a few inner threaded aluminum posts and all I had to do was cut them to the length I needed. To find posts, just search on Tower Hobbies or A Main for ‘’post’’ and a lot of inner threaded aluminum posts will come up for servo trays, steering bellcranks etc. Just find the ones that would be easy for you to cut to the right length. I found these from Axial and I think they would work without any modification to them.

http://www.amainhobbies.com/product_...mm-Post-Grey-2

Two 8X12X3.5 mm bearings

http://www.amainhobbies.com/product_...all-Bearings-2

8x10 mm shims and 6x8 mm shims to center the CD perfectly in the tranny.

http://www.amainhobbies.com/product_...mm-shim-kit-10
http://www.amainhobbies.com/product_...6x8mm-Shim-Set

1.5 mm thick carbon fiber. Aluminum or other kinds of metal can be used also. Even a simple washer of the right size.

http://www.fullforcerc.com/Carbon.htm

Four M3x6 mm flathead hex socket screws. No link here as I am sure everyone has a place to get them, hardware store, LHS or online.

Here’s a link to some carbide bits that will help tremendously in drilling and counter sinking the holes in the ring gear

Drill bit
http://www.usacarbide.com/catalog/pr...ducts_id=17778

Counter sink bit
http://www.usacarbide.com/catalog/pr...ducts_id=17916