Inside the CEN Diff`s

NITRONUT

Well-Known Member
Messages
185
Location
Lindenhurst N.Y.
One of the problems that savage owners have to face sometime or other is differential failure. Sometimes it can be caused by bearing failure, weakness of the plastic diff cups or improper shimming of the ring and pinion gear.

One of the best things you can do is find a new set of diffs like the ones used by CEN Racing furnished by my buddy BLAZER X.

LETS TAKE A LOOK

First thing you can see is that the entire diff is metal, no plastic here folks!
After opening the diff housing the first thing you will notice is the way the bevel gears are mounted in the housing.
There are NO cross pins to hold the bevel gears.

Here is a look of the typical setup of bevel gears used by HPI in their diffs.

Now lets look at the difference.

You can see the HPI`s on the bottom and the CEN bevelgears on the top. There are no cross pins holding the bevel gears in place. Instead the gears are held in place by the square holder that does not allow any flex in the drive trainand allows the bevel gear to rotate in the holder.
Here is what all four bevel gears look like installed.


Now we get to see the dreaded bearing failure common to the savage diff`s

Allmost everyone has seen this!

YES THAT THING AT THE BOTTOM OF THE PIC WAS A BEARING...LOL

Now we can see the difference in the bearings next.

The CEN on the right uses a smaller shoulder allowing a thicker bearing that should handle the abuse we put our trucks through. The stock HPI on the left uses a much thinner bearing that just cant handle the torture.

All of the diffs have good quality O-rings to keep the diff fluid in and from what I can tell shouldnt leak a drop.


The last thing you have to do is fill them up screw the tops back on and throw them in!


As soon as the weather gets a little warmer and I don't have to freez my but off in the garage I will post the instalation how to`s in Part 2.​
 
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Larsenracing

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Manufacturer
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3,849
Location
Worthington, MN
Nice job bro. It's nice to see the real difference in these. Now do you have the part numbers needed to do this like the whole diff number and the spiders number along with all necessary parts to replace the stock crap.
 

NITRONUT

Well-Known Member
Messages
185
Location
Lindenhurst N.Y.
Nice job bro. It's nice to see the real difference in these. Now do you have the part numbers needed to do this like the whole diff number and the spiders number along with all necessary parts to replace the stock crap.

Unfortunatly Blazer X built them for me but I will get the info and post it.
 

MTBikerTim

Brushless Power!!!
Messages
868
Location
Australia
Nice. The bearings in my nitro savage which still runs stock diffs have gone already after putting less then 4 litres of fuel through them. Not impressed. I still have some spare diffs (from converting the electric savage) so hopefully I might get a bit longer run time out of these.

For part numbers I looked up the GST manual.
 

Larsenracing

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Manufacturer
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3,849
Location
Worthington, MN
So from that do you use everything inside the plastic case like diff, all cups, and all bearings with no modifcation to the stock 21 or X bulkheads?

And is the front and rear Cen GST diff different, If so the rear one is the only one that works right?
 
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Jeremy

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4,389
Location
tacoma wa
Yea use everything but the case,and the cup joints .. you use your stock hpi cup joints all you need to do is drill the hole a little bigger so you can use a larger pin. other then that its just shimming the the pinion ... .. front and rear are the same
 
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NITRONUT

Well-Known Member
Messages
185
Location
Lindenhurst N.Y.
So from that do you use everything inside the plastic case like diff, all cups, and all bearings with no modifcation to the stock 21 or X bulkheads?

And is the front and rear Cen GST diff different, If so the rear one is the only one that works right?

To install the CEN diffs you may need to add some shims and a few 8mm washers to bring the pinion gear closer to the diff housing. As for the front and rears they are the same. Bolth are functional.
 

Larsenracing

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Manufacturer
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3,849
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Worthington, MN
Thats great info SJ. Are these a different gear ratio than the stock setup?

I just noticed that the ones on ebay are listed front or rear not both, so I was just making sure.
 

Blazer X

Well-Known Member
Messages
973
Location
Hoosierland
http://www.hpieurope.com/piw.php?lang=en&partNo=86278 ......stainless replacements...ultra hard and will burn up about 15$ worth of drill bits.
http://www.hpieurope.com/piw.php?lang=en&partNo=86064 .........regular replacements.....I like these just because they are easy to drill out and replace.
The last two sets I built for guys I used stainless 8mm washers from hardware store to shim
pinion close to diff. Then I fine tune gear lash with these.......http://www.hpieurope.com/piw.php?lang=en&partNo=86598
 
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Larsenracing

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3,849
Location
Worthington, MN
So just to get this straight all 3 cups need to be replaced with the stock cups or the ones Blazer X listed or is it just the 2 outdrive cups?
 

Double-O-Zilch

BASHMASTER
Messages
908
Location
San Antonio, Texas.
Nice job Nut! I like pictures. Was thinking of putting the Cen diffs in my project savy and the pics help with figuring them out. Kudos to Blazer for keeping these diffs going.
 

Blazer X

Well-Known Member
Messages
973
Location
Hoosierland
You can use your original input cups, you will not be able to run the grub screw anymore because the cen pinion runs a flat not a thru hole. Just use the cen set screws on the stock savage cups and tighten them down with red loctite.
 

NITRONUT

Well-Known Member
Messages
185
Location
Lindenhurst N.Y.
So from that do you use everything inside the plastic case like diff, all cups, and all bearings with no modifcation to the stock 21 or X bulkheads?

And is the front and rear Cen GST diff different, If so the rear one is the only one that works right?
Here is what you need.

part #GS005
GS002
GS001
GS060
and this is what you will get


You will be useing the stock HPI drive cups and just drilling the hole bigger to fit the drive pin through so you can use the CEN bevel gears on the HPI cups.
 

Larsenracing

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Manufacturer
Messages
3,849
Location
Worthington, MN
You guys are great. This is awsome information. I will deffanetly run these on my basher.

Do you know if the gear ratio is the same as the stocker or different?
 

Blazer X

Well-Known Member
Messages
973
Location
Hoosierland
If you are curious of the cost of these diffs and want some built, I have put together 5 pairs now and average about 75$ for parts. I buy them as cen take outs like what SJ posted and buy the new savage cups. Let me know if interested.
 

NITRONUT

Well-Known Member
Messages
185
Location
Lindenhurst N.Y.
You guys are great. This is awsome information. I will deffanetly run these on my basher.

Do you know if the gear ratio is the same as the stocker or different?
From what I can tell the ratio is a little steeper that the stock,(more speed). When I put the diffs in I will list the exact ratios.
 

Double-O-Zilch

BASHMASTER
Messages
908
Location
San Antonio, Texas.
Scroll down for gear ratio.

https://www.hpisavageforum.com/forums/showthread.php?t=2153&page=2&highlight=gst+diffs

Originally posted by JT.

You will need to change the spur/CB to get the same ratio as if you were using stock diffs.
For comparsion in 3rd gear
stock is 14.61:1 when using 17/49 gearing
CEN is 14.45:1 when using 15/49 gearing
(Thanks to lizard for doing the math)
 

MTBikerTim

Brushless Power!!!
Messages
868
Location
Australia
Blazer your obviously using the wrong drill bits :p. I was able to drill straight through the hardened out drives with one old drill bit no problem. I was a bit worried though as it was the only bit of that size I had.
 

Blazer X

Well-Known Member
Messages
973
Location
Hoosierland
Very well could be, they are Irwin cobalt drill bits. I also had a couple of ace brand, but both of them were smoked.
 

Crash

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Messages
3,693
Location
Chicago
I had a heck of a time getting the Cen drive cups open, the screws were extremly tight and not very durable for wrenching loose (factory sealed)

I also had no drill bits up to the task of drilling the drive pin holes out either.

once done they are great, it takes some skill and know how to mod them to work in a savy.
:resp:
 

NITRONUT

Well-Known Member
Messages
185
Location
Lindenhurst N.Y.
I had a heck of a time getting the Cen drive cups open, the screws were extremly tight and not very durable for wrenching loose (factory sealed)

I also had no drill bits up to the task of drilling the drive pin holes out either.

once done they are great, it takes some skill and know how to mod them to work in a savy.
:resp:
The one thing I had to do on one of the diff`s was to use my brazing torch set very low and heat the crap out of the diff to soften up the thread lock. That crap was tough!!!!!! I took the diff apart holding it with a welding glove and when it cooled I retapped the bolt holes to get all the thread lock out and used some new hardware.
 

MTBikerTim

Brushless Power!!!
Messages
868
Location
Australia
As nitronut said the diffs need to be heated to get the screws out without damage. I used an electric heat gun but you can use an oven or anything. The trick is that the screws come out easily once hot enough. If they aren't coming out easily then stop trying and keep heating or you will break or strip a screw. They use a stupid amount of loctite on these diff for who knows what reason.

The drill bit I used was a Sutton drill bit but being an Australian company I doubt they are available in the US. I also set the drill to a very low speed and took it fairly easy.
 

xtremsavage

OG HPISF MEMBER!!!!!
Messages
1,840
Location
Jacksonville, Fl
In case anybody is in need of more info, that hasn't been mentioned yet, you also need to fill the Cen pinion gear with JB Weld or something similar because the Cen pinion is hollow all the way through the center. If not filled, it will allow more dirt and sand to get inside your diff housing. Well, more than usual. Also, the drive cup pin holes that need drill larger, the size it needs to be is 3mm. If you don't have or can't get a good qaulity drill bit,, then you can use a 1/8th drill bit. I went through a bunch of drill bits also. The only ones I found that went through easy but, could only be used that one time, were Dewalt Titanium drill bits. And believe me, those drill bits are extremely strong.
 
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