Diff upgrade options

[rock-rod]

Considering parts for HPI are scarce, what options do I have to upgrade these diffs? I am running the stock Savage SS (non-X) and yesterday managed to destroy a set of spiders in the rear. These diffs use the plastic cups but they do have the 4 spiders. The break occurred with the stock k4.6 motor. I have a not-quite-broken-in picco .28 that I would love to run but I need to get this truck a little more durable. Heck my revo is more reliable.

Should I go CEN Diffs or does it make sense to get a pair of flux diffs (pinions aren't available) and swap the ring gears over so I at least get metal cups. For the cost it seems the CEN diffs are the way to go but I am curious as to what others have done. It's frustrating parts are so hard to get now.

 

[cbaker65]

Considering parts for HPI are scarce, what options do I have to upgrade these diffs? I am running the stock Savage SS (non-X) and yesterday managed to destroy a set of spiders in the rear. These diffs use the plastic cups but they do have the 4 spiders. The break occurred with the stock k4.6 motor. I have a not-quite-broken-in picco .28 that I would love to run but I need to get this truck a little more durable. Heck my revo is more reliable.

Should I go CEN Diffs or does it make sense to get a pair of flux diffs (pinions aren't available) and swap the ring gears over so I at least get metal cups. For the cost it seems the CEN diffs are the way to go but I am curious as to what others have done. It's frustrating parts are so hard to get now.

That is weird that you manage to destroy a 4 gear spider ,you can rebuild that ,but with a stiffer lube ,a heavier lube should relieve
a lot of the stress & strain on the diffs!...

 

[rock-rod]

Back in business. I found an old hpi vorza diff I had from another project and the diff cup is identical to the one in the savage. A quick ring gear swap, a few shims and silicon fluid and all better for now. Gotta love cross compatibility. If it breaks again I will do the CEN diff mod.

 

[rock-rod]

And broke the rear diff again. Same type of failure which stripped a few teeth off the side gear inside the diff.
With that said, my recently acquired CEN diffs are now installed. the process was pretty easy and required swapping the drive cups from the savage diffs (had to enlarge the pin holes) and then using shims to position the pinion gear in the right place. It all worked out great and I gained some top speed because the CENs are a higher ratio. Hopefully this fixes the diff issues once and for all.

 

[olds97_lss]

I had that happen on my flux, blown spider gears. Luckily, I don't throw away much and I have a couple old savage diffs around with alloy cups. The previous owner of my flux broke screw heads off the cup on the rear (only had 3 holding it together) and stripped the heads out of the screws on the front.

I'm thinking I'll have to go the CEN route as well as I'm unable to find pinions. I have a spare sintered BP diff, but I installed my last 9T pinion recently. A bonus with the CEN diffs is that you run 8x16 bearings on both ends of the diff.

By "swapping drive cups", do you mean the ones that go out to the wheel axles? Did you have to do anything special to the pinion to fit the center cup? Do you use the HPI wheel cups and center cups?

My pinion after a not so nice landing...

 

[rock-rod]

You have to remove the cups (or stubs, whatever you call them) from the hpi diff, drill the pin holes out to accept the CEN pins, and then install the now modified cups into the CEN diffs. The reason is because the CEN cups won't fit the huge hpi dog bone ball ends. You will need a 1/8in carbide bit to drill through the hardened cup.

The CEN diff itself drops right. The pinion will need some shims to move it close enough to the ring gear. The overall OD of the CEN diff is less than the hpi diff. I used a combination of 16mm and 8mm shims to get it set right.

It's funny because I found some old posts with guys saying the CEN pinions will break teeth off like that. Guess there's nothing truly bulletproof in a brushless monster truck.

Oh and lumidave on eBay is selling flux diffs and pinions. I actually bought a full set just in case the CENs didn't work out.

https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/381995025600

 

[olds97_lss]

Thanks for the link! That's the guy I bought from last time and he was out of stock after that. Going to order a spare now!