RCBasher.com - HPI Savage

Savage (sav-ij) -adjective: fierce, ferocious, cruel, untamed, barbarous, wild, rugged, bloodthirsty

HPI Savage Tips, Mods, and Fixes

Installing Bulletproof CEN Nemesis Diffs in your Savage

RCBasher.com members LJ and Oslo recently completed the CEN diff mod upgrade and have been raving about it! If you check out the pics, you'll see why. These diffs are BEEFY! Check 'em out.

What's it take to slap some CEN diffs into your Savage? It's not a drop-in installation, but it's not too difficult, either. Follow along as we walk through the steps. Thanks for sending in the pics and info, LJ and Oslo!

The best (cheapest) source for these assemblies seems to be eBay. If you pick up some complete CEN Genesis/GST/Nemesis/Arena diffs off eBay or from a reseller, this is probably what you'll receive. They won't drop into your Savage like that, so you'll have to open them up. It's easy to pull the plastic housing apart, but you'll likely have to heat the remaining assembly in an oven or heat it up well with heat gun. CEN uses some very strong threadlock on the diff screws you'll have to break down with heat. You might be able to heat up each screw individually with a soldering iron, but there are sixteen of them, so that method will likely take a while!

Here's what they look like once completely dissassembled. What are the differences compared to the Savage diffs? Big, cast blocks behind the spider gears (similar to, but meatier than, the Savage X's), no spider gear cross pins, a separate bolt-on cup end, and blessed 8x16x5mm bearings instead of those doomed-to-fail 10x16x5s the Savage uses.

Here's a comparison shot showing the differences between the pinion gears from CEN and those found in a Savage. Click the image for a larger view and additional text. The only thing I don't like about the CEN pinion is that it's hollow, which means dirt can get into the ring/pinion area. It's suggested that you fill the hole through the pinion gear with something like JB weld or other epoxy to keep dirt out. Check out the size of those teeth on the CEN gear!

Here are a couple comparison shots of the ring gears. Notice the CEN uses a 9-tooth pinion and 26-tooth ring gear, resulting in a 2.89:1 ratio. The Savage uses a 13-tooth pinion and 43-tooth ring gear, which gives a 3.31:1 ratio. That means just swapping to the CEN gears will give you a 14.5% "taller" ratio, meaning it'll accelerate slightly slower but give a higher top speed, assuming the engine can make use of the resultant taller ratio. If you want to maintain your existing overall gear ratio, you'll need to get a clutch bell with two fewer teeth. That'll get you close to where you were before. If you want it exactly the same, do the calculations and see what you'll need. If you want me to figure it out for you, click the "Contact Me" link and let me know your current bell/spur combo and I'll get back to you with a recommendation.

Here are the spider gears and blocks along with some Savage spiders for reference. Not too different, but again, the CEN units definitely look a bit tougher.

As was mentioned earlier the Nemesis and GST diffs don't come with bevel-gear cross shafts, however the CEN Arena TR diffs do. Cross shafts aren't required to hold the gears where they belong, but they can be beneficial when it comes to filling the diffs with your favorite silicone diff lube. Oddly, without the shafts, the lube tends to work its way into the vacant spot in center of the diff and stay there, not doing much for limiting diff action. If you add the cross shafts, it fills much of the center area not giving the diff lube a place to reside undisturbed, which brings back the limited-slip action you were after in the first place!

Here's where the "work" needs to be done to make the CEN diffs fit your Savage. You'll need to drill out the cross pin hole in your Savage outdrives so the CEN drive pin will fit. The smaller Savage pins won't work properly with the CEN bevel gears and the CEN outdrives won't work on the Savage, so you need to use modified Savage outdrives. Simply enlarge the cross pin hole diameter to 1/8".

The bearings you'll probably receive with your CEN diffs are metal-shielded rather than rubber-sealed. They'll work, but they don't hold out dirt too well and will need replacement sooner than later. I'd recommend going to rubber-sealed bearings instead. Since they're only $0.99 each at the SMF Store, it's cheap insurance. Also shown here are the differences between a CEN diff cup and a Nova diff cup and at the bottom of the image is a CEN end cap with a Savage outdrive installed.

LJ needed a 2.18mm shim between his pinion gear and inside bearing for proper mesh between the ring and pinion gears. That's a good starting point, but you'll have to experiment a little bit to get the exact mesh you want since there are always slight manufacturing differences from one part to the next.

Here are a couple nearly-complete CEN diffs! One has the ring gear installed and the end cap separate, while the other has the end cap installed without the ring gear attached. Now fill them about halfway with your favorite diff lube, lay down some gaskets, and attach the ends. Don't forget to threadlock those screws!

Assembly is complete!

Now install them in the Savage bulkheads and attach the drive cup to the pinion gear. The only difference here is you'll need a regular set screw rather than the stock screw pin, or else cut off the pin portion of the screw pin. That's because the CEN pinion gear's shaft has a flat on it for the set screw to contact while the Savage's original pinion gear has a hole all the way through it for the screw pin.

That's it! Now go beat on your Savage.

Feedback is always welcome, please click the "Contact Me!" link if you'd like to leave some.