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Diffs

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newnitrofan

newnitrofan

Hardcore RCTalk User
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Location
Irving, TX
RC Driving Style
  1. Bashing
Hey guys, Just got the savy back together after a bad esc and a bad slipper clutch. Took it outside for some minor bashing. It was running great as I was taking it easy with it given the problems I have already had with it. Well I gave it a few hard throttles and one from a dead stop. How cool was it to see it do a flip right there like that from a dead stop. (this is a savage flux by the way). Anyways I hear some gear noises from it as it rolling forward. Took the rear diff apart and found that I have fried the pinion gear already. These are supposed to be the bullet proof diffs toughest hpi makes. Anyways just wondering what you guys are doing to toughen these things up. I am growing more and more frustrated with this truck. More broken parts than run time.
 
Nope, the stock Flux diffs are just the "better" diffs than the old school ones. They are machined vs cast.

The new diff gears have fewer/larger teeth on the pinion/ring gears.

Take a look at post 15 here:
https://www.rcnitrotalk.com/forum/showthread.php?t=71195

I posted photo's of all HPI savage diff gears from the 21 savage through to the really new HD gears.

Regardless of all that, proper shimming and bearing upkeep are what make diffs last. If you can, chucking the stock plastic cases and going with alloy will allow you to shim perfectly and they won't flex/warp like plastic cases allowing the gears to get out of line. Then it's just a matter of keeping an eye on the bearings.
 
Ya know I'm feeling like a savage virgin here. I so never remember shimming my savage 21 at all after I did the 4 spider diff mod. I got the slipper clutch thing now, makes sense. Shimming the diffs How on earth does all that work, after a lot of consideration I am going to be going with the bulletproof diff pack from hpi. I am also going to be renewing the diff oils in the diff itself. Think I'ma run bout 30K in the diff oils. I will wind up backing off about a quarter turn on the slipper. I read on a review I think off of tower's website to run a fully tight slipper.
 
Basically, you just put shims behind the pinion gear or on the ring gear to get the gear lash adjusted to where there is almost no play at all. Plastic cases usually are molded "tight" but have a tendency to flex under pressure.

I re-did my savage diffs last winter with the diffs you have now (XL diffs), but I used alloy cases. I also shimmed them really well. I ran about 6 gallons through them this summer and they look like new still. 4+ of those gallons was with an LRP28S3 which is an engine that's hard on diffs.

I've replaced the center axle cups twice and the rear center axle once, just to let you know the wear happening. I'm actually due to replace the cups again as they are getting very notchy.

This is a decent "shimming" walk through for the older savage style diffs/cases:
https://www.rcnitrotalk.com/forum/showthread.php?t=47876&highlight=shim

A shot of one of mine after rebuilding them:
2008-1108-AlloyDiffCase01.jpg
 
Any diffs have to be shimmed right. You have way more power now than a nitro. Probably just lucked out before. The power also hits a lot harder and faster. Full torque from a dead start.
 
Last edited:
Guys really thank you. It has honestly been so long since I have done all this it's not even funny. I have the bullet proof diff gear and pinion on order. I'm in the process right now of trying to get the top bevel gear off of the diff casing. It so won't come off. I think I understand it right, I have to heat up the diff cases to get the bevel gear to come off. Matt you are making perfect sense when you say way more hard hitting power. The flux is one powerful truck. I'm fixing to throw the diffs themselves in the oven for a bit.
 
With the diff cup, I usually just hold the ring gear on it's edge on the table and whack the cup with a hammer handle. Whack it, turn it, whack it... usually pops off after a couple "taps".
 
Ok that worked and worked well, had to substitute the end of a mag light but yeah that did it. Guess It's a good thing this all happened, once I got those things apart I seem to have found pretty much no diff oil in the diffs. The front diff actually had screws that were not tightened all the way around the top of the bevel gear. I swear HPI used to make a quality kit. I'm just gonna have to vote that mine was made on a Monday or Friday or somethin.
Metalhead Matt said:
Learning and teaching's a good time. Sounds like you need to change your screen name to newbrushlessfan!:)
Click to expand...

Ya know I'm gonna have to say that was messed up but dam it did I laugh. Good one
 
That's wild that the screws were loose. Do you mean the ring gear screws? You bought it new? I've known to check RTR vehicles out good before you run them but that's going a little deep.
 
Yeah I thought that was a little much myself. I honestly just bought the truck bought a week and a half ago. Couldn't even say I have more than four runs on it. It's not like I'm way hard on rc vehicles either. Bought a revo bought a month ago and have a half gallon through it with no issues. Yeah it was in face the screws holdin down thre ring gear. Like I said it had to be a monday or friday car or maybe someone was goin thru a divorce and just didn't want to work that day. Think I have also learned not to buy the display model at the LHS. I want to go in there and ask how long the thing had been sittin there now.
 
Got everything soaking in a simple green bath right now. Guess it'll be a few days before I can get the parts. I so know my lhs won't have anything like these bulletproof dealies in stock. Oh well off to play with the revo and lil revo over the weekend.
 
Ehhh, good ol illinois. It's always good to be cold this time of year for sure. I always hate traveling through there. Anything off of the great lakes sucks.
 

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