Savage diff

[-=ADA$=-]

My friend has problem, hes eating rear diff very quickly ( he put one set in yesterday, adn today its gone :D) and last set was kyosho mp 7.5 set, so what would be good replacement gear set for rear diff, 2 bevel gear (stock) didnt work, 4 bevel aluminum (kyosho) didnt work either, so what would be good, but not very expensive option?

[mavrick0]

The guys I run with have been doing the same thing but they took my advise, ditched the diff oil and went with grease and haven't had any problems at all. I've run my Picco .26 savage this way with the stock HPI gears and not blown them. And keep in mind the guys I run with are running either the Picco .26 or Sirio .27's. As for the type of grease, we are all running automotive synthetic wheel bearing grease.

[kitty]

My recommendation is to go with two sets of stock spider gears with Ofna cross pins and either a quality diff grease or higher weight silicone fluid (10k minimum). Other bevel gear sets will fit, but they don't hold up due to not being hardened enough for use in such a heavy truck. Unfortunatley, finding decent full sets of the stock HPI gears is a problem. If you get lucky, you'll find packages with four good gears (enough to do one diff). It's a hit-or-miss proposition, as some packages are mixed good and bad gears and some all bad gears. Apparently HPI has had problems with having good gears made. The best thing to do is get them from a LHS and look them over closely before purchasing them. If the teeth on them look pointed, skip them. If they aren't a darker colour (the better hardened gears), skip them. Look for the darker ones with teeth that have flat tips.

Best wishes and luck to your friend. If you or he has any questions about doing the 4-spider mod, feel free to ask me. =)

[-=ADA$=-]

thanks ill surely tell him to go with stock 4 spider gear and use greas, cause last time ( kyosho gears) he had 50k wt oil, and it still stripped, so ill tell him to stick with greas, so he will have to buy two sets ( 2 large gears, 2 small ones) to put in one diff, right?
Thanks!!

[kitty]

Yes, he'll need two additional sets of the gears (HPI #86032), but only the small spider gears will be used. The reason his stripped, in spite of using 50k fluid is because the gears weren't hardened and not meant for use in a heavy truck like the Savage. To my knowledge, they do fine in the buggy they were intended for.

One other thing I recommend when doing diffs for the Savage is different screws that hold the ring gear to the diff case. The stock size is M3x12mm self-tapping screws. Changing to M3x18 (anywhere between 15 and 20mm length, really) *machine thread* screws will hold the ring gear on much better. The self-tapping screws tend to back out. When they do, the mesh between the ring gear and pinion is lost and usually the pinion loses the battle and can also toast the pinion bearing.

Another potential cause for failure is overfilling the diffs. When using silicone fluid, the diffs should only be filled to where the fluid is just shy of the top of the top cross pin (top, as you are looking down into the diff while filling it). I fill mine to about the middle of the cross pins. If too much is used, when the fluid gets sloshed around inside and expands, it can leak out. Once it's all gone, the gears have little to no lubrication. Even grease will squish out if there's too much in the diff. One thing to consider about using grease in the diff versus silicone fluid is that a thick, heavy grease will give more of a "posi-lock" effect, which can put more stress on driveline components, especially when used in the front diff. A good quality white lithium grease or RC diff grease (white/clear silicone grease) should work okay without being too sticky. And don't forget to use a bit of grease inside the gearbox (the two halves that house the differential assembly and pinion gear). Don't overpack that either, and a lighter grease (not silicone fluid in this case) is good. To help keep the grease from squishing out between the gearboxes and the skid plates, run a thin bead of silicone RTV or Shoe Goo around the bottom of the gearboxes. I'm partial to Shoe Goo myself; it sticks so well that the next time you remove a skid plate, you have to pry it a bit. I've only had a minimal amount of grease leak out of my gearboxes and that all came from between the bearings and the pinions.

[-=ADA$=-]

ive just messaged it to him, your very helpful, thank you, he had two sets of HPI gears, so he put 4 spiders in rear diff, some where darker some not, but because were in europe ( order everything at TH) we cant pick "better" gear sets :( bad luck, but ill remember about it!!

[kitty]

Hopefully the ones that aren't as dark as the others are still hardened, but don't hold your breath. I can understand it's harder to get such parts in Europe; there have been times when it's been hard to find them here in the US. One of the best ways to check the gears is to look at the teeth on them. If they are pointed, they're from a bad batch. If they are flat at the tops, those are the ones you want. In the future, seeing that you order from TH, it would be a good idea to get a few spare sets, just in case. Best wishes to you and your friend!

[-=ADA$=-]

thanks, he had a set of gears so he put them in, and well check tommorow if they hold, and do you think that maybe alu gearbox would help solve the problem? i thought maybe its because stock plastic housing flexes and allows gears to strip. Also ive asked him if there were shims behind spider gears and he said there wasnt, and he didnt put them in, cause i he thought if thats how they set it up in factory its ok, now i told him to shim gears so the mesh will be better. Hopefuly it will help. Thank You for help very much, and BTW how are your black died wheels?? I remember i leved the way they looked!!

[kitty]

Don't shim behind the spider gears. Any shimming should be done on the outside of the diff case (on the output shafts), to keep the side-to-side movement to a minimum. A number of guys have had pretty good results using aluminum diff cases, but if the small spider gears are bad, nothing will help that. I've seen pictures of alloy diff cases after a number of gallons of wear on them that had some wear, though it looked pretty marginal. In my opinion, the alloy diff cases are much better than the softer plastic stock ones, though some have been trying out the diff cases made for the Hot Bodies Lightning buggy. They're using the stock HPI internals and Ofna cross pins, but the Lightning cups have a metal insert in one end that helps with wear and the plastic is a bit harder. Word so far on those is that they are holding up pretty well and still are a bit cheaper than alloy cups. One nice thing about the Lightning diff cups is that they use larger side bearings which are the same size as the pinion bearing.

If your friend really wants to go all out, I recommend alloy gearboxes (that house the diffs) and maybe alloy bulkheads. When my diffs get worn, I'm probably going to go with the Lightning diff cups with 4 spider gears, Ofna cross pins, alloy gearboxes and do a shim job between the diffs and gearboxes. Right now I'm using stock diff cups with the 4-spider mod and they're holding up okay, but I've only run a gallon and a half through the engine. Time will tell.

Oh, almost forgot. You asked about the black wheels. Over time they have faded to a funky blackish purple. The next time I dye chrome rims black, I plan to leave them in longer and remove the wheels before dousing my truck with Simple Green for cleaning, as well as avoid getting denatured alcohol on them when doing spot cleaning. I think part of the fading has been due to getting Simple Green on the wheels while cleaning my truck. I know that it'll strip chrome off wheels, so I'm not surprised.

[-=ADA$=-]

thanks for input, we have run his savage and diffs are still ok, which is wierd cause usually they got blown in 10 minutes, so for now its ok, but i told him to switch to alloy diffs but he probably wont change to alloy bulkheads, and diff housing, he doesnt want that. we'll see how long will present setup last

[kitty]

The good side of alloy gearboxes and bulkheads is that there is not the flex that the plastic parts have. If you shim the diffs and use at least the alloy gearboxes (not diff cups, but the case that surrounds it), there won't be the side flex that can cause ring and pinion failure. So far my ring and pinion have done fine. Apparently I was lucky enough to get solid, well made gearboxes. It all comes down to what you're willing to do and spend to beef up the diffs and surrounding bits, and a lot of that decision can come from what engine you run.

[-=ADA$=-]

well my friend (not me unfortunately) runs stock S25 engine, and he could get diff cups, but not cases, he said he will only upgrade to aluminum things absolutely necesary, and diff cases doesnt seem like this for him, and his ring and pinion gear is fine also, just spider gears, but they'r also fine ( still :D) but thanks for help i (and he) appreciate it alot!!

[billysss]

hpi has released a new 4 spider gear meshing. It wont break even with a Sirio 27. The stock spiders have only 2 spiders, try the 4 spiders with crossing pins, color black, perfect,

[kitty]

billysss, what do you mean by "hpi has released a new 4 spider gear meshing"? I've been offline for a few days, so if they are now using 4 spider gears in the diffs, I must have missed reading about it.

[-=ADA$=-]

yea, same here, but ill surely tell about it to my friend,but for now his diffs are ok, holding up nicely, and although he bent alloy shock tower, diffs are OK