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RC8T Diff Problem HELP

Old 02-15-2010, 05:07 PM
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malice23
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Default RC8T Diff Problem HELP

My friend runs an RC8T and we tore it down yesterday to rebuild the diffs. I refilled the diff with oil as per the manual and everytime I screwed it all together the diff would not move and stay locked. I tried a few things but each time those last 4 screws went in the diff would not move at all, Finally I removed 2 shims from each end and it moves fine and I thought this solved the problem. However after reinstallign everything the diff moves freely but when you turn one wheel the other side is supposed to spin the opposite way as all diffs do, but the wheels spin in the same direction. Occasionally they spin in opposite directions but I dont trust it. The diff feels smooth and it seems like the removal of those shims worked. I think the extra shims compressed all the gears together way too tightly and wouldnt allow them to spin freely. I personally run an Xray truck and NEVER had this issue so its kinda strange. If anyone has advice Im all ears. We worked on this diff for like 2 hours with no luck and now that its installed we are hesitant to run it. Has anyone had this problem? Could it be the gasket also?
Old 02-15-2010, 06:55 PM
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bhcvc
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Default RE: RC8T Diff Problem HELP

The shims were definitely what prevented the diffs from spinning freely. Don't overshim as it creates a high load for the motor.

As for the spinning, I have a ST-1 which uses the same diffs and don't have that problem. Sounds like you may have under-shimmed it now. If you're running pretty thick fluid and the gears aren't well meshed due to too much space, then the fluid will be viscous enough to move the other wheel the same direction. Once in a while, the gears gets close enough to each other to engage and actually spin the correct (opposite) direction.

When you shim the diffs, don't just remove all or use all the shims. Play around with what works best. There's 6 gears to shim, so it can take a while. Usually, I just run a shim behind each small gear. But in some of my diffs, I've had to use a shim behind one of the large gears.
Old 02-15-2010, 07:47 PM
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malice23
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Default RE: RC8T Diff Problem HELP

I literally took it apart the way it was, refilled the oil and put it back together and it just didnt go back together the way it was supposed to. The only shims I removed were the ones under the large bevel gears. Under those gears are 2 shims. I only removed one and it seemed to have worked but now I have the weird rotation problem with the wheels. when spun in one direction they spin correctly (opposite directions) and when spun in another direction they move together in the same direction. Im wondering If I should add a smaller shim to the shim under the bevel gears but one that is slightly thinner.
Old 02-15-2010, 08:36 PM
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brazoski
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Default RE: RC8T Diff Problem HELP

I do not think the problem is the shims. You should have 2 shims under each large gear and one on each of the 4 smaller gears. Make sure the large gears are all the way down on the cross pin on the output shafts. Also make sure you are getting the screws holding the ring gear on in the screw holes. I have put a diff back together and got the screws in the slot beside the screw holes and had the problem you are having.
Old 02-15-2010, 09:09 PM
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malice23
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Default RE: RC8T Diff Problem HELP

Oh man...I was loooking at that also but maybe youre right. It would make sense that the gears felt a little rough if the screws were in the way. Im going to try it again tomorrow night.
Old 02-15-2010, 11:42 PM
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malice23
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Default RE: RC8T Diff Problem HELP

Well tried it again tonight and I tried everything. Shims, making sure the holes were aligned for the screws etc. Each and everytime I screw it together it just binds up. I think the diff mightve gone without oil for such a long time that maybe the gears inside just got messed up or grinded to where they dont mesh right. either that or the diff itself is of poor quality. I dont know if the RTR and the FT it have the same diffs but it sure is weird. Im going to take apart the rear diff tomorrow night and see how that one works and compare. So far Ive wasted a whole bottle of 7k fluid on this front diff. Sooooo mad.[:@] All I can say is Ive been running the same diffs in my XT8 since June and they are mint. What in the world is wrong with this thing?
Old 02-16-2010, 03:00 AM
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DrDremel
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Default RE: RC8T Diff Problem HELP

Hmm....this sounds weird to me? I durability tested my 8t's diffs and got 7 gallons on em before I deemed it time to service em (they could have gone longer, but wasn't worth the possible damage). This is very strange. I've been running AE forever and never have had to replace the diff internals..... Yes the RTR and FT use the same diffs btw.... I guess you could replace the internals, should cost you guys all of 7 dollars.... Man that's weird.... I would say to replace the washers as the book says and check to make sure you're putting the diff in the case in the right direction (meaning the bevel gear is on the correct side and not contacting the chassis).


Man that's weird tho.....
Old 02-16-2010, 11:30 AM
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malice23
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Default RE: RC8T Diff Problem HELP

It s not only weird its insane. My fingers are chaffed from screwing and unscrewing. I looked at the manual and there was an O-ring that slides on to the outdrive before it hits the bearing. On the outside of the diff case not the o-ring that seals the inside. When I originally removed the diff there was no o-ring attached to the outdrive except for the one thats on the groove of the outdrive, that really small one. So I even experimented with adding an 0-ring but still no luck. I can tell you this though...that part of the diff that gets screwed into the diff housing does look like it may be at fault because it looks to me as if the outdrive shaft, after its slid into place, sticks out too far. When the bevel gear is placed over the pin above the shims the shaft in the middle looks like it protrudes a bit far as compared to the other bevel gear below in the diff housing. But like I said I experimented with shims non-stop last night and I cant find a solution. The only thing Im going to do tonight is remove the rear diff and see if its the same situation with that diff as well. And the problem is not when the diff is installed on to the truck. Its the diff itself when its assembled. Typically when youre done you screw it all together and then twist each outdrive to see if the play is correct and make sure its not bound up. But in this case, no matter what, it binds up when fully assembled.

Could it bee too much oil or that a new gasket is needed? I filled the diff with oil as the manual said, to the top of the gears. My Xray diffs have different directions though. In those directions, it says to fill the diff half way and it seems to work just fine, I tried that also with no luck.
Old 02-16-2010, 12:29 PM
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Default RE: RC8T Diff Problem HELP

sounds like they got ran dry and the spider and pinion gears are worn past service limits ... i have been a AE guy since 1985 and have never seen a AE diff wear out, but from what you're describing they are though.
Old 02-16-2010, 12:31 PM
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malice23
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Default RE: RC8T Diff Problem HELP

Thats the only thing I can think of.
Old 02-18-2010, 01:24 AM
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bhcvc
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Default RE: RC8T Diff Problem HELP

If the gears are really that badly worn, you should be able to see it fairly easily. Instead of just refilling, take everything apart in that diff and clean all the parts of the oil. Inspect the gears for wear (you should be able to see if the teeth are worn away) and also the diff case. If the large gear is sitting too deep, that may be a reason why it seems stuck. If it ran dry (with no grease or oil), that would make a lot of friction and heat which could melt or warp the diff case (I've seen this in other diff cases...never on AE though) which would not allow the gears to mesh right nor allow the large gear that's inset too much into the diff case to spin properly. The o-ring on the outdrive won't affect the way your diff spins. It just helps to keep dirt out.
Old 02-18-2010, 01:31 AM
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malice23
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Default RE: RC8T Diff Problem HELP

I finally got around to the rear and center diffs. I took them out and cleaned them as I did the front diff and they went together very smoothly, however they had more oil in them than the front did. I was done with both diffs in less than an hour and Ive just concluded that the front diff was just damaged beyond repair. Both the front and rear have fresh oil in them and they spin smooth as silk. I told my friend the issue and he ordered a new front diff. New shims, gaskets everything new. So, now that Ive disassembled and reassembled this thing about a million times it shouldnt take too long to build and install everything lol. Well I guess that was the issue in the end. It ran dry for too long and the gears either got warped or deformed or it couldve been the diff housing or shims or a number of reasons. Bottom line though I wasnt going to spend another day on it because it drove me mad. Reminded me of when I first got into RC and had trouble with the worst vehicle in history, the t-maxx 3.3 lol. That thing broke more than it ran. So before I had a relapse I just tossed the diff and had him order a new one. Problem solved I guess.

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