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Noob question about differentials

PKMSM

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The main question is, is there supposed to be a little air left in the differential casing after assembly, or is the case supposed to be completely filled with no air left?

The manual says to fill the diffs with oil up to about 1mm from the top, but that looks to be before the top sun gear is in place, so when the top gear is in place the oil is pretty much at the very top of the case. The manual is also clear about not overfilling, which would only be possible if a little air is supposed to be left in the case. I have seen on some threads that people run their thumbs over the top gear at the end to ensure the diff isn’t overfilled, but it’s hard to see from the pictures whether at that point there is a little air gap left at the top or if the diff is filled all the way and flush to the top with the second sun gear in place. I’d appreciate it if someone could clarify this for me.

I filled my diffs all the way with no air on top whatsoever, and didn’t squeegee the top with my thumb before assembling them (with a little oil standing on top of the sun gear since at end I added more oil with the top gear in place until the case was completely full). At this point I’m wondering if I’ve done things correctly or whether I should take the diffs apart and take a little oil out before finishing the build and running the car (ET48).

Also, do you change the gaskets every time you open the diff? Normally I would but the car hasn’t been run yet. if I’m going to open the center diff I’ll change the plastic gear with a metal one that I bought after I assembled it, so I’ll probably change the gasket on that one, but I was wondering what the general recommended practice is.
 

Billl DeLong

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no air, run thumb over top gear installed, I've never had to replace a gasket ever, but I've seen folks over tighten the screws on their diffs which have pinched and destroyed their gaskets after causing a leak because they over tightened the screws on the diff, facepalm.
 

PKMSM

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Thank you. Just curious though, if there isn’t supposed to be any air in the diff when assembled, then how can it be overfilled?

Unless the note is there to warn that if you fill it all the way to the top before putting the top gear on, it will overflow and make a mess.

3020A6F1-000F-448B-8561-9FF905C31F95.jpeg
 

Billl DeLong

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The problem with the illustration in the manual is that it doesn't show the top gear installed like this

49466657728_820db4cf3b_c.jpg



you don't need to recess 1mm with the top gear in place, makes it so much easier and foolproof this way to get consistent rebuilds every time ;)
 

PKMSM

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Definitely agree about the manual, would have made it so much clearer this way.

It doesn‘t seem that I need to redo anything. There was no air gap in my diffs. I didn’t do the thumb swipe, but that’s splitting hairs at this point. At least now I know the correct procedure for the next build.
 

Supradude26

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I agree and build / rebuild my diffs like others have said, with the top sun gear already in the diff when putting the last bit of fluid in. I typically leave no more than 1 mm of room just for the output shaft, pin etc as I know for a fact nobody will ever notice whether or not a diff is 100% full or 1mm was left when filling. On all my pre 2.0 vehicles I also use the M2C aluminum diff housings which have the screw holes milled down into the housing so if I ever overfill one then any excess oil will be pushed out the last screw hole. I then use a thin tool to remove enough so the last screw doesn’t pressurize the fluid when installing it. As far as gaskets go, I only replace them when they are either broken, leaking or appear to be close to failing. Typically it works out where I’ll for whatever reason open a diff 5 or more times before needing to replace the gasket.
 

Kalden

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I made the mistake of overfilling. Didn't do the squeegee with my thumb as mentioned earlier in this post - just went straight ahead and tightened down the screws. Well the diff leaked as soon as it spun up from the output shaft side so had to do a complete rebuild. Fortunately the kit had enough spare fluid leftover to another fill. Re-used the old seals no issues and now no leaks.
 

Kalden

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One Futher note is if the diff leaks it is most likely not to stop as the silicone fluid has now displaced the grease on its way out. The grease is insoluble with silicone and acts like a thick medium barrier between itself and the o-ring.
 

CDT221

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I just started my Et48 2.0 build. Accidentally left a bearing off one side and had to dump all the gears out of the diff case. That's besides the point, I am just now realizing from your other posts that we were supposed to use the black Moly grease for the diffs. I used silicone grease. Do I need to pull them apart again?
 
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I just started my Et48 2.0 build. Accidentally left a bearing off one side and had to dump all the gears out of the diff case. That's besides the point, I am just now realizing from your other posts that we were supposed to use the black Moly grease for the diffs. I used silicone grease. Do I need to pull them apart again?
The black grease usage is minimal and to be used on the outdrives shafts and seals for best sealing of the diff.

Do you need to rebuild your diffs? Probably not… the seals should still be able to fulfill their function. I use bike fork grease for it and would bet most guys never bother about it. Curious to read other’s opinion as well on the topic.
 

CDT221

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The black grease usage is minimal and to be used on the outdrives shafts and seals for best sealing of the diff.

Do you need to rebuild your diffs? Probably not… the seals should still be able to fulfill their function. I use bike fork grease for it and would bet most guys never bother about it. Curious to read other’s opinion as well on the topic.
I watched the tekno build vlog and one of the reps use copper grease and silicone grease. Less worried now
 

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