when is it necessary to shim the diffs?
 

ive been running a 7xl 4s with no probs, but decided to add heavier diff oil for the front and rear on my 3905 emaxx. im going with 85k front and 50k rear. stock is 30k. do you guys generally shim when you raise the wieght in oil, or when you go brushless, or only when you change out your diff case to another? or why? do we have to buy shims or does it come with shims from the factory?

i just check them all the time, just to keep slop and slack out

The art of shimming is to have as less play as possible, without binding. :)

But you got to watch for heat and expansionThere should be a barely noticable movement between the gears. Hard to describe, If you think you got a gap do it again till you are sure there is a small gap

Heat and expansion on a plastic cup and case? you can't shim them that tight, the gears aren't that perfect.

Unless traxxas has changed, they don't shim anything stock. Some diffs are closer fitting than others just due to variation in the way mass produced assembly line parts and their tolerances fit together. I've had tmaxx diffs that didn't need shimming, I've had others with way too much play in desperate need of shimming. Kinda on a case by case basis. I have no experience with 3905 diffs however, they're supposed to be better (?).

I think I've seen one of your threads linked with the how-to shim diffs on trx forums - thats a good place to start. Need to understand what you're trying to accomplish to start with. You want to remove side to side play of the diff in the case and at the same time, set the height of the ring gear to get proper mesh with the pinion. Last step is to shim pinion to get its depth on the ring gear right. End result is a slightly tight diff that has little to no play. My experience with them is after a short break-in period, they become very smooth and give a nice trouble free, reasonable service life. Its not hard, but requires of patience to get right.

McMaster-Carr has good selection on shims - where I bought mine.

Easy way to handle this - buy/keep spare diffs on hand and when it starts the dreaded clicking noise - replace the one making noise. Your bl setup is putting a lot more torque on the diffs and diffs supporting structure than it was ever designed to handle - that's what causes the prob.

i havent opened my diffs, im waiting for the oil and gorilla snawt to come in. just wanted to know why people are even shimming diffs in the first place. hopefully my stock diffs come pretty well fitted so i dont needa mess with it. i dont think i should since it was running fine stock, and just becuase im switching out the oil, i dont think i should have a need to shim now.

Shimming diffs reduces the slack between the pinion and spur, reduces the chance of stripping the gears

The 3905 diffs are "supposed" to be better than previous diffs. I'm not sure exactly what that means though. Maybe trx specified some tighter dimensional control so there is less possible slop between the way they go together. There's a lot to be said for "if it ain't broke, don't fix it" too. Run it til something starts making noise, then worry about it. It could be a long time.

I bought a pair and didn't open them, just installed them. When I get caught up, I'll think about pulling one and taking a look.

The more expensive diffs like UE - shimming is critical to getting the most out of your investment. I wouldn't even think about not shimming a set of those.

bb16- you meant pinion and ring gear, not spur, I got your back! Shimming makes the difference between stripping a gear and not stripping a gear. You need the cases open anyway to change oil, i would absolutely shim if you can pull the pinion in or out at all with the case closed, as much as you can move it is as much slop that you've got, basically room to fail. Conversely, too tight a mesh will cause heat and failure. It's trial and error getting a good mesh, but is well worth the time spent.

is there anywhere where i can buy a huge bag of random shims? like to suit anything? this would allow me to experiment. how accurate are the shims? like is a 0.1mm shim the same as one next to it?? always wondered about that...