Just wanted to find out what exactly does it mean for a transmission to be improperly shimmed?

Also, how does shimming relate to transmission longevity vs. smoother shifting if at all?

When rebuilding a tranny using a rebuild kit, how does one make sure it's shimmed properly?

Thanks in advance..

 

in our transmissions, there are 3 shafts (we'll consider the differential a shaft for the purposes of this thread) that have a bearing on each end (6 bearings). those bearings need to have the proper preload or end play on them so they will last and not wear out prematurely. ball bearings need end play (i.e. a little bit of 'looseness') and conical/tapered roller bearings need preload (i.e. they need to be compressed a little bit). the input shaft has ball bearings, the differential and main shaft have conical/tapered roller bearings. (there are other shims inside the transmission too, but they don't have to do with the bearings on the shafts which is what you are asking about, so i won't bother discussing those.)

bearing shims are required to set the proper preload or end play on the bearings, because during manufacturing there are very slight differences in how the cases are cast and machined, how bearings are machined, and how shafts and such are machined. when the trans is assembled, these slight differences can make the difference between bearings lasting a good long time, or failing prematurely. hence shims are needed to keep everything consistent as far as preload or end play on the bearings is concerned.

improperly shimmed means you have a shim of improper thickness behind one or more bearings. if you don't shim shafts correctly, the bearings are going to wear out fail sooner than they would if they were correctly shimmed. it won't really affect shift quality initially, but once you have bearings that are severely worn, you could start seeing degraded shift quality. once your bearings are that worn out though, you have much bigger problems than worrying about how smooth your shifts are (like worrying about whether or not you are going to get stuck on the side of the road because some major part in your trans is about to fail catastrophically).

you make sure the trans is properly shimmed either by using a dial gauge as per the FSM, or by using the solder crush method. i have posted on here about the solder crush method so you should be able to find an explanation. basically you take some little pieces of lead solder and put it behind the bearing race and bolt the trans case together, spin the shafts, let it sit for like 20 minutes, unbolt the case and take the little pieces of lead solder out and measure their thickness with a caliper, and then refer to the table in the FSM to figure out how much thickness you need to add or subtract to get the proper shim thickness. then you have to find the proper shim and install it behind the bearing race and bolt the trans back together for installation into the car (finding the proper shim is often easier said than done for someone who is just doing their own trans because nissan is always backordered on these things for weeks if not months. guys who build lots of transmissions usually have a stockpile of shims they can select from).

 

thanks for taking the time to explain it.

I found your post regarding the solder method here: http://forums.maxima.org/4th-generat...uilt-pics.html

(That's just in case someone else has the same question down the line)

 

Yep! The solder crush metod's what I used to determine the right shim. MY oem bearing were toast. Been about 10k since the rebuild and theyre holding up good...wish you luck sir!