700R4/4L60/4L60E Input and Forward Drum Service


well since the input and reverse drums of this family of transmissions overlap and are very similar for nearly 20 years, i didn’t see the point in dividing up the articles. as you have probably seen this picture before, but this is the input/reverse drum assembly out of a 4L60e, but is identical for the most part to any of the other trannys. this is pretty much how it comes out of the tranny. you can also see the 2-4 band in the picture.



rev drum

the job of the reverse drum in this transmission is primarily to be a holding member for 2nd and fourth gear to create planetary action. the 2/4 band basically locks against the outside of the drum in an identical fashion to a low 1/reverse band in a th400. the sheet metal teeth on the outside of the drum engage your sungear shell. to the inside of the drum is your reverse clutch pack. it is typically a 4 disc clutch and can be serviced pretty much like any other clutch pack out of a th350 or th400.  basically the 2/4 band is dissengaged , the clutch packs of the input drum are dissengaged, and the front of the input input drum drives the reverse drum to give reverse.

the outside of the drum is known for excessive wear and if damaged should be replaced during a rebuild. it will wear just like steels in a clutch pack….primarily during the 2nd gear shift. if it is minor wear you should take some emery cloth and at least scuff the surfaced up so the new 2/4 band gets worn in correctly for proper shifting. overall pretty simple but not to be overlooked.

theinput input drum assembly is next. outside of the low 1 holding clutch to the rear(same as a th350), and the reverse drum clutch, this houses everything else pretty much. in a nutshell it seems complicated as hell, but it really isn’t that bad. most of the pistons sit to the rear and all the clutches sit to the front….and there is really only one spring you need to compress to get everything out. pictured here is what the drum looks like as it comes out. you can see the cast sprag to the front that engages the reverse clutch pack. check this quick for gouging and wear. at the base of the input shaft is your sealing rings. now they used several sealing rings over the years. it is pretty much an industry standard that whatever it did have: put the one piece teflon sealing rings back on it during rebuild. and yes it does require some special tooling.

youS3700040 pretty much flip the thing over and drop out the clutch packs. first thing is you remove the outer snap ring, 3/4  pressure plate, clutch pack,  and apply plate(yes there is a pressure plate on both ends). this is your 3/4 clutch pack assembly and can have up to 6 discs/steels. you may also have return springs that fit in 4 spots to the outside of the clutch pack between the drum and pack. these are return springs. some builders leave these out to make the shift quicker and better clutch apply…..sometimes they were never in there to begin with. if they are there, i recommend putting em back in unless there has been a shift kit installed of some kind. the outer pressure plate thickness can also vary so if it is damaged, make sure you get the right thickness to insure proper clutch pack clearance. these are known as a problem area and they make specialty cluch set ups for these like the Raybestos Z-pack to remedy failure in extreme applications. depends on what your application is for during rebuild.

once that is out of the way, S3700039the next one down is the forward clutch. pictured here is a second snap ring….which sits below the 3/4 apply plate. once this is removed pretty much everything else will fall out as far as clutch packs so don’t panic. this is where if you have never done it, you remove the ring….slam it on the bench….and panic when all sorts a shit come out. this is what a diagram in an atsg manual is VERY helpful. 

anyways out comes the snap ring, forward clutch pressure plate(which is also variable thickness) and  forward clutch pack with bottom wavy steel come out with the forward sprag assembly usually. there is at least 4 clutches and steels the sprag assembly should be set aside for later inspection.  below the forward clutch is your overrun clutch set up. the backing plate for the overrun clutch is also your apply plate for the forward clutch…..so if you can take note on how it came out so you don’t get frustrated during re-assembly. the overrun clutch is only 2 small discs and two steels.

at this point, you are down to removing your pistonsS3700037. this is the only real spring pack you need to remove. as you can see here, i made up a small tool with a few bolts and a piece of flat steel, then chucked it into my lathe and used the drill chuck as a compressor. there is a tool for doing this….i’m just a cheap shit and this cost me 20 minutes of time.  now, when compressing the spring pack here only compress it like an 1/8″ or less! there isn’t a lot of free travel under the spring clip here so just push it enough to relieve tension on the clip…then wrestle the clip off. i believe the reason for the lack of compression space was to keep the material as thick as possible in that area of the drum. the area of the drum below the retaining clip is a fail point and there is even aftermarket sleeves offered by sonnax to beef up this area for heavy applications and big power.

once the clip is out, you pretty much can pull out your pistons. S3700036the first set out is the overrun clutch piston(center), the forward piston(middle: holds the overrun piston), and forward piston housing. this picture by the way if how i put em back into the drum as an assembly….they may come out individually.  below the forward piston housing is another return  spring, the tin apply ring(looks like a cheap piece a shit with long fingers) and the 3/4 apply piston. now….you’re empty and you can clean, inspect, and rebuild. now, depending on the year and model, your input drum may have aluminum pistons with seals, molded seals that are made into the piston(require piston replacement), or a mix and match of both styles of pistons. most people prefer the molded pistons for the 3/4 apply piston, however  i think it really depends on the year and the application…..and your budget.

here is a photoS3700031 of the 3/4 piston to the right- apply ring, and forward piston housing.






here S3700029is a picture of how the piston assembly is layed out from right to left, right being the bottom and left being the top. you can buy all this as an assembly from your parts suppliers if you have chronic damage of want to use modern pistons in an older 700R4






hereS3700025 is the forward clutch sprag assembly.  this can be a fail point and should at the very least be popped apart to check for cracks and what condition it is in. a double cage 29 element sprag is preferred for these and some of the early trannys had a weaker sprag. enough years have gone by though that if it has been rebuilt, someone probably already revamped it. it basically is a one-way lug type sprag that locks in gears 1-3 and overruns in overdrive….basically driving the entire transmission. so if there is a problem, you don’t check it….you’ll be in deep shit. the two disc overrun clutch meshes with the smaller side of this assembly and when locks gives you engine bracking, otherwise the clutch will overrun allowing you to coast. 

re-assemblyS3700027 is not that bad after you do it once. first, install the 3/4 piston. i use a feeler gauge but some kits come with an installation tool….and there are specialty install tools as well. once in, put on your apply ring and spring, then what i do is assemble the rest of the pistons as pictured earlier and install them as an assembly. then re-install your springs. the first clutch pack in is your overrun clutch pictured here. i am pointing to the seal here that MUST be replaced. this seals the center of the input drum to the output shaft. don’t forget to put it in. i usually put it in first before any clutches. it’s splined on the outside and is hard to miss in the rebuild kit. also when putting the apply plate for the forward clutch on this, make sure you put it in right. the rest of the input drum is pretty much stack and go. like any other tranny, you should soak your clutches. on these trannys i prefer to always replace your steels with your clutches it’s money well spent!

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