TH400:Accumulators

316600931one issue that must be addressed on ANY build with one of these piece a shit plastic accumulators is that it MUST be replaced. the picture here to the left is a typical cracked piston from a mid 70’s bop tranny. starting in the mid 70’s most of the gm trannys had these in the valve body. the older ones had an aluminum pistons which are the replacement of these. over the years as trannys are rebuilt in the name of normal service, good builders replaced these with aluminum. so it is hit or miss as to whether or not you will have a plastic or an aluminum piston in your valve body. one thing is for certain, if you rebuild a tranny and don’t swap this out knowing full well it can crack, melt, and fall apart….you’re an idiot. so, this is first and foremost an obvious performance upgrade for any th400.  when this cracks it makes 3rd gear sluggish and you usually have to gun the throttle to get it to move in reverse.

3 setNow, the spring below the piston was different from one th400 to another….BOP’s may have had a softer spring vs a chevy truck would have had a stiffer one. quite often once there is severe abuse, these springs will break. so for heavy duty/race applications i chose to remove the spring render the accumulator useless. this omit’s a fluid cushion and make for a more crisp shift for the 2-3…..as well as reverse depending on your build.  you can accomplish this in a couple of way. if you have an older piston you can simply take the spring out and install it upside down. newer aluminum pistons have 3 tabs toward the top so you will have to grind those off to fit the valve body bore. the way i chose to do it is to install the piston in the bore completely, then install a 5/16″ set screw in the oil port next to it to block oil flow. the picture above is pointing to the port you tap and plug. i like doing it this way simply because your piston is installed correctly in the bore and the servo piston to the low 2 band will seat against the top of it correctly. now, if you have been paying attention……yeah the set screw might leak a bit, but the rest of the plumbing of the tranny via the servo will equal that of the 3rd gear circuit and the servo spring holds the whole works down .

the 1-2 shift accumulator sits inside of the rear servo. this is one of the places where bypassing an accumulator can effect numerous things. i have tried sealing off screws in the case-omitting accumulator piston seals, and all sorts of chicken shit. i wouldn’t suggest doin anything like that unless otherwise specified in an aftermarket shift kit or valve body per manufacturer. Gm had 2 or 3 different large servo springs over the years…not the small inner spring on the servo shaft itself the larger outer one….the most common are a blue spring and a yellow spring. i have found that the yellow springs were most common in heavy duty chevy truck applications and seem to be stiffer, so i prefer using that one and saying piss on it. the softer the spring-the softer the shift…and vice versa. it is not uncommon to find these springs busted upon disassembly. there usually isn’t much point in doing much with the 1-2 accumulator other than a large servo spring change in my book.

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