TH400: Check ball locations,shift kits, and mid plates

this is one of the things that a lot of guys will struggle with on both a th350 and th400, especially with all the rumors of guys leaving this one out-leaving that one out-drilling plates. well first off, for anyone who does not do this every day, you should really get yourself an ATSG manual, which is an Automatic Transmission Service Guide. they are available for most transmissions across the board and have very helpful charts and specs including check ball locations. i must also stress that although you can leave out check balls and drill ports, a QUALITY shift kit like the ones from trans-go are the way to go as they have not only a custom mid plate, but new springs and check ball locations to get the job done right. some kits also include extra springs for the low 1 band piston to severely limit accumulator action and/or make apply more aggressive.

but, if you are a cheap bastard like me, you want to try doing it yourself. well, yes you can, but it may or may not bring good results depending on the wear of the valve body springs you are using.  this is where a good kit with the springs will make a difference. i have fairly good luck with the th400 fabbing up my own stuff  so that is what we will be doing here.  on the th350’s, usually just buy a shift kit for street/strip, but fabbing your own kit on a th350 can be done.  i have never really persued it much to be honest and don’t see the point. i’m rambling again…..anyways, figure out what you want to spend and what you want to accomplish, then make your decision there. if you’re going to buy a kit, don’t waste your money on a cheap one that simply omits a few check balls and gives you a tin plate for a mid plate….that’s what we are going to do here.

bomber check ball


these are the 6 check ball locations found in most th400. i did a rather shitty job with a paint program to number them, but you should be able to get the idea here. 1 is for the modulator, 2 is the check in the 2nd gear oil passage that leads to the low 2 band, 3 is the check for oil going to the intermediate clutch(which gives you 2nd gear, 4 is the check located above the accumulator port(which to be honest i forgot wtf it really does), 5 is the check that bobbles between the low 1 and reverse oil ports to keep the tranny from locking itself solid while in low 1, and 6 is the 3rd gear check ball.

the only real critical check balls you must have under any and all conditions is numbers 1 and 5, the rest are there to block the port during clutch application but unseat to releave pressure rapidly. so, obviously by omitting 4 of the 6 check balls, you application and shift will be dramatically quicker….sometimes too quick. when i build 1st/rev or full manual set ups i only use these two check balls since there is no real point in having the rest of em if you aren’t shifting is there? for an aggressive street/strip i also will only use these two check balls and drill the 3rd gear port  in the mid plate to a 3/16″ diameter. you then calibrate your shift by modify’n the governor and adjusting the modulator valve so you get a hard shift at the right time. what we are doing here is controling the atittude of the shift, not the timing of the shift point, that’s the job of the modulator valve and/or the governor.

check ball number 6 can be omitted during ALL rebuilds, and even suggests that in the ATSG manual for a th400. on some of the 77-80 bop short shaft transmissions, there was no check ball port in the mid plate(even thoug there was a check ball for some odd reason), so to increase flow of oil to the 3rd gear you must drill the mid plate at this location for aggressive street/strip rather than the port shown in the picture below.


i found this picture that illustrates  both the second and 3rd gear oil ports in the mid plate of a th400. you are looking at it here from the upper valve body- or rather the case side of the mid plate. you will notice that while the 2nd gear port has the larger check ball  port next to it, while the third gear oil port does not. that would be the number 6 check. all oil is metered through this 3rd gear port for 3rd gear both in apply and disengagement, therefore you really don’t need the check ball. for agressive street or full manual valve body, you would simply leave out the 2nd gear check ball and drill the 3rd gear port with a 3/16″ drill bit. if you wanted to step up you shift to “firm” it a bit, you could leave the 2nd gear check ball in and drill both 2nd and 3rd gear ports to something like a 1/8″ or less to meter the oil orifice that controls the rate of apply. obviously the larger the port- the quicker more harsh the shift becomes. some shift kits even suggest just leaving 5 of the 6 check balls than drilling ONLY the 3rd gear il port to a whopping 5/32″ to give added oil flow for heavy towing conditions. i can only assume this is to improve holding of the 3rd gear clutch that drives you down the road most of the time.

now, modify’n the mid plate and leaving out check balls will only do as much as the rest of the tranny is built for. by that i mean you still have fluid accumulators for both the 1-2 and the 2-3 shift. so if they are still functioning, the shift with a 3/16″ hole may be just what you want whereas if you disable the accumulators(which is another article), a 3/16″ drill size may seem VERY aggressive.  in several builds for heavy towing, i will disable the 3rd gear accumulator, use 5 check balls, and omit the center seal from the direct clutch(here again another future article). depending on the mid plate you may or may not need to drill the 3rd gear port as well. you must know what the other components in the system are going to do before you go apeshit drilling everything out than….ooops.

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