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It’s a bit funny but I have probably explained the following procedure to a few friends a half-dozen times in the last few weeks. And honestly unless you too are a diehard drag racer using a GM style automatic transmission and converter setup, the information here is probably worthless. But then again, to a lot of those that do use that setup there seems to be a huge mystery to this procedure. Actually it’s all rather simple. Why this might apply to some of the latest GM stuff, I am specifically talking about Powerglides, TH350s and TH400 transmissions. And this procedure should be used every time you have the transmission serviced or maintenance on the converter performed. You should also use it if you are changing the flexplate for the engine or the bell housing/transmission case.
First things first. The convertor has to be seated properly in the transmission before anything else can happen. I like to call it the “3-step drop”. What needs to happen is that as you place the converter in the transmission, you need to make sure that the splines of the transmission are engaging the internal splines of the converter and that the converter hub properly seats within the drive tangs of the transmission fluid pump. It’s actually pretty easy although some converters can be a real bear getting them to make that final seating. What you will feel is that the converter “drops” or moves back further on the input shaft as you rotate the converter back and forth. The first drop is almost negligible and is simply the converter hub aligning itself with the outer portion of the transmission pump. The next drop is significant and typically moves the converter back about 1/2″, this indicates that those splines have now engaged each other. The last and final drop again is about 1/2″ and will be the hub engaging the fluid pump tangs. Now at this point, the converter is completely engaged in the transmission but if you were to run it this way, you would find that you will destroy your transmission pump in quick order. This brings up the procedure that needs to be used.
With the transmission installed in the car and bolted up properly to the engine, it is time to take a measurement. Depending on the combination of flexplate, the thickness of the converter mounting pads, the bell housing or transmission case and whether a rear engine plate is used, this measurement needs to end up being somewhere between .125 and .187. With the converter still pushed back into the transmission, we want to measure the distance between the flexplate and the mounting pads of the converter. The easiest way I have found to do this is to take a set of feeler gauges and insert a stack of them until the stack is just snug between the measurement points. Once you have that measurement, you can then take a dial caliper and measure the thickness of the stack or lacking a dial caliper, you can add up the feeler gauges. This measurement will be the distance between the flexplate and the mounting pads of the converter. As an example, let us say the measurement is .234 – well that is a little bit too much as if we were to pull the converter forward and simply bolt it to the flexplate, we would run the risk of pulling the converter hub out of the drive tangs for the fluid pump and in turn we would have an inoperable transmission. What we need to do is a little bit of math. If our desired minimum clearance is .125, we deduct that from the measured distance, which in this case is .234. That leaves us with .109 as the excess distance. We now need to find some hardened washers or spacers to take up the extra clearance. Using the dial calipers again, we should be able to find washers that come close. You can also do this with feeler gauges although it is a bit awkward. Again as an example lets say we find 3 washers that are .090 in depth – three are required for the GM transmissions and we want to make sure that they are all the same. That still leaves us with .019 extra clearance but if we add that to our desired .125, we come up with .144 which is well within the maximum amount of .187. We can now bolt the converter to the flexplate with the .090 washers between them and we have the proper clearance for the transmission to fully perform its job function.