What we’re looking for here is a clearance between the converter mounting pads and the flexplate mounting pads of roughly around 0.125-inch. With the converter fully seated in the transmission, this 0.125-inch clearance allows the converter hub to pull slightly out of the transmission pump for proper operating clearance since at high-torque stall speed, for example, heat will cause the converter to expand. However, excessive clearance of more than 0.250-inch doesn’t offer sufficient engagement of the converter into the front pump, which can cause serious damage. As you can see, this is a relatively small window for proper operating clearance.
In the photos below, the distance from the converter mounting pad to the bellhousing is 1.150-inch, while the engine to flexplate distance is 0.817-inch. Subtracting the flexplate distance from the converter pad depth leaves 0.333-inch. (1.150 – 0.817 = 0.333-inch). This is 0.083-inch greater than the 0.250-inch max spec. So, for this particular combination, we would need a 0.080 to 0.100-inch washer between each of the converter lugs and the flexplate in order to position the converter properly.
The opposite situation can also occur where there is either too little or no clearance between the fully seated converter and the flexplate pads. This situation is a problem because when the converter is bolted to the flexplate, insufficient clearance here will force the converter into the front pump, which will cause massive damage especially to the transmission pump. Worse yet, this preloaded situation will also force the crankshaft forward causing severe damage to the crankshaft main thrust bearing.
The resulting tight combination will be catastrophic to both engine and transmission. Generally, this is most often caused by either the converter being not fully engaged in the front pump or a poorly built flexplate with improperly positioned converter mounting pads – or a combination of problems.
It should also be noted that spending a little more money for a high quality flexplate offers multiple benefits. Many cheap replacement flexplates suffer from poor quality control and can exhibit horrible out-of-roundness and run-out. Not all street engines need an SFI-spec flexplate, but a high quality flexplate like a B&M unit for example, will deliver the kind of accuracy that makes a transmission installation far less stressful.
The world is full of cheap automotive replacement parts that look attractive but can cause damage if builders aren’t on top of their game. Don’t let this happen to you.
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