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The task of bolting an automatic transmission to an engine seems simple enough. But unless you’re paying attention, it can be fraught with problems for both the transmission and the engine. Let’s start with attaching the flexplate to the engine. Always make sure the raised portions of the converter mounting flanges are positioned toward the converter. It’s very easy to install the flexplate backwards, and you may not notice this until the trans is already in place.
Many enthusiasts still cling to the idea of using cheap star lock washers underneath the flexplate or flywheel bolts. The issue with these washers is that they often crack, especially when used more than once. If this occurs, the washer can slip out from under the bolt, which reduces the torque load on the fastener and then all kinds of ugly things happen. A far better idea is to use a set of high quality flexplate bolts along with a light application of thread-locking compound and the proper torque spec.
Before bolting the trans to the engine, take a moment to measure the distance from the engine bellhousing flange to the raised converter mounting pad. For a small-block Chevy, this distance will be roughly 0.900 to 1.00-inch. Next, make sure the torque converter is fully engaged in the transmission. This step is critical. Once you’ve done that, measure the distance from the converter mounting pad to the transmission bellhousing flange. For many GM transmissions like the TH350 and 700-R4, this distance is 1.125-inch. Here are the specs for the most popular domestic V8 engine transmissions:
Powerglide - 1.125”
GM TH350 - 1.125”
GM TH400 - 1.185”
GM 700-R4 - 1.125”
GM 4L60E - 1.125”
GM 4L80E - 1.030”
Ford C6 - 1.125”
Ford AODE - 1.030”
Chrysler 727 - 1.250”
Just a half-dozen bolts typically hold your automatic transmission's flexplate to the crankshaft. That's plenty even for high-horsepower engines, if you do it right. Make this connection count by using the correct fasteners and precise torque specs.
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.
See the full line of B&M transmission components now
We used a length of angle iron as a straight edge to measure the distance from the transmission bellhousing flange to the converter mounting pads with the converter fully seated in the transmission. The distance on this TH400 was 1.150-inches.
Measuring the distance from the engine bellhousing flange to the converter pad height on this engine was 0.817-inch. The difference between these two measurements exceeds the 0.250-inch recommendation so we’ll add a 0.100-inch thick AN washer in between each pad to move the converter into the proper position.
Not all street cars need an SFI-spec flexplate but they can still benefit from a high quality flexplate like this B&M unit to help ensure proper clearances are met and maintained. These flexplates offer much higher quality standards than cheap, offshore replacement parts.
B&M automatic transmission components
B&M flexplates
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