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LS Tech and Moderator
11,745 Posts
Discussion Starter · #1 ·
Alright been slow around here thought I would post some tech.

So you have your new motor and want to run a early auto transmission with it.

There is lots of confusion on this! I need this spacer or that flexplate or this converter.

Really all you need is the right Stock GM flex plate and spacer.

The Gen III/IV has a different crank offset vs the Gen I SBC.

Basically its .400 in. from the were the SBC crank flange location.

So to use the 4l60 with the Gen III GM changed the torque converter by adding a nub on the center hub that contacts the crank to support the TC.

Gen III converter
**need to update picture**

SBC converter note smaller shorter hub as well a small bolt pattern

So on Gen III engines with a stock 4l60/65/70 GM uses a dished flex plate the one one the right note the shorter crank bolts.

On Gen III engines using the 4l80 trans which uses a converter with the Gen I style center hub GM used a Flat flex plate and a spacer mounted between the crank and flex plate. This mounts the TC flush with the center hub.

The flat flex plate is on the left note the longer bolts.

Dished 5.3L flex plate mated to a SBC Gen I converter. Note the hub is un supported

Flat 6.0L flex plate and spacer on a SBC converter. Note hub is supported by the spacer.

So you want to bolt your 700r4/TH350/TH400 to a Gen III motor.

Well your in luck!

You have two options using stock GM parts.
1)Use a the dished flex plate and add the spacer after the flex plate. The spacer serves to just support the center of the SBC TC only.

GM PART # 12563532
CATEGORY: Engine Flywheel

GM PART # 12563533

2) Use a 6.0L flat flex plate with the spacer between the flex plate and crank. Note:the spacer is part of the flex plate ie its installed onit and should come off with it.

The spacing of the two flex plates when attached to the crank are the same IE the teeth of the both flex plates are in almost the same location. The dish of the 5.3L flex plate makes up for the spacer. So any Gen III starter should work.

Another cheaper option vs the GM spacer is to use the dished flex plate and a bushing spacer for $20 form Pace Perf.. It rides behind the flex plate in the crank and supports the hub of the verter.

Now you will still have to elongate the flex plate to TC holes on either option as the spacing for the Gen III TC is metric. Not much will be need, test fit it first on the TC.

And from what I have seen you just can't use a Gen III 4l60 TC in an earlier trans.

There is one kicker. Some early 99-2000 4.8L and 6.0L had a longer crank flange to make up for the .400 inches. If your motor happens to be one of these all you need is the flat flex plate and remove the spacer.

LS Tech and Moderator
11,745 Posts
Discussion Starter · #3 ·
This should be a sticky. Very nice information, alot of people ask how to do this. Thank you for taking time and posting this.
I added it to the LS FAQ sticky thread. I hate to many sticky's at the top make the forum cluttered, so I grouped most of them into the FAQ at the top.

This isn't the only way to do this. But this uses off the shelf parts cheaply.

1,321 Posts
i am so out of touch

really..... more info
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