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Old 02-22-2010, 04:43 PM   #1 (permalink)
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TECH: Gen III/IV "LS motor" to Auto Trans

So you have your new Gen III 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 longer nub on the center hub that contacts the crank to support the TC. As well as going with a metric TC spacing.

Gen III converter with long hub

(need a better picture as this is not mine)

SBC converter note smaller shorter hub

Gen III engines with a stock 4l60/65/70 GM uses a dished flex plate. Note its the one on the right with the shorter crank bolts. The flat flex plate is on the left note the longer bolts.

On Gen III engines using the 4l80 trans which uses a convter 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 and it slides into the spacer to support it.

The flat flex plate has the spacer on the fwd side facing the crank.

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.

Spacer from GM parts direct or local dealer
GM PART # 12563532
CATEGORY: Engine Flywheel
GM LIST: $52.86
OUR PRICE: $31.34

GM PART # 12563533
GM LIST: $6.86
OUR PRICE: $4.06

Another option for a spacer is the bushing spacer it doesn't need longer bolts and just sits in the end of the crank. So its a lot cheaper option $20 vs buy the GM spacer and bolts.

PRW1800346-LS Engine to Old Style GM Trans Crank Spacer

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.

Now you will still have to elongate the flex plate to TC holes 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 a couple of kickers. 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.

And you really have no way of running a DBW TB with a TV cable. Unless you mount the TV cable to the DBW pedal itself. Its easier just to use a Gen III trans.
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Last edited by Bo185; 02-23-2013 at 08:53 AM.
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