Automatic Transmissions

TH35O, 700R4, TH400, or Powerglide Transmisions – BOLT UP

First off, there are NO issues with physically bolting any of the GENIII engines to these transmissions.  One less bell-housing bolt is used as the newer GENIII blocks are missing the “1 o’clock” hole.  The issues are solely with the torque converter bolt pattern, and snout depth.  Depending on what vehicle your engine came from, you may or may not need an adapter or flexplate modification. 

5.3L & 6.0L LQ9 ENGINES:  Most, if not all 5.3’s & 6.0 LQ9’s will need a spacer a longer bolts to be added behind the flexplate as they came with a “dished” flexplate for the 4L60e / 4L65e transmissions.  All GENIII flexplates use the old TH400 converter larger bolt pattern diameter.  So, if you are going to use a TH350 or 700R4 with the standard torque converter you will need to elongate the flexplate converter holes inward, making a smaller diameter pattern.  Or, you can run a TH400 converter on your TH350 or 700R4 and it will bolt right up.  Elongating the holes is usually the most cost effective.

6.0L LQ4 ENGINES: Most, if not all 6.0 LQ4’s came in 2500 series cars and trucks, which had the 4L80E transmission.  A spacer & longer bolts are NOT needed, but the torque convert bolt pattern is still an issue as with the 5.3L when using the smaller TH350 & 700R4 Converters.  If you are using a TH400 Converter, then it will bolt right up and NO adapter or modification is necessary.

700R4 Specific: First off, it is not feasible to use a 700R4 with a DBW engine as there is no throttle body mechanism to hook the TV (throttle valve) Cable to.  There are some Internet threads about hooking it directly to the DBW gas pedal, but I don’t recommend it.  For a DBC engine the 700R4 is a little unusual because it has this TV (throttle valve) cable that needs to be hooked to the throttle body with VERY specific geometry to work properly.  The TV cable is NOT a “kick down” cable.  It controls transmission valve body pressure relative to throttle position (load).  A bracket that supports the end of the TV cable will need to be fabricated and attached to the intake manifold near the existing gas-pedal cable bracket.  A universal TV cable from Summit Racing: ATI-702050 works well.  Follow the diagram below for correct throttle body modification.  A cable mounting point needs to be attached or welded to the bottom side of the throttle body 1.10” from the throttle body pivot point.  At full throttle the TV cable should be completely pulled tight. If you are uncomfortable with this procedure have a qualified transmission shop adjust it.  If not properly adjusted serious transmission damage WILL occur, and quickly.

700R4 TV Cable Diagram.jpg

FLEXPLATE MODIFICATION: Using a mill to elongate the holes is best, but a Dremel tool and 1 bit per hole will usually get the job done too.  Check torque converter bolt alignment and snout fitment into flexplate before installing in vehicle.  Shop around for these parts, you can save yourself some money!

Spacer – GM Part# 12563532 ($49.95)

Bolts – GM Part# 19257940 (6 needed, $5.95/each)

4L80E TRANSMISSION: If you want to use a 4L80E transmission with full ECM control it is most cost effective to start with a 6.0L LQ4 2500 Truck motor as it came factory with this transmission and it is no problem.  FYI - 4L60E & 4L80E transmissions are NOT directly interchangeable.  The wiring pinout of the trans plug is different, and 2 VSS’s are used versus 1 on the 4L60E.  Besides the wiring difference, the ECM also requires a segment swap as that portion of the transmission control is completely different.  It can be done, but I do not do this conversion.  If you require a 4L80E transmission with any engine other than the 6.0L LQ4 here are a few options:

Manual Shift kit / Vacuum Pressure Modulator 4L80E-3 & 48-MOD:

This option is the cheapest but you loose all “auto” features and reverses shift pattern.



Automatic Shifters

There is a large selection of shifters and styles available.  For offroad use I prefer a “gated” shifter with a modified gate.  With a modified gate, I see no need for a manual transmission valve body, as full automatic mode is nice for basic trails & cruising.  The modified gate will allow you to instantly find reverse with no confusion, but still allow full automatic mode when in “drive”.  These are 2 of the most economical gated shifters that work great for rock crawling and general offroad use, both around $150.00:

            B & M Sport Shifter, $154.95 ------ BMM-80776

            TCI Thunder Shifter, $149.50 ------TCI-616131