American Automatics

Specialising in Classic American Transmission Rebuilding

Chevy overdrive 700R4 and 4L60's

This gearbox type is very popular for upgrading an old 3-speed TH350 or TH400 to an overdrive. There are two main benefits from this gearbox, a nice low ratio first gear of 3.06 to 1 and a forth gear of 0.7 to 1. This means quicker off the mark and low revs at crusing. Additionally the torque converter locks up giving you direct drive in 4th under light throttle. Ideal for motor way trips to shows. This drop in engine rpm can give a good fuel saving as well as stopping you thrashing your car at 70mph. Normally you will crusie 70 at about 3000rpm with the old 3-speed and this will drop to around 2100 with the overdrive.
Main design changes and evolution
This gearbox has 3 main variations by year.
1982-1984 - small 27 spline input shaft. No auxilary valve body.
1985-1987 early- 30 spline input shaft. No auxilary valve body.
1987 late to 1993 early - 30 spline input shaft. With auxilary valvebody. Identified by arched casting line through oil cooler fittings.
1993 late up is 4L60E - electronic version of the same mechanical design. ECU controlled. Wont work without one. Has large round multiplug on the side and NO round governor cover, no TV cable.

Minor other variations
There are four main case styles available. Regular V8, Corvette casing, K casing, and small V6. The large V6 4.3 engine has the same bolt pattern as V8s and is normally found in day vans. The small V6 is normally found in 3rd gen Camaros and Pontiacs. The Corvette casing is particular to C4 Corvettes only. It has no bolt holts for a regular 2 bolt mount and mounts by the special tail housing. The K case is normally found in 4x4 vehilces such as the Blazer. It has a slightly different shape bellhousing face by the dust shield. It has a dust shield which is made of cast aluminium and has 2 bars which bolt up to the engine block for additional strength. You may have read that you should look for an 'MD8'. Let me put you straight. I have seen this on many variations. It means spit. It is sometimes there and sometimes not. I believe it means 'Medium Duty 8 cylinder', which is what they were mostly designed for. There were NO factory heavy duty versions of this gearbox ever made. Heavy duty and high performance is only achieved by aftermarket upgrades of which there are tons.
This section is a work in progress......