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LS Swap Guide: GM 3 Speed / 4 Speed Parts List

10/16/2019
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As hot rodding evolves and more modern fuel injected engines become readily available, the want and need to swap these engines into your older hot rod becomes more desirable. Whether you want the added horsepower, fuel economy, or throttle response, it is now a viable option.

Let’s say you do decide you want to swap out your SBC with a low mileage fuel-injected 5.3L or any other LS-based engine. A couple of common options are to source one from a local salvage yard or as a new crate engine. You can have the new efficient engine and keep the look and feel of your stock GM Manual 4-speed transmission such as the Saginaw or Muncie “Rock Crusher” that you love.

Considering you already have a 10 spline 4-speed transmission, there are only a few items you will need: BBC Bellhousing, Clutch, LS Flywheel, Pilot bearing, and a Hydraulic throw-out bearing. Now we can go over in more detail the exact parts required to make this swap as easy as possible.

The first piece we recommend locating will be the Bellhousing GM casting # 621 from a BBC/SBC manual transmission using a factory 11” flywheel/clutch setup. These are typically found in 70’s GM cars and trucks. This Bellhousing has the correct transmission bolt pattern except for one upper bell housing bolt hole to adapt a 4-speed to an LS-series engine. Classic Industries offers an adapter Bellhousing that is basically a 621 housing with all bolt holes that are required when using an LS series engine.

One thing that will need to be replaced will be the pilot bearing or bushing. In this case we will be using an LS7 Pilot Bushing GM which can be sourced at your local parts house for roughly $15. This pilot bearing offers the correct I.D. to match up with your GM 4-speed transmission, along with the correct O.D. to match up with the LS series engine.

Tech Tip: LS blocks have a rear casting journal with a 7/8 plug. If grease/bread method is used to remove the old pilot bearing, you risk pushing that plug into the pan causing a severe oil leak.

The clutch you will need for a GM 4-speed Transmission. The flywheel required will come from a 2005 Silverado 1 ton truck (GM PN# 12561680 & Sachs PN# 1050) which can also be sourced at your local parts house or GM dealership.

In order to complete your swap, you will also need some miscellaneous bolts and hardware. As for the mounting hardware we will utilize the factory LS flywheel bolts and the standard BBC pressure plate bolts. Most of this hardware is offered by Speedway Motors multiple vendors:

Another decision to make in transforming your hot rods efficiency is whether to update from mechanical to a hydraulic throw out bearing or continue to use your mechanical throw out bearing linkage setup. If you already have a functioning 4-speed car, the mechanical clutch linkage might be the easiest and cost-efficient route. While you can still upgrade to a Hydraulic Unit at any time in the future, now is an opportune moment since the car is disassembled and ready. This upgrade to hydraulic will provide a softer pedal feel with just as much pressure as the conventional mechanical.

A few other items needed to upgrade the clutch linkage to hydraulic would be...

  1. Speedway Motors Bleeder Kit to allow access to properly bleed the air out of your hydraulic system.
  1. Speedway Motors Master Cylinder with Reservoir, which is a ¾ bore to allow full travel of the throw-out bearing
  1. Speedway Hydraulic Clutch Release Bearing Set-Up Tool, provided with the throw-out are a series of shims to set up the correct spacing of the bearing this tool helps to find the correct depth and how many spacers are needed.

Now with the shopping list provided you will have everything needed to connect your GM 4-speed to your LS based engine. While this is one of the many steps in a LS swap we hope this provides you with an alternate transmission option other than the more conventional transmissions such as T-5, T-56, or the automatics 4L60E, 4L80E. Be sure to check out some of the other articles in our Tool Box that deal with LS swaps.

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