Q&A

Mailbag: Mating an Older GM Automatic Transmission to an LS Engine

(Image/Summit Racing)

Q: Can I use an older GM automatic transmission with an LS engine?

A: Yes.

There are three exceptions in the LS engine family for what we’ll discuss here (the LS4, LQ4, and LR4), but mostly, yes. The LS4 is the one LS engine designed for front-wheel-drive, transverse-mounted applications.

For details about the LQ4 and LR4 engines, check out Mating a GM Automatic Transmission to an Early LQ4 or LR4.

With that important disclaimer, let’s get into it.

GM Automatic Transmissions

Solution for Mating an Older GM Transmission to Your LS Engine

At minimum, you will need a new flexplate, and there are two styles of adapter flexplates available:

  • A flat flexplate with a spacer and longer bolts.
  • A cone-shaped flexplate with a pilot extension.

It’s also recommend that you replace your torque converter. It isn’t required, but the power band of the LS engine is different than the engine you’re replacing so you may need a different stall speed.

To select a flexplate, choose the correct converter bolt pattern for your torque converter.

Transmission ModelConverter Bolt Pattern
Powerglide10.750 in.
TH35010.750 in.
200R410.750 in.
700R410.750 in.
4L6010.750 in.
4L60E10.750 in.
TH40011.500 in.
4L80E11.500 in.

Notes

Some flexplates include the spacer, bolts, and extension (if required). Please check the notes on the flexplate you choose.

  • If you need to order the spacer and bolts separately, check out SUM-704000.
  • If you need to order the pilot extension separately, check out SUM-704004.

Many of the adapter flexplates are drilled with multiple converter bolt patterns. Please confirm the bolt pattern on the applications tab.

Shorter Crankshaft on Most LS Engines is Why Flexplate Adapters Matter

All LS engines used the standard GM bellhousing bolt pattern. The dowel pins are in the right spot and five of six bolt holes will line up.

Most GM transmissions will bolt to an LS without an issue.

The problem with the LS engines is the short crankshaft. It is 0.400 in. shorter than previous small block and big block engines.

That means the older torque converter pilot is too short to engage the crank.

Some early LQ4 and LR4 engines used a longer crankshaft than other LS engines, which is why they’re compatible with the older transmissions without spacers or extensions. However, special adapters are required when mating an older transmission to most LS engines.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racings tech department, in which there are hundreds more. Click here to see them all.

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3 Comments

  1. If your Lq4 was in front of a 4l80e it’s a simple bolt up. No needed changes to the flexplate as it comes with the factory spacer.I swapped to a powerglide with no changes.

  2. is there a way to incorporate the TH350 kickdown to the LS?

  3. What is the correct bolt size to bolt up a 4L60e to a 5.3 engine for both bolting to bellhousing and also the torque converter to the flex plate?

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