How to adapt LS Engine to TH350 or TH400 1


Adapting an LS engine to a TH350/TH400 is not only possible, but quite easy to do. Read more to see how with lots of pics!

I want to clear up some confusion here. I did A LOT of research on bolting the newer LS motor to the older style TH400 transmission during my swap, most of which just confused me even more. I documented what I learned and hope to provide some clarity for you here!

What parts do I need and why?

If you get a 4.8/5.3 engine with a dished flexplate the ONLY part you need is a Hughes Performance HP3795 Transmission Adapter. The reason you need this is because the LS motors have a crank that is .4 inches shorter than the older Chevy engines. The dished style of the flex plate makes for the same bolt pattern, but the Hughes flexplate adapter spaces the crank out .4″ to make up for the shorter crank.

You will also need to have a carbide burr in order to elongate the flywheel holes for the slightly larger bolt pattern on the TH400 torque converter. Something like this:

Still confused? Here are LOTS of pictures:

LS to TH400 adapter
Hughes Flexplate Adapter on the snout of my TH400 torque converter.

The spacer is needed because the snout on the center of the torque converter is what centers it in the crankshaft. This reduces the chance of vibrations/damage.

LS to TH400 adapter
Notice the 3 bolt circle on the newer flexplate is a smaller diameter than the older style torque converter. No biggie, grab a drill/die grinder, a carbide bit, and elongate the holes to fit the larger bolt pattern. This is pretty easy as the flexplate is relatively thin.
LS to TH400 adapter
5.3/4.8 dished flexplate bolted up to my TH400 torque converter after elongating the flexplate holes by ~.5″
I then bolted the flexplate back onto the crank and torqued it to spec.

Warning, make sure you use Locktite on the 6 flexplate bolts and follow proper torque sequence alternating side to side.
First time through torque bolts to 15 ft/lbs
Second round torque to 37 ft/lbs
Third and final round torque to 74 ft/lbs

Just for good measure I always mark critical bolts with a paint pen after I final torque them. This also helps as a good indicator if the bolt ever moves because the lines won’t match up.
Close up of the Hughes Flexplate adapter. Lightly sand the bore on the crank then tap the spacer in with a rubber mallet.
All the spacer does is provide proper crank stickout for the snout of the torque converter to slip into and center on.

You can find the LS adapter on Amazon for pretty cheap considering how easy it is to install and that no other additional parts are needed.

This is an excerpt from my Ol’ Blue LS Swap buildup because I still have a lot of people that ask me about it and it was buried inside my buildup. You can read about the rest of the LS Swap into my ’71 Chevy here: 5.3 LS Engine Swap into Ol’ Blue 1971 Chevy – Part 1


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