LS & LT Crankshaft Flange and Flexplate Guide

GM made several changes to the crankshaft flange design throughout LS and LT engine production. Knowing which flange your engine has is critical to purchasing the correct flexplate (automatic) or flywheel (manual transmissions). The “bolt” count refers to how many bolt holes are on the crankshaft flange that attach the flexplate or flywheel.

4 Types of Crankshaft Flanges

• 1) 6-Bolt Dished: Fits 99% of LS engines on the market. This is the most common flange used on nearly all Gen 3 and Gen 4 LS engines. 551356-LS or 551056-LS
• 2) 6-Bolt Flat: Found only on 1999–2000 6.0L LS engines. These engines have a longer crankshaft that extends farther toward the transmission. This design requires a flat flexplate .
• 3) 8-Bolt: Used on LSA and all Gen 5 LT engines. Our SFI-certified 8-bolt flexplate 551838-LT01 or 551056-LT.
• 4) 9-Bolt: Found only on LS9 engines. This is the least common flange pattern.

Matching the Transmission & Torque Converter Pattern

Now that you have selected your flexplate style, the next step is making sure your torque converter bolt pattern matches. Chevy used several converter patterns over the years, depending on transmission type. Choosing the wrong one can prevent proper alignment and cause flexplate or pump damage. If you have a transmission from an LS powered vehicle, then typically the flexplate and torque converter already match, so no parts are needed.

Common Torque Converter Bolt Patterns

• 10.5” Pattern: Early small-block Chevy converters found on Powerglide, TH350, and some TH400 transmissions.
• 10.75” Pattern: Common on Powerglide, TH350, 200-4R, 700R4, and 4L60 (used with SBC or non-LS 4L60E).
• 11.06” Pattern: Found on LS-based 4L60E converters. This is the correct pattern for most LS engine swaps.
• 11.5” Pattern: Used on heavy-duty TH400 and 4L80E transmissions. Many use six bolts instead of three.

Torque converters can have 3 or 6 bolt holes, depending on whether they are standard or heavy-duty models. All of our ICT Billet flexplates can be used with either 3 or 6 bolt and will fit all 4 bolt patterns from 10.5" to 11.5", making it a breeze to mix and match transmissions. We offer adapters and spacers to fit any combination you have.

Setting Proper Torque Converter Depth

Once you know your torque converter bolt pattern, the next step is ensuring the converter is built correctly for your engine. If you are ordering a new converter than you can have it made to your exact combination of torque converter bolt pattern and crankshaft depth. The depth matters since the torque converters lug rides inside of the crankshaft hub to maintain center. Once your transmission is bolted to the engine, slide the converter to the flexplate, then check that the converters hub is inside the crankshaft hub. Next make sure that the converters pads are touching the flexplate. This ensures that you are not preloading the converter to the flexplate, which will cause damage.

You can easily extend the torque converter’s crankshaft lug to reach the LS engine using our 551165 adapter. Another way to achieve this is by using a bolt-on ring style adapter, but those may interfere with the torque converter body and therefore not useable.

Flexplate Bolts

Using the proper bolts is key to keeping your flexplate attached to your engine. We offer bolts in standard length or extended length if you are using a thicker flywheel or flexplate adapter. To prevent your engine from rotating while installing your flexplate bolts, use our flexplate holding tool.

Why We Recommend SFI Flexplates

When building a high-performance engine, safety matters as much as horsepower. A flexplate failure at high RPM can send metal fragments through the floorboard, dash, or even the windshield. This is why NHRA requires SFI-rated flexplates on any vehicle running 9.99 seconds or quicker in the quarter mile or exceeding 135 mph. Our Made in the USA ICT Billet, billet steel flexplates are tested and certified to handle up to 20,000 RPM and 3,500 HP with an SFI 29.2 rating. Upgrading to an SFI-rated flexplate, bellhousing, and transmission case provides proven protection and long-term reliability.