Spec GM F40 Aluminum Flywheel
The SPEC SCLS4AF40 is an aluminum flywheel for a very specific build: bolting a GM LS4 5.3L V8 to the F40 six-speed manual transaxle. SPEC lists it for LS4 engine swaps into the F40 gearbox used in the Pontiac G6 and in Saab models, and files it under swaps that need custom components. If you are searching for a flywheel for a stock G6 GTP, Saab 9-3 Aero or other F40 car with its original engine, this is not that part; it is for builders putting the transverse LS4 from an Impala SS, Grand Prix GXP, Monte Carlo SS or LaCrosse Super behind a manual.
From the factory the LS4 was paired with automatic transaxles in the cars listed above, so those cars never had a manual flywheel. The flywheel is the piece that makes the swap work at the crank: it bolts to the LS4 crankshaft, carries the ring gear for the starter, and gives the clutch disc its friction face at the right position for the F40's release system. Factory F40 cars use a dual-mass flywheel to damp gear noise, so a single-mass aluminum part like this one changes the feel. Aluminum keeps rotating mass low, so the V8 revs and falls faster, while idle gear rattle can increase compared with a dual-mass unit. SPEC's product page lists a weight of 15 lb.
Confirm the combination you are building: an LS4 block, an F40 transaxle and the adapter and clutch parts that tie them together. SPEC sells matching LS4-to-F40 clutch packages for the G6 gearbox, so choose the clutch at the same time and make sure the disc spline and diameter fit this flywheel. On a running swap, rule out other causes before replacing parts; our slipping diagnosis guide and flare vs slip guide help separate real clutch slip from other drivability problems. Our lightweight vs stock flywheel guide covers what to expect from an aluminum wheel on the street.
SPEC lists SCLS4AF40 for "GM-any LS4 engine swap requiring custom components" to the G6 F40 six-speed gearbox and to the Saab F40 six-speed gearbox. It is not a direct replacement for a factory F40 car with a four- or six-cylinder engine. Because swaps vary, check bellhousing adapter, starter location and clutch package against this flywheel before final assembly, and contact us with your engine and transaxle details if anything is unclear. Background on factory F40 cars is in our Pontiac G6 transmission guide and Saab 9-3 transmission guide. More drivetrain parts are in our hard parts collection.
Our manual vs automatic repair cost guide helps budget the clutch and gearbox side of the project. The flywheel is new, in stock, ships the same business day from the USA and carries a 1-year warranty.
| Brand | SPEC |
| Part number | SCLS4AF40 |
| Fits | Spec GM F40 Aluminum Flywheel |
| Type | Flywheels |
| Condition | New |
| Ships | Same business day (in stock), from the USA |
| Warranty | 1-year |
No. SPEC lists SCLS4AF40 only for LS4 V8 engine swaps into the F40 six-speed. Factory F40 cars keep their original engine's flywheel.
The transverse 5.3L LS4 came in the 2005-2008 Pontiac Grand Prix GXP, 2006-2009 Chevrolet Impala SS, 2006-2007 Monte Carlo SS and 2008-2009 Buick LaCrosse Super, all with automatics.
SPEC offers LS4-to-F40 clutch packages for the G6 gearbox. Match the disc spline, diameter and pressure plate to this flywheel and to your release system.
The F40 is rated for 295 lb-ft of engine torque, below the stock LS4's 323 lb-ft. Builders who run the combination should expect hard launches to shorten gearbox life.
GM specifies a synthetic 75W-85 GL-4 manual transmission fluid, about 2.8 liters, per GM documentation and owner manuals.
