A dual-mass flywheel splits the flywheel into two masses joined by springs, so it absorbs engine vibration before it reaches the transmission. A single-mass (solid) flywheel is one piece: simpler, often lighter, usually resurfaceable and tolerant of more torque, but it passes more vibration and gear rattle into the driveline. Which one suits you depends on how the car is driven and how much noise you will accept.
How a dual-mass flywheel works
The primary mass bolts to the crankshaft and the secondary mass carries the clutch. Arc springs and a damping system between them soak up the torque pulses each cylinder firing sends through the crank. That is why many modern manual cars and most manual diesels use one: it keeps idle smooth, cuts gear rattle at low rpm and protects the gearbox from shock loads.
Dual mass vs single mass at a glance
| Dual mass | Single mass | |
|---|---|---|
| Vibration and gear rattle | Low | Higher, especially at idle |
| Weight | Usually heavier | Often lighter; lightweight versions rev faster |
| Resurfacing | Generally not recommended | Usually possible within limits |
| High torque or hard launches | Springs can wear out sooner | Tolerates it well |
| Replacement cost | Higher | Lower |
Dual-mass flywheel failure symptoms
- A rattle or knock at idle that changes or stops when you press the clutch pedal.
- Judder or vibration as the clutch engages, especially pulling away.
- A clunk on take-off, when lifting off the throttle or when switching the engine off.
- More rotational free play or rocking between the two masses than the maker allows, found with the transmission out.
Some of these overlap with a worn clutch, so compare with signs of a bad clutch and clutch chatter before buying parts.
Can you resurface a dual-mass flywheel?
Most flywheel makers advise against it. Machining the friction face of a dual-mass unit can upset its balance and does nothing for worn springs, so a worn dual-mass flywheel is normally replaced. A single-mass flywheel can usually be resurfaced if it is within its minimum thickness and free of cracks.
Converting to a single-mass flywheel
Conversion kits pair a solid flywheel with a sprung clutch disc, which takes over some of the damping job. Expect more gear rattle at idle and more vibration through the cabin, in exchange for lower cost, easy resurfacing and better tolerance of extra power. It is a common choice on cars that see hard launches or added power, and a poor fit for someone who wants a quiet daily driver.
Clutch kits with a flywheel
Changing the clutch is the right time to deal with the flywheel, because the transmission is already out. For the Subaru WRX, the ACT Xtreme Clutch Kit w/ Streetlite Flywheel ($1,254.00) pairs the clutch with ACT’s lightweight Streetlite flywheel, and the ACT Heavy Duty Performance Street Disc Clutch Kit w/ Flywheel ($1,204.00) covers WRX, Legacy GT and Forester XT. Check the listing’s fitment for your year and transmission. For clutch grades, see stage 1 vs stage 2 clutch; for service life, how long does a clutch last?
More guides: clutch master cylinder symptoms.
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