Piston Slap: An Exedy-ous From Logical Clutching?
James writes:
Hi Sajeev,
I’ve got a 2003 Impreza WRX (blob-eye) that’s covered 96,000 km now. I bought it two years ago from the original owner, who seemed to take very good care of the car (every receipt since new). The car has been almost faultless.
About 4,000 km ago, at 92,000 km, the original clutch was clearly on its last legs. Under wide open throttle in 3rd gear, the clutch would slip and struggle to get power down. So off it went to a local WRX specialist to have the clutch replaced with a new kit: new pressure plate, machined flywheel and new Exedy OEM (standard, not heavy duty) replacement clutch.
However, when I got the car back, it felt completely wrong.
With the OEM clutch, you could roll and creep around the place smoothly when you took off using just the clutch. This new clutch makes a terrible shudder, and the front end vibrates pretty bad. The car does take off smoothly with throttle applied, but it won’t be very smooth if you catch the clutch in a bad mood. The clutch also has a bad habit of shuddering on downshifts, especially at lower revs.
My mechanic told me to give the clutch time to “bed in” and said it’d get better. After 1,600 km, with little to no improvement, they admitted the clutch might be a faulty unit, and replaced a new clutch kit free of charge.
Since then, the clutch has felt better than the first replacement — but not by much after 1,700 km. I went back to my mechanic and, while they said “it wasn’t perfect”, they wanted me to bring it back in another 1,000 km to see if it hadn’t improved. However, it just seems weird that the second clutch is still displaying the same problems. What could be the issue here? Bad flywheel? Improper installation? Another faulty clutch?
Sajeev answers:
I was concerned the Exedy WRX clutch might have the same issue as my go-to aftermarket clutch: the “King Cobra” for small block Ford products. The unit in my brother’s Fox Mustang (light, by cable-operated standards) is very different than the newer one in my Fox Cougar (much heavier). Not enough to bother me, but I digress.
Exedy says their Pro-Kit clutch is “ manufactured on the same production line as OEM clutch components.” The Internet also agrees. And the Internet is NEVAR WRONG!
That means one of your final questions might contain the answer, but swap “flywheel” for “pressure plate with a bad spring.” Or maybe the rod between the pedal and hydraulic cylinder is out of whack. And yes, perhaps there’s a batch of bad clutches from the factory! I’d read this thread first, then discuss the condition of the pressure plate and, lastly, consider blaming it on a bad batch of clutches.
Bonus! A Piston Slap Nugget of Wisdom:
It’s theoretically possible — but not probable — that a leaky rear main seal could contaminate the clutch. It could cause clutch chatter if oil reaches the friction material. Considering its improbability, plus the seemingly well maintained, stock (?), unabused nature of your WRX, this doesn’t seem likely.
[Image: Shutterstock user Somchai_Stock]
Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.
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Way back when, having a Corvair Corsa, with clutch shudder after 'my' replacement - my shop teacher lent me an alignment tool. He told me the shudder was because the clutch was slightly, off center with the flywheel and transmission shaft.
Hey guys, original poster here. I originally sent this through a few months ago, but sadly Sajeev wasn't able to run it sooner. But this means that I can clear up a few things here. I can shed some light on why the original clutch needed to be replaced at 94,000km. Believe it or not, but the car was owned by a Christian rock-loving, mother of two. But this family clearly had petrol running in their veins - they had previously owned many T-56 equipped Holden V8s, and the WRX was replaced with a Legacy GT Spec B. But the car was driven mostly around town in stop-start traffic when I'd bought it at 67,000km. So that meant the wear would've been higher than a more lightly driven freeway car. Who knows, maybe the original owner enjoyed launching her WRX? Second of all, I learnt manual on this car. While I did burn the clutch a bit at first, I was fine by the 2nd week of driving it. My driving mostly consisted of driving to and from work in urban areas. The biggest factor to the early clutch replacement would be the dyno tune on the car. With a bigger turbo back 3" system, it went up from 125awkW to 168awkW. When the clutch was starting to go, it was occasionally chattering when taking off at the lights (this is probably down to flywheel abuse), and the clutch was starting to slip in WOT in third gear. This is often symptomatic of a tuned car, on a OEM spec clutch, when you ask the clutch to deal with 35% more power. If I'd reverted to the stock tune, the slipping in 3rd gear would've disappeared with less power. I could've easily gotten another 10-20k km if the car wasn't modified. The slipping wasn't massive, and you would barely notice it in normal driving, but I couldn't be bothered dealing with a slipping clutch in a performance car. Let it be known that while I'm not the easiest driver in the world, I do have mechanical sympathy. But if I'm going to have a turbo rally bred car, there will be the occasional 4k RPM launch, where I'll feather the clutch a tiny bit to avoid sending a massive shock through the gearbox. The other posters are correct, Subarus do have weak clutches to prevent people dumping the clutch and breaking gearboxes. In short (or TL:DR), car driven in 95% urban situations, used to learn manual, then moar BOOST and hard driving equals short clutch life. ----- Anyway, moving on - in the time before this was posted, there was a solution to this problem. With the flywheel machining issue, my mechanic (a trusted WRX and EVO specialist in Adelaide) was replacing the used flywheel with a pre-machined, ready to go flywheel. They are a high turnover garage and can't have a car sitting in a hoist for 2-3 hours while a flywheel is machined. Unless they were all warped, bad flywheels, this narrows this out. After the issues with the 2nd replacement clutch, I went back and my mechanic said he was going to cover the costs and pay for an OEM clutch direct from Subaru, rather than Exedy. I was happy with this outcome as I didn't want another Exedy clutch going in, and either did my mechanic - after forum searches, this seemed to the be common fix. However, when I went back to pick up the car, my mechanic said he'd looked at the Exedy and Subaru clutch, and advised me he didn't think it would make a difference as they looked identical. The internet says otherwise (AND WE BELIEVE EVERYTHING ON THE INTERNET!), but he seemed convinced that they'd be the same. In the end, they put an Exedy HD clutch in, which fixed it! There was no shuddering. However, this created other problems, as the clutch was noticeably stiffer (a bitch to drive in traffic...), and I never wanted a HD clutch as they apparently destroy stock 5 speed Subaru gearboxes. Sounds like it was the Exedy clutch not being right, but lesson learned, demand OEM or go through this again. It was pretty annoying they put in the HD clutch without consulting me, but it didn't really matter, because in the end I sold the car about a month later to someone interstate, who was happy to pay a premium due to the condition and mileage of the car. They ended up driving it back 2200km/1400 miles back home. Go figure. I ended up buying a Legacy GT which recently had the clutch replaced at 146,000km.