Clutch help please

[AZPete]

SOLVED! Scroll down to Post #14 for solution.

My clutch is slipping and I need some help to find why. I've got an 818S with a 2006 WRX 2.5 L with a new OE clutch disc and a new OE pressure plate (Subaru calls it a cover). This has a push type actuator.
Symptoms: at heavy acceleration the engine revs quickly with relatively less increase in speed; smells like clutch burning after several episodes; no tire skidding sound or smoke. At normal acceleration the clutch acts and feels normal. The "grab point" of the pedal is about half way up. The pedal stroke and free play are to FSM specs.
Diagnosis: must be either hydraulic or mechanical - right?
Hydraulic? My first thoughts were that the clutch master cylinder was not allowing fluid to flow back, or lines were crimped, but I have measured the precise position on the release lever at the relaxed (engaged) position, then pumped the clutch over 20 times, and the release lever returned in the same position. Hence, residual hydraulic pressure is not the cause. Next, I removed the operating cylinder (slave cylinder) and the release lever remained in the same position. Hence, the operating cylinder is not keeping the clutch from fully engaging. Right?
Mechanical? I replaced the disc, pressure plate and release bearing before installing the engine & tranny months ago. If I have to remove the transmission to fix the clutch, I'd like to know what might be the problem before I remove rear body panels, suspension, wiring, cables, just to look. Here are photos of the clutch plate and pressure plate. Do you see any problem?

http://i210.photobucket.com/albums/bb180/AZPeteCobra/new%20clutch%20disc%20100.jpg (http://s210.photobucket.com/user/AZPeteCobra/media/new%20clutch%20disc%20100.jpg.html)

http://i210.photobucket.com/albums/bb180/AZPeteCobra/new%20pressure%20plate%20100.jpg (http://s210.photobucket.com/user/AZPeteCobra/media/new%20pressure%20plate%20100.jpg.html)

[Hindsight]

You didn't mention whether your flywheel was new or used and re-surfaced. If you re-surface the flywheel and the machine shop takes it down too far or doesn't machine it properly, it will not allow the pressure plate to provide it's full clamping force.

[Flamshackle]

Also another common problem is people can easily put too much grease on the inside surface of the thrust bearing and under the centrifugal force it spins out and onto the flywheel face.

All the best

[AZPete]

Thanks guys for your help. Hindsight, I had not thought of the flywheel thickness but my flywheel is used and I didn't have it turned. The flywheel came with the engine from AJW so it's possible it was turned by some previous owner. If I have to take it all apart I'll mic the flywheel, for sure. Flamshackle, I don't think I greased up the thrust bearing too much but that would cause slipping for sure.

[michael everson]

I dont see how the thickness of the flywheel would affect the clutch. If the pressure plate is bolted to the flywheel and the clutch disc is sandwiched in between, then it shouldn't matter.
Mike

[Hindsight]

The WRX flywheels are dual surface. The surface where the PP mounts is raised. Clutch surface is recessed. If someone machines the flywheel and only machines the recessed part (or machines both, but not in the exact same ratio), the pressure plate loses clamping force.

[Junty]

Hey Pete, I also have a similar problem, couldn't complete second run on dyno due to clutch slippage in 5th gear. I also fitted new Clutch and Pressure plate - standard OEM. I did a little research and it would appear that standard Subaru pressure plate has only about 1100 ft pounds of hold pressure. There is a wide range of non-standard and performance upgrades for the clutch and pressure plate. My initial thought was that in a light car (818) and being 2wd as apposed to 4wd clutch was the least necessary upgrade. Hindsight however shows that I should have upgraded. A friend of mine in Aussie has a 3600 ft pound pressure plate - specially built for 500Hp 4wd WRX. I don't think that this is necessary for an 818 - however I have found on Ebay upgraded pressure plate, ceramic clutch, bearings and 12 pound flywheel as a kit for about $230.
I'm using the existing setup for as long as I can - then replacing with light flywheel, upgraded pressure plate and sprung ceramic clutch (sprung to keep car steetable).

[AZPete]

Very interesting, Junty. I get slippage in the lower gears anytime I tromp it. I think though that other 818 guys would be having problems also, if the OE clutch plate and disc are causing it. So, I think I'll wait and see if others say they are having the same problem. :confused:

[Junty]

Thanks Pete, I decided to proceed with upgrade sooner than later. just ordered an XTD stage 3 kit from Ebay. Includes spigot bearing, release bearing (push type), 6 blade sprung ceramic clutch, 30% upgraded pressure plate and a 12 pound chrome molly fly wheel. $215 delivered to my US address. It will take perhaps til December before I have this fitted - I'll keep you informed of progress...

[STiPWRD]

My wrx (light weight) flywheel has one level surface for the clutch contact and pressure plate attachment points. So, unless it's badly worn, overall flywheel thickness shouldn't affect clutch engagement. I do think it's a good idea to have the flywheel turned anytime the clutch is replaced. I knew I was going to be running some extra power, so I upgraded my clutch to this one from fx racing, rated for 373 hp, 361 tq., cost was $169 on ebay.
470894709047091

[AZPete]

I agree that with high hp an upgraded clutch is probably needed but my 06 WRX engine is bone stock so I'd think the OE clutch should be fine. Unless slipping is a known problem (doubtful) I think my problem is either bad parts or bad installation.
Could I have installed the release bearing reversed?
The OE clutch plate has slightly thicker lining (.3 mm) on the pressure plate side, per FSM. If I installed it reversed would it cause slipping?
On inline engines a leak at the rear main bearing can put oil into the bell housing. Can that happen in a Subaru boxer?

[flynntuna]

There's an access cover behind the flywheel to remove the Pistons that can leak mimicking a main bearing seal leak.

[STiPWRD]

Pete, if the clutch and crank shaft were not aligned, there's a chance you dislodged the input shaft bearing that goes in the middle of the flywheel when installing the transmission onto the engine. This would result in the input shaft being unsupported at the front end and over time, mess up the oil seal at the transmission/input shaft (due to excessive motion of the shaft). This happened to me once so I always use some loctite when pressing in that flywheel bearing. Basically the inner bell housing gets covered with a layer of transmission fluid.

The best way to find out is to remove the trans and see your yourself.

[AZPete]

I found the problem! But, it was not any of the things suggested above (thanks for trying, guys!), not any of the causes in the factory service manual, and not about bad parts or stupid installation. Checking for oil on the inside of the bell housing, I went to remove the rubber dust cover and discovered that it was pinched between the release lever and the edge of the hole in the bell housing. When I cut and pulled the dust cover out the release lever moved forward slightly - Voila! In a test drive the clutch does not slip under heavy acceleration. The rubber dust cover was not bunched up but appears to have been installed correctly with the lip around all edges of the opening, so I don't feel stupid. It was keeping the release lever from full clutch engagement. I've cut the dust cover so it won't restrict the release lever but covers the opening.

[Hindsight]

Glad you figured it out!

[skullandbones]

Wow! That was a stroke of luck. It would have been a shame to tear the thing down after so much hard work.

Continued good luck,

WEK.