[krustissimo]

I started a thread a few months ago in the diagnosis forum about a problem I was having with weird chattering in the engine area at low RPM acceleration. My guess (now hope) is that it was the DMF failing but I found out there are hardly any replacements available anymore and they run about $2500! I didn't want to run the super-lightweight 13lb aluminum one but luckily I found an alternative in the Spec 86S steel single-mass flywheel and ordered one for somewhere around $500.

 

I'm only partway through the installation but I figured I'd share what I've learned so far. Before I ordered I called Spec to confirm it should work with the original clutch, etc. and they were very nice and helpful: bottom line was that it should be a direct swap. The flywheel arrived in just a week or so and it looks very well made. It weighs in at 26 pounds (I think the old one was 32?) and I mated the pressure plate to it to be sure the bolts line up, which they do. It also comes with a nice smooth pilot bearing already inset, plus the 9 flywheel bolts you need (which saves like $50 so that was nice!)

 

A quick note on the bolts: the originals are longer by 2cm or so and use the wacky "three square" pattern. The replacements use an 8mm allen head which seems to have a deeper recess so hopefully fewer problems with stripping (though luckily I didn't have any issue getting the original flywheel off.)

 

With the old and new flywheels sitting side by side on a flat surface I noticed the new one was just a tiny bit taller. I put them on my table saw's cast iron top and rigged up a dial gauge to confirm. This measuring setup isn't very precise but it shows that the friction surface on the SP86S is 2-3mm further from where it mates to the engine than on the original. Same results for the recessed surface around the friction area where the pressure plate mates to the flywheel.

 

I'm hoping this won't cause any issue. I figure the effect will be that the throwout bearing rests 2-3mm further towards the back of the car, but still well within its normal travel range. My guess is this will move the clutch activation area a bit higher/earlier when pressing the clutch pedal, but maybe I have this backwards or the hydraulics will adjust in a way that it doesn't affect anything.

 

I'll find out soon! As of right now I have the new flywheel installed and torqued down with no issues. The rest of the re-assembly I've done several times already so if there are no surprises I'll be testing later today or tomorrow.

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[krustissimo]

Finished up last night and went for a test drive this morning. Good news is the old problem (rattle/vibration on low rpm throttle-up) is fixed. So it was in fact the DMF causing that issue. More good news is that the car drives pretty nicely with the solid flywheel. Its been up on the quickjack for months so I can't really compare before and after, but overall I'd say its easier to drive than before this whole issue arose: smoother clutch engagement and less precision needed to prevent luggy starts. The clutch engages pretty low to the floor, but it feels natural and I don't see any issue there; I'm not going to try to adjust it. Clutch resistance is really low but that actually showed up on my first rebuild attempt and I think it has to do with greasing the fork bearings or something. I don't remember what it was like 20 years ago when I got the car.

 

The bad news is there is some kind of rattle in the mid 2000 rpms, sounds like from the gearbox. I guess this is what everyone was talking about with the 13lb flywheel? I'm going to change the gear oil (its been ages) and drive a while before making any conclusions.

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[tejon007]

Best wishes with the repair.



We have a non-dual mass flywheel on the shelf for when the original fails. So, what you did is OK by us.



Since you've already installed it (but, maybe others will find this useful), hate to mention it, but it seems that it's always a good idea to have the flywheel and pressure plate dynamically balanced. Haven't had one done in about 3 years or so. It was something right around $100, but had to travel around 150 miles round-trip twice to take and retrieve from the shop.

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[vliegwielolaf1234]

I replaced mine a few years ago , with a new DMF was a 1000E here in Europe  , since i did not want to give up in comfort the rattle issues etc , i think the DMF will last longer than it's owner now .

It can be done , but balancing things would be the way to go , i think the engine might suffer since the s2 has a harmonic balacer op front if i remeber correct , maybe that will do the trick , fitting one of those .

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[krustissimo]

Went for another drive today and the gear rattle (assuming that is what it is) is pretty bad. It only happens between about 2200 and 2800rpm and is louder in higher gears than lower ones. Not sure if it does any harm or if its just an annoyance but I'd like to at least reduce it if there's any way.

 

Reading about DMF->solid flywheel conversion for other cars it looks like a common practice is to also switch the friction disk from the normal plain one to one with springs in it. Anyone know if there is such a disk available for 968s? I'm hoping maybe yes since the pressure plate and friction disk are dimensionally the same as in the 911.

 

Also any thoughts on using a higher viscosity gear oil? I haven't changed the gear oil yet but was planning on doing that today. I think my current oil is about 10 years old though it only has like 10k miles on it (she's a garage queen, obvs!)

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[krustissimo]

Re: balancing, I know it would be a good idea but is it likely to affect gear rattle? And how effective can it be, given that the clutch connects to a two-part driveshaft which itself isn't likely to be well balanced? The driveshaft has that blocky clamp thing on it which doesn't look symmetrical, though of course it is a relatively small diameter compared to the flywheel and clutch.

 

One thing I noticed on the pressure plate is that it has some drillings around the outside rim that look like they were factory balancing adjustments. I re-used that original pressure plate rather than put in a new one I have in reserve (because those are rare and expensive, too!) and I tried to keep the original orientation relative to the flywheel's position sensor gap. However the bolt drillings on the new flywheel are a little shifted from the original so it isn't quite right, plus my markings on the plate+flywheel had gotten faint so there was some guesswork involved. And anyway it probably doesn't do any good since they weren't balanced together, but I figured what the hell.

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[tejon007]

Are you sure you fixed the original problem?



Did you check to see if the DMF had actually failed after you replaced?



Did you use the stethoscope to track down the "rattle"? (torque tube rattle, engine mount rattle under tension/acceleration, etc)



Can't figure out if you're sure it's a noise that changes with engine speed or with car speed when the transmission is in gear?

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[krustissimo]

I'm quite sure it was the DMF now, though that was just my best guess before I made the swap. Unfortunately I don't have the experience to tell from feel by rotating the two halves, but the symptoms made sense and I couldn't find any other possible explanation. Most tellingly, the problem went away completely with the new flywheel. The original problem happened regardless of clutch or gear position or whether the car was moving: it was a sort of rattle or clicking sound from the back of the engine area when revving up from idle. On an actual start it would have both the rattle noise and a bit of a judder as the clutch engaged.

 

That original problem is completely gone and everything seems totally fine except when in gear (and therefore of course in-motion), and it only makes the sound when throttling up or engine braking through the 2200-2800rpm range. It seems to get slightly louder progressively as I go to higher gears. I wouldn't say it varies much as a function of RPM; the sound mostly seems to just go on or off when in that range. I would say the loudness increases when accelerating harder or engine braking harder.

 

From reading around I'm pretty sure its caused by backlash chatter in the gearbox due to the small irregularities in thrust from the engine and/or the drivetrain not being balanced. I think its basically a resonance thing and that's why it only happens in a certain RPM range (possibly it would also happen at 2x the rpm range, though I haven't tried that yet). If its due to a balance issue it should be pulsing the gearbox at 42hz at 2500rpm, or four times that if it is from the engine's inherent irregularity. So I'm thinking maybe if I can isolate the dominant frequency of the sound it might give a clue whether balancing would help or not.

 

However I can't test quite yet as I discovered the crush washers were completely missing from the gearbox when I went to change its oil! I'm waiting on an amazon delivery of a crush washer assortment to put in the new stuff, hopefully this afternoon. The old oil was surprisingly dark but I didn't see any significant metal bits in it. Unfortunately the drain plugs on my '92 don't have magnets in them for some reason (very weird! I was actually thinking of gluing some in myself.)

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[krustissimo]

New 75W-90 gear oil is in and I went for a brief test drive. It seemed like the problem had completely gone away at first but then it started doing it again (probably after the oil had warmed up a bit.) I couldn't stay out long enough to get it very hot so I can't say if it is better, worse or the same as before.

 

Anyway it is at least driveable and I hope/believe it isn't doing anything too bad to the gears. I think I'm just going to live with the noise and put some miles on the car for the first time in forever. I just took a look at "bring-a-trailer" and the values of 968 cabs are shockingly low (like $15k) so if that's really all its worth I think I'm going to stop babying it and drive it into the ground.

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[Bruckner]

Interesting and useful account of a problem we'll all face sooner or later. Mine is due for a replacement, it is loud at idle and the bounce is excessive.



The Spec steel option seems to be the next best thing to genuine DMF, but I would replace the clutch disc with a spring loaded unit, never researched which would work. There's also the 944S2 harmonic balancer that's easy enough to install, URO makes a replica part, not sure if quality is good. All this adds up the cost but DMF is expensive. Frustratingly, LUK and Sachs still produce the almost identical DMF units for 964/993 series, for less than 1K$, whereas 968 DMF is only sold by Porsche who no doubt bought the last batch produced.

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[jsheiry]

Pete at RS Barn and engine balancing company BHJ put together something better than the S2 balancer.  Pete no longer sells them to my knowledge but you can purchase from BHJ in CA.  Not the cheapest thing in the world but perhaps still cheaper than a new DMF.

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[Bruckner]

Pete at RS Barn and engine balancing company BHJ put together something better than the S2 balancer.  Pete no longer sells them to my knowledge but you can purchase from BHJ in CA.  Not the cheapest thing in the world but perhaps still cheaper than a new DMF.

I'd be interested to know which disc Pete used along with his own single mass flywheel and harmonic balancer, for a street driven car that is, and whether this setup is quiet. Spec steel FW + BHJ harmonic balancer + spring loaded clutch disc isn't much cheaper than new DMF. No easy solution...

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[Waylander]

You really dont want to use a 911 plate and cover, unless your a body builder and need a bigger left leg and a cracked firewall

And I need a DMF too, but cant buy the Spec unit in the uk

Post would be out of this world

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