Replacing vs. resurfacing stock 1G AWD flywheel

[XC92]

I'm taking the winter off time to research various tasks I'll have to perform on my '92 Talon TSi AWD manual come spring (way too cold to do anything serious on the car for me right now, especially with an 18" blizzard about to hit tonight and temps in the teens).

One is figuring out whether to resurface or replace the stock OEM flywheel, which has already been resurfaced at least once and possibly twice.

I know all about getting the step height right and how to measure it with a dial indicator or depth gauge, so I'm good there. What I'm still wondering about, though, is the following:

1. What's the minimum safe friction surface thickness the OEM flywheel should have? I couldn't find it in the FSM or googling this site or online.
2. If it's been resurfaced past this point, should I shim the back (crank) side to get it back to the proper distance to the fork/fulcrum so disengagement isn't messed up, and if so how?
3. Or is this a bad idea because a too-thin flywheel could potentially overheat, crack and even break apart under hard shifting?
4. If it can be resurfaced, the thickness has to be uniform radially, from the outside in, right?
5. If I do need to get a new flywheel, or simply want a new new, I'm considering the ACT Streetlite, which I've heard great things about. But would it be too light for the kind of driving I do, which involves a decent amount of stop and go and between 20 and 40mph on streets, and no racing, launching or tracking? I.e. would it just not feel "right", and prematurely wear out the clutch? Or is it a mid-weight flywheel and not a true light one?

 

[jpmxrider489]

If your flywheel has been resurfaced already. You could have it done again. But if you are unsure, I would replace it. Can you provide pictures of the flywheel and your clutch? I cant speak to if a lighter flywheel would be hard to drive for you. I am not at stock power levels. However, I did drive the ACT streetlite flywheel and street disk for the clutch. The car does rev up a little faster but its still manageable once you get used to it. I had my flywheel resurfaced twice. After two times, I found out that was my reason for uneven clutch wear. Since then I have went from my ACT flywheel to a Fidanza flywheel. The Fidanza has a replaceable surface which is nice. No more machine shop. If I were you, I would look into buying that. That's just my opinion. For the labor involved in doing this job, If your concerned about the minimum friction surface, I would probably just buy new. I had a bad experience. I would not do anything with shims. Hope this helps.

http://www.teamrip.com/flywheel-step/#:~:text=The ideal step height for,to be less than ideal.

https://www.roadraceengineering.com/clutchandflywheeltech.htm

 

[97gsxIA]

1. What's the minimum safe friction surface thickness the OEM flywheel should have? I couldn't find it in the FSM or googling this site or online.
   .608-.612
2. If it's been resurfaced past this point, should I shim the back (crank) side to get it back to the proper distance to the fork/fulcrum so disengagement isn't messed up, and if so how?
   Hell no.
3. Or is this a bad idea because a too-thin flywheel could potentially overheat, crack and even break apart under hard shifting?
   The amount of heat that has gone into the flywheel over its life-span compromises it's integrity more than it being a couple hundredths of an inch thinner. Shimming is just a bad idea here, period.
4. If it can be resurfaced, the thickness has to be uniform radially, from the outside in, right?
   Uniform is uniform. You should be able to get the same measurement from everywhere you can put the dial on (clutch mounting pads to flywheel surface)
5. If I do need to get a new flywheel, or simply want a new new, I'm considering the ACT Streetlite, which I've heard great things about. But would it be too light for the kind of driving I do, which involves a decent amount of stop and go and between 20 and 40mph on streets, and no racing, launching or tracking? I.e. would it just not feel "right", and prematurely wear out the clutch? Or is it a mid-weight flywheel and not a true light one?
   The lighter flywheels rev up faster and fall off faster. You can compensate for both via driving habits and things like TPSdelta in tuning software. Its fairly negligible. Might feel different at first then you get used to it. Nothing wrong with running the streetlite on the street.

 

[XC92]

Thanks. But for #1 that's the step height, not the thickness of the friction surface. However your other remarks appear to indicate that it's not something to worry about.

Btw the point of shimming isn't to artificially "thicken" the friction surface (which wouldn't happen anyway as only the non-friction hub part would get shimmed), but to keep it within the proper distance range from the TOB, fork and fulcrum, so that engagement/disengagement happen with enough play at either end.

Every resurface brings them slightly closer to the crank, and lets the MC & SC out a bit more, the same as the way the clutch disc wearing out does. So I thought that as the clutch disc wears down, eventually this could cause engagement issues, at which point you have to either replace the flywheel or shim it. Why is shimming such a no-no?

 

[XC92]

If your flywheel has been resurfaced already. You could have it done again. But if you are unsure, I would replace it. Can you provide pictures of the flywheel and your clutch? I cant speak to if a lighter flywheel would be hard to drive for you. I am not at stock power levels. However, I did drive the ACT streetlite flywheel and street disk for the clutch. The car does rev up a little faster but its still manageable once you get used to it. I had my flywheel resurfaced twice. After two times, I found out that was my reason for uneven clutch wear. Since then I have went from my ACT flywheel to a Fidanza flywheel. The Fidanza has a replaceable surface which is nice. No more machine shop. If I were you, I would look into buying that. That's just my opinion. For the labor involved in doing this job, If your concerned about the minimum friction surface, I would probably just buy new. I had a bad experience. I would not do anything with shims. Hope this helps.

http://www.teamrip.com/flywheel-step/#:~:text=The ideal step height for,to be less than ideal.

https://www.roadraceengineering.com/clutchandflywheeltech.htm

The clutch is new, an SBC Stage 2 DD, which comes with a modified Exedy sprung disc (i.e. not a puck). Here's a photo of the flywheel after I cleaned it off just before I reinstalled it:

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Btw, those 3 "dimples" on top, along the trans side of the flywheel, I assume that they're to properly align the pressure plate, right? If so, I assume that it's impossible to properly install the PP without aligning them correctly, right? I assume I did this correctly but since it was my first clutch replacement I just want to make sure I did it right.

 

[pauleyman]

The friction surface uncut is above the surface of the cast ourer surface. I don't recall a depth you can machine to but if you're significantly below the cast surface get a new wheel. For the 99% streetcar stock weight flywheel is probably easier to deal with. I've had both.
New flywheel give you a fresh surface, fresh distances etc.
I disagree on a new flywheel not needing machining including the replaceable face units. Different clutch manufacturers require different step heights. Meet those requirements.
I don't care for the shim idea anywhere but I understand why people do it.

So
You've asked a lot of questions during your journey.
Opinions overflow but at the end of the day you're the one that has to live with the car. What you plan to do with it both now and in the future should heavily influence how you repair and/or modify the car.

 

[97gsxIA]

Sorry, I must have glossed right by #1.
You're right the thickness of the surface isn't (necessarily) as important because by the time its worn down to be too thin, the step height would be out of control and irrecoverable.
The thicknesses of surface material probably vary from stock to aftermarket, but probably not a lot of resources around to tell you on what is safe/unsafe.

They do make specific flywheel shims to get step height back into spec, and I wish I could give you some science behind why I think its a bad idea, but its more of a what has been passed down from teacher-student for me.
Its like this- they make shims for pivot balls to get better clutch engagement but I think you'll find the majority of knowledgeable people on here will say that its a load of crap and is just bandaid.
There's many things one can do to get better clutch engagement- but the tried and true is using your pedal adjustment with components that are within applicable spec.
Everything outside of that is at the expense of the user's comfort. Would a flywheel shim work? Shit, probably- why not The math makes sense and people have done it. Would I do it? No. So consider it an IMHO kinda thing.

As far as installing the PP, the alignment dowel pins that you see on the steps in between the fasteners properly locate the pressure plate to the flywheel. If everything lines up with those pins and the fasteners, you're set. Don't forget to use your clutch alignment tool for the disk. Ezpz

 

[XC92]

I only took head-on pictures of the flywheel but from the looks of them it appears that there's still plenty of friction surface before it matches let alone goes below the outer surface, so it looks like I'm good there. Probably several machinings left in it by this standard.

Although I'm still someone concerned about how each machining takes the friction surface further away from the trans, TOB, fork, etc.

But if it's still within spec range and the MC/SC are still able to compensate for this, up to where the clutch disc is almost worn out, then I guess I'm good.

Btw to be clear I meant shimming between the flywheel and crank, not the fulcrum. Not saying that I'm going to do it, just that this is what I meant. And we're talking no more than what was machined off, which is probably around the thickness of a coke can.

And yes, I do ask a lot of questions, but it's because, not being an experienced car person who's developed a good "feel" for how to do things (and having an engineering background), I need to go by hard specs. For something as potentially dangerous as an improperly repaired or maintained car, I'm willing to risk being a bit annoying. Apologies if that's been the case.

 

[XC92]

Sorry, I must have glossed right by #1.
You're right the thickness of the surface isn't (necessarily) as important because by the time its worn down to be too thin, the step height would be out of control and irrecoverable.
The thicknesses of surface material probably vary from stock to aftermarket, but probably not a lot of resources around to tell you on what is safe/unsafe.

They do make specific flywheel shims to get step height back into spec, and I wish I could give you some science behind why I think its a bad idea, but its more of a what has been passed down from teacher-student for me.
Its like this- they make shims for pivot balls to get better clutch engagement but I think you'll find the majority of knowledgeable people on here will say that its a load of crap and is just bandaid.
There's many things one can do to get better clutch engagement- but the tried and true is using your pedal adjustment with components that are within applicable spec.
Everything outside of that is at the expense of the user's comfort. Would a flywheel shim work? Shit, probably- why not The math makes sense and people have done it. Would I do it? No. So consider it an IMHO kinda thing.

As far as installing the PP, the alignment dowel pins that you see on the steps in between the fasteners properly locate the pressure plate to the flywheel. If everything lines up with those pins and the fasteners, you're set. Don't forget to use your clutch alignment tool for the disk. Ezpz

Perhaps I'm not explaining myself properly. Even with the step height at the proper value, which for many setups is between 0.608-0.612", every time you machine the flywheel down, it moves its friction surface away from the trans.

Keeping the step value correct just assures that the PP spring is within its proper working range between full engagement and full disengagement. But the flywheel's friction surface is still going to move further away from the trans with every machining and the TOB and fork will still need to move further towards the crank to disengage properly with sufficient room for the pedal to move towards the floor, at least 55mm for the 1G AWD's.

I suppose that you could partially compensate for this by bringing the step value closer to its upper limit. But you're probably going to run out of that after a machining or two and it'll change the engage/disengage points and feel.

Thus a very thin shim on the crank side of the flywheel. I'm not saying that it's a good idea, just that it would bring things back into alignment in this specific respect.

Look at it this way. Here are the various relevant specs:

F: Distance of flywheel friction surface from crank plate
S: Step height (distance of flywheel friction surface from PP mount points)
C: Clutch disc thickness
K: Position of outer fork within trans opening under SS spring "preload"

We know that as the disc wears down, C decreases and K moves towards the passenger side. But this also happens each time the flywheel is resurfaced, assuming that S remains the same, because F decreases. At first this might not matter much, but as C approaches minimal thickness, it could potentially cause disengagement issues. A shim on the crank side, whatever other issues it might introduce, would fix this.

Note that a shim on the crank side of the flywheel would be linear, as opposed to a shim on the fulcrum, which changes things in an angular manner and is thus more complex to calculate.

 

[97gsxIA]

And yes, I do ask a lot of questions, but it's because, not being an experienced car person who's developed a good "feel" for how to do things (and having an engineering background), I need to go by hard specs. For something as potentially dangerous as an improperly repaired or maintained car, I'm willing to risk being a bit annoying. Apologies if that's been the case.

What you're talking about doing is approaching what most people (who are experienced and have the "feel") would consider improperly maintained or repaired. And that will be their opinion based on experience.
There's always going to "look" like there is plenty of friction material left for machining. Its a chunk of plate that is centimeters thick that you're machining thousands of an inch off of. There's a reason the surface thickness isn't in a FSM- because no one has taken it down far enough with shims to necessitate writing that figure.
You can engineer your way to rationalization with all the math in the world, it comes down to what you're comfortable with doing. You don't have to convince me, you're convincing yourself.
If you trust the math, then shim it to your heart's content. No one will stop you.

I've seen pieces of flywheel the size of a nail go through a bell housing, linkage, hose, firewall and embed into the roof liner of a car. Experiences like that and lessons I've been taught guide where I put my trust. You take solace in math. So use your math and if it works, it works, if it doesn't, then youre on your way to generating your own "feel."

Roll the dice, or spend the money- one way or the other you'll learn something.

 

[pauleyman]

Perhaps I'm not explaining myself properly. Even with the step height at the proper value, which for many setups is between 0.608-0.612", every time you machine the flywheel down, it moves its friction surface away from the trans.

Keeping the step value correct just assures that the PP spring is within its proper working range between full engagement and full disengagement. But the flywheel's friction surface is still going to move further away from the trans with every machining and the TOB and fork will still need to move further towards the crank to disengage properly with sufficient room for the pedal to move towards the floor, at least 55mm for the 1G AWD's.

I suppose that you could partially compensate for this by bringing the step value closer to its upper limit. But you're probably going to run out of that after a machining or two and it'll change the engage/disengage points and feel.

Thus a very thin shim on the crank side of the flywheel. I'm not saying that it's a good idea, just that it would bring things back into alignment in this specific respect.

Look at it this way. Here are the various relevant specs:

F: Distance of flywheel friction surface from crank plate
S: Step height (distance of flywheel friction surface from PP mount points)
C: Clutch disc thickness
K: Position of outer fork within trans opening under SS spring "preload"

We know that as the disc wears down, C decreases and K moves towards the passenger side. But this also happens each time the flywheel is resurfaced, assuming that S remains the same, because F decreases. At first this might not matter much, but as C approaches minimal thickness, it could potentially cause disengagement issues. A shim on the crank side, whatever other issues it might introduce, would fix this.

Note that a shim on the crank side of the flywheel would be linear, as opposed to a shim on the fulcrum, which changes things in an angular manner and is thus more complex to calculate.

Your last point is not correct. Shimmimg the fork is the same as shimming the flywheel. Both would move in the same plane as a straight line parallel along the axis of the crank. If one really wanted to control they would know the distance of the flywheel surfaces relative to each other and shim exactly how ever much is machined off. The distance is the measure of the friction surface to the crank mating surface. I think I would ignore the step height here unless it is significantly different than stock otherwise it's always with a general range as already mentioned. On a stock(ish) pressure plate it probably doesn't matter and it's also why I don't shim at all.
I also pay attention to hydraulics, measure without fail and if there any components I don't trust they are repaired or replaced.

 

[1990TSIAWDTALON]

The input shaft doesn't care about any of this you know. It has plenty of spline room. Resurfacing only moves the disk a fraction more away from the front bearing on the shaft.