When to resurface a flywheel??

[euro911sc]

How do you know your flywheel needs to be resurfaced? If it were never resurfaced what is the spec that determines it needs to be? (this is not the same as the wear spec that means its time for a whole new flywheel) There is no spec on making this resurfacing decision in the spec book, just a wear limit. From my 3.0 spec book:

The flywheel bearing surface on the drive plate can be machined on a lathe, if there is serious scoring or considerable burnt spots.

Keep lathe cut as small as possible.

Wear limit of flywheel thickness:

911 SC 8.5 mm
911 Turbo 9.9 mm

Mike (TerribleHondo) was helping me put the crank together on Sat. and he asked me if I was going to resurface the flywheel... I said, 'No, I never have either.' His experience lies with domestics and apparently he resurfaced every time the tranny was separated from engine... I have done that 4-5 times and never thought about it...

He pointed to the micro cracks:



Cracks are evenly spread around the flywheel and radiate radially. None are very deep that I can determine, but I'm now questioning the re-use of this piece. There are also slight grooves radially that I could feel with my finger, but not really measure...

Should I just media blast it and slap it back on? clutch was replaced at 78k and I have about 90k on it with 5 track days.

Best regards,

Michael

[304065]

I just now saw that you replied below. I can merge if you want.

I think you're right, I think the triangles denote a surface roughness spec.

If you think about what the flywheel surface does, it provides a surface for the clutch disc to mate. Accordingly, it needs to be very flat, not warped from heat, and free of surface imperfections.

Given both those thoughts I would consider looking for a new flywheel. I don't think cracks are just a surface phenomenon, like blue heat distortion or scratches would be-- they indicate that the metal has changed dimensions enough for the cracks to propogate.

Flywheel Specifications

[911pcars]

Excessive localized heat can cause the friction surface to work harden, and those areas can be harder than the tip of a carbide tool bit. The result is that the tool bit skips over the work-hardened surfaces = uneven surface.

I would recommend finding a machine shop that uses a grinding wheel to refinish the flywheel; same with brake rotors and brake drums.

Sherwood

[euro911sc]

John:

Yeah I didn't get much response on that other thread so I started one of my own By the responses I think I found a collective hole in the knowledge base.

Sherwood:

Good point on work hardening. I'll have to look around for a surface grinder that works on rotating surfaces.

I may just simply go for a new lightened fly, but Bruce Anderson recommends against it in his book... The motor spins up fine (after 18 months of not driving it I think I remember it )

We'll see what it costs to resurface it with a grinder. any guesses?

-M

[Steve@Rennsport]

You should look for someone with a Blanchard Grinder, preferably with a magnetic table that makes it easier.

I never resurface flywheels, instead preferring to replace them as I cannot afford any comebacks,...

With all due respect to Bruce A. (who I've known for a long time), light flywheels work very well as I've not only installed hundreds of these but have had one on my own car for 27 years.

[JP911]

Steve-
Do brand new flywheels still need to be checked, and possibly lightly machined, before being installed?

[911pcars]

If you're the type that requires engine parts be balanced, you'll probably want to perform the same with a new flywheel, just to make sure. It has the largest diameter of anything that rotates with the engine, and any imperfection will result in vibration and decreased engine life.

Sherwood

[JP911]

Great explanation, Sherwood. I have a receipt for flywheel balancing from my engine builder (whom I trust implicitly), but was wondering why he would do that on a brand new part.

[911 tweaks]

any ballpark $$ #'s for balancing a flywheel??

Also, what stk and lightened flywheel brands/models do you recommend, Steve, for a stock/slightly mod SC engine??
Thanks! Bob

[Steve@Rennsport]

Jon:

Brand new OEM (not Chinese copies) are pretty good and do not need to be resurfaced,...



Bob:

I use aluminum ones for performance applications.

[TERRIBLEHONDO]

Steve why not resurface instead of buying a new one? I have never had an issue on domestic cars. If we had to turn it to much then we would replaced them. Is there something different with resurfacing a Porsche flywheel? Not enough meat on the flywheel to turn it and have success?

[Steve@Rennsport]

Originally posted by TERRIBLEHONDO
Not enough meat on the flywheel to turn it and have success?

Yep,...thats the problem and leads to clutch slippage when the disc wears a bit.

I simply deplore having to do something over again due to trying to pinch pennies,...

[euro911sc]

hehehe well... I guess I have a new crank stand It's a little too big for a desk paper weight

Will I have to go with the stock fly replacement? from PCA Stock Class Rules:

I. Substitution of any other flywheel for a stock dual mass flywheel will count as a “prepared”
change and the vehicle will progress up one stock class if it has not already done so due to other
“prepared” modifications.

Though, I don't have a dual mass fly... so does this rule count?

Best regards,

Michael

[cgarr]

Isn’t that groove in the flywheel the wear limit indicator? If you grind the face and shoulder the clutch should have the same clearance and same pressure.

[Walt Fricke]

A problem I hear of when having 911 flywheel reground is that sometimes the grinder doesn't take the grind/cut all the way out to the edge. If a little lip is left out there, it will leave the pressure plate farther away from the flywheel surface than it should be (by the amount of the cut that left the lip).

I don't quite know what to make of surface heat check cracks. Brake rotors develop them also. They can't be good. If it were on, say, a crank shaft you'd not give two extra seconds thought before junking it. But I've not had a rotor fail as a result of these - excessive wear or runout seems to come first. And I don't think any of my flywheels have developed any as deep as the one you showcase. Plus I'd be concerned about the overheated areas. Are there any blue spots on the back of the flywheel?

Steve's customers would blame him if a flywheel had to come back out prematurely, and would conveniently forget they had pressured him into saving modest sums against his advice. How you pinch your own pennies when you do your own work has a different dynamic, at least with me. I'll cut corners if I figure the worst that is apt to happen is that I'll have to pull things apart in another couple of months as long as nothing catastrophic has intervened.

That being confessed, having a flywheel as a crank stand is a very good thing. Nice use for all those old bolts, too. If that crack (or any other, though I suppose you showed the worst one) won't grind out before you reach the wear limit, with the work done by someone who fairly regularly grinds flywheels of this sort, I'd get another flywheel. In fact, I think I would do that anyway.

Walt Fricke

[euro911sc]

Walt,

No blueing at all and I'm not thinking there was a lot of heat in this thing... I'm pretty smooth on clutches so I would be surprised if there were. I'll be getting a new fly... just a matter of determining if I can go light or have to stay stock.

-M

[Walt Fricke]

M

Despite your flawless style, the dark areas on the disk surface in the photo look like they were caused by heat . Plus if I am correct (which is hardly a certainty where engineering matters are concerned) that those cracks were caused by heat, there is only one likely source of enough heat to do that.

If you lived in San Francisco, I'd say it was from slipping the clutch on hills, but you live in a flatter place.

Doubtless due to the previous owner.

Walt

[euro911sc]

*laf* I'm flawless Walt Driving is like golf... if you hit a bad ball it must be the club... if not the club, the ball, if not the ball, your partner... but, never oneself! Car was in western Virginia (not WV) prior to its life with me. I know its has seen a very hard life: missing material in the tranny where 2nd blew up, lots of RTV where it should not be in the engine, etc... I've auto-Xed it and tracked it so this may simply be a product of the driving style.

Correct that the cracks have to be from heat... what else could they be from? I'm just curious if this kind of wear is normal? Something the engineers saw during their original testing and could not get around? (unlikely, but...) They look like they are very shallow and I'm actually not sure they would make a material difference in strength... I'm not willing to find out, but more curious.

Here, we see visual evidence that the part needs to be replaced... BUT, if no cracks/blueing/etc., what tells me I need to resurface/replace?


Best regards

Michael, you local smooth operator

[KTL]

That's heat alright. We we just last night compariing brake rotors on a race car vs. my car and same type of cracks were the discussion.

I think the key to deciding whether or not to junk the flywheel if you choose not to resurface but are still in spec is the surface profile of the flywheel. Your wheel may be fairly smooth despite the heat evidence? Sometimes flywheels will have significant grooves or valleys from the clutch disc material abrasively rubbing the flywheel. I'm sure TERRIBLEHONDO could show you one of these some time?

With heat evidence like that I would not resuface, no matter what the surface condition of the wheel. I'm no expert but i've seen a dozen or so flywheels in my days of wrenching on cars and even the most abused wheel didn't have heat cracking like that. But i've only done street cars, so track cars are more likely to see this kind of high heat abuse from someone who slips the clutch MUCH too much.

[911pcars]

"...But i've only done street cars, so track cars are more likely to see this kind of high heat abuse from someone who slips the clutch MUCH too much."

Slipping clutch. Perhaps exacerbated by not double-clutching.

Sherwood