Aluminum Flywheel 6 Bolt

[crummasel]

Hi there,
i have got 2 Aluminum Flywheels for the older 6-bolt 70,4 crank.
One has expectedly cracks in the mounting area but that one has obviously been mounted without the 6-bolt-washer. I got it from an eBayer in the US reluctant to exchange or regulate; thats life and hopefully 6 weeks diarroea for him... ;-)
The other one is as new, seems to have been mounted once and it's immaculate.
Maybe Fidanza?
However: i am planning a well tuned 2.4 with full aluminum clutch system (frame and plate), clutch plate with rubber absorber for 2.2L pulled clutch system).
What do you think about usage of that flywheel?
Thanks,
Robert

[KTL]

6 weeks diarrhea.............. LOL that's hilarious! Best laugh i've had in days

I'd be reluctant to use the aluminum flywheel on the 6 bolt 70.4 crank. The cranks were of course suspect with the iron flywheels, so i'd be leary about the aluminum taking a beating from the harmonics.

One of the tricks people have done with the iron flywheels is increase the torque on the bolts (substantially) but I wouldn't recommend that on an Al flywheel because you'll smush the aluminum.

I have a Fidanza flywheel for my 3.2SS in my racecar (3.0L 9 bolt 70.4mm) crank and I was hesitant to use it. Its still sitting in my storage closet..... Not saying Fidanza flywheels are bad. Just not sure about them for hard use.

[Flieger]

I actually think that a flywheel with less inertia like an lightened steel unit or an aluminum one would help with the torsional vibration and fretting problem.

[crummasel]

Originally Posted by KTL

6 weeks diarrhea.............. LOL that's hilarious! Best laugh i've had in days

I'd be reluctant to use the aluminum flywheel on the 6 bolt 70.4 crank. The cranks were of course suspect with the iron flywheels, so i'd be leary about the aluminum taking a beating from the harmonics.

One of the tricks people have done with the iron flywheels is increase the torque on the bolts (substantially) but I wouldn't recommend that on an Al flywheel because you'll smush the aluminum.

I have a Fidanza flywheel for my 3.2SS in my racecar (3.0L 9 bolt 70.4mm) crank and I was hesitant to use it. Its still sitting in my storage closet..... Not saying Fidanza flywheels are bad. Just not sure about them for hard use.

Ok, i am happy to have at least generated a laugh...

But honestly: the cracked one has obviously been mounted without ANY shim, marks clearly visible.
And IF the flywheels are a risk: How to prevent a fail?
More than that, i am intending to use the full aluminum clutch (OEM/NOS) also lightweight frame and plate.

Anyone with experience?

Thanks,
Robert

[Steve@Rennsport]

FWIW,...I've used Tilton aluminum flywheels on 6-bolt cranks for 34+ years and some of these things revved beyond 8K.

It certainly changed the MOI of the rotating assembly and raised the RPM that shook the bolts loose, but never failed.

Still have one in my street car that has been in service quite successfully since 1979. Too bad they no longer make them. I cannot say how the Fidanza ones hold up over the long term, but I would certainly have any used one Zyglow'ed before use.

[crummasel]

@ Steve:
Maybe you can identify which make of flywheel it is:





http://imageshack.us/photo/my-images/844/img2012082600260.jpg/
And how did you mount (torque) the bolts?

Thanks,
Robert

[kwikt 911]

I've been using the Fidanza on my 6 bolt 2.4L 911 with no problems at all for the past 6 years. I torqued the 6 bolts using the factory specs with a drop of loctite on each bolt with the cover washer. My rpm limiter is set at 7100 rpm's and I consistently bounce off this while autoxing. Drivability wise..... there isn't that much difference than a stock flywheel.

[KTL]

Your flywheel is a Fidanza. Giveaway is the hand-engraved part numbers and the riveted steel insert. Looks similar to mine.

[crummasel]

Thank you all,
i will try it with specified torque and loctite.

[crummasel]

Hi all,
As all the longterm project is doing some steps ahead, i am picking up this thread again. Meanwhile the project has "grown" to a 2,7L mildly tuned carburetted street engine with some 9,5 comp. and mod solex cams.
Actual concern is the reverse perspective: assuming a proper mounting may withstand any shortcomings and simply holds (still in doubt) what about the 2.7 crank which in some threads is said to be prone if cracks due to torsional vibrations? Any comments on that point given a proper balancing of crank, flywheel and clutch?
Lightweight is still the topic so said flywheel plus AL clutch w/AL pressure plate?
Comment to that: It should be revving up quickly but unlikely that it will see more than 7000.
Thanks for any enlightened comments,
Robert

[PFM]

Crumm,

FYI ARP now makes a nice set of flywheel bolts for the 6 bolt flywheels. Still thinking of adding an AASCO to my 914 project.

[crummasel]

PFM, thanks for that hint but i believe the std bolts will do the job. More in doubt for me if the "meat" (that is the flywheel portion of aluminum in the 6 bolt mounting area) will do it.
But more than that: question for me is, if it is said that the 70,4mm 6 bolt crank tends to break when it is not "damped" by weighty rotational masses at high revs, can one say that with drastically reduced rotational masses one is still fine up to, say, 7500 rev/min assuming all the setup is well balanced?
Any comments on that point?

edit: kwick said he is using his engine up to 7100 with that flywheel..
also steve is positive on that point below 8000.. any more experiences?

[HawgRyder]

Robert...
Doesn't the flywheel have a steel .... Washer? (circle with 6 holes in it) that goes under the bolt heads.
I know my stock 69S had something like that.
That would spread the load of the bolts over a larger area so more torque could be applied.
Bob

[crummasel]

Yes, true, there is one to be applied and i assume it will help. The cracked flywheel i have obviously has been mounted without from a previous ownes. But again: Assuming the mountage does its job, i am now looking at the crank when used w/o significant inertia damping and where the rev limit to be set.

[racing97]

The 215 mm flywheels for 2.2 2.4 2.7 engines don't only break due to the Bolt Circle but also due to the heavy duty Pressure Plate assemblies pulling the flywheel towards the transmission.


Regards

[crummasel]

So besides of consideratons concerning the mounting area itself no negative on the crank itself regarding torsional vibrations on the crank below 8000 as steve says (kwick below 7100)?

[Walt Fricke]

The 70.4 mm 6 bolt crank developes a 4th order vibrational node right at the face where the flywheel bolts on. Porsche tried to use a harmonic balancer on the race 2.8s, but without notable success. I am told their approach to making the bolts stay tight (the harmonic, at high enough RPMs, causes the bolts to back out)was to replace them after every race! One guy told me he tack welded the bolt heads to the (steel) flywheel, and the tack broke.

The USA mechanic hot rod approach was to tighten to 150 lbs/ft. I've never had a bolt break - and I reuse them - in my 8,200 rpm motors, both 2.7 and 2.8. And very careful application of red Loctite, so none goes anywhere except on the threads - zero on the mating surfaces.

And I haven't had any cranks break. Porsche had this happen on the 2.8s, and had them made with larger radius fillets where the journals meet the counterweights, and had to shave the bearings a bit narrower.

So I'd not worry about cranks breaking due to an aluminum flywheel.

What about the torque? Well, other than remembering the words "4th order harmonic," I don't know enough about this stuff to speculate on what having a lighter flywheel/clutch assembly would have on that harmonic. If rotating mass (MOI)is part of it, and less raises the critical speed, then less mass would be good. And maybe less bolt torque would do the job. Because factory torque does just fine, from what one hears, for motors not spun above 7K.

I am dubious that even 150 lbs/ft of torque would cause the aluminum to crush enough to hurt anything. Yes, it is more malleable, but so it compresses a little more than steel would? What's that going to harm as long as it compresses evenly (as it should).

I am afraid I would have to think the guy who used an aluminum flywheel without the thick 6 hole "washer" piece was an idiot. I'm happy to leave off the washer-like plates from CV bolts - the boot sheet metal crushes, but so what - the bolts still hold and the CVs are too stout to be affected. But not here, where you know there have been problems, on a steel flywheel, and shear lunacy with aluminum, where you need to spread those loads across the entire annulus.

I do think I may know why that guy did that. You should measure the thickness of the alu flywheel at the bolt holes, and that of a stock steel flywheel. I bet the alu is thicker. I ran into this when mounting an aluminum flywheel (of unknown origin, obtained in a trade, but probably a Fidanza prototype or the like) on a 70.4 crank for use with a 5.5" clutch. Because the flywheel was thicker here, the bolts did not engage as deeply into the crank. It seems that INBUS doesn't make the reduced head inhex bolt in any longer length, so I had to make due with regular Allens. So their heads interfered with the clutch disks, and that's no good. There are two thicknesses of the 6 hole washer, but the thinner of the two didn't do the trick either. Cutting the bolt heads down meant I couldn't get the torque I needed with a hex tool. I switched to a set with one disk with a reduced hub thickness, and got things to work without interference, although other issues have kept me from running up track hours. I think I compromised on the torque, maybe 130 lbs/ft.

Which is a long winded way of saying deleting the washer would gain more thread engagement with the crank using the stock bolts, but that's not the way to go about it.

In short, if one of your alu flywheels is in good shape, it doesn't sound (especially with Steve Weiner's experience) like you will have a problem if you can get it mounted properly without clutch disk interference.

[crummasel]

Hi Walt,
thank you for your considerations, fortunately my kitchen door goes directly into my workshop so i just measured the mounting area thicknesses:
Stock 8mm
Fidanza 8.8mm
What is the application (and partnumber) of the thicker washer?

Thanks,
Robert

[Walt Fricke]

Robert
I don't buy these things, I accumulate them by breaking motors or something. I had four in my semi-organized parts bins, so I measured:

Thin = 3.045mm (probably 3mm nominal). Thick = 4mm on the dot.

I believe the thick ones were from early motors, like the 2.0 and 2.2, but don't know exactly. A guy could make a pretty good guess as to the change by spending time with parts books (available on line for free) to see when there was a change in numbers. But I'd guess you have the thin one already, so you can't use the difference to make up for the thicker flywheel. Maybe 0.8mm less thread engagement isn't enough to worry about?

[crummasel]

Honestly i am absolutely not worried about the thread engagement as i have several screws and among them also longer ones which engage good and im am loctiting them anyway. Re the washer: do you have one of the thicker ones you could send me?
Thanks, Robert