Urgent technical note: Fiat/Lancia TC cast-iron Flywheels

Crank, rods, sealing, pistons, block, flywheel etc
Guy Croft
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Urgent technical note: Fiat/Lancia TC cast-iron Flywheels

Post by Guy Croft » April 1st, 2009, 9:28 am

With immediate effect a new regime is in place at GCRE (that you would be wise to copy) and that is the non-destructive (NDT) crack testing of OE flywheels in every case. Part of the reason for an abnormal incidence of cracks around the central region on items I have examined lately is age, the material is just 'weaker' than it used to be.

Primary reasons for cracking:

1. Severe imbalance of fw or assy
2. Embedding of fw bolts into fw and subsequent loss of preload
3. Incorrect torque setting (over/under) on fw bolts - easily upset by incorrect lubrication regime.
4. 'Swashing' of fw friction face. This can be balanced out - but it puts an fluctuating and very adverse axial load across the radius of the fw every time the clutch is engaged. It can be caused by a poor quality regrind of the item or bent crank/flange out of true.
5. Cracks inherent in fw - precision grind and crack test is advisable on each rebuild.

Cracks in the friction face are not uncommon and usually disappear after grinding.

PH 00.181 prep (3).jpg
A crack right round the central region. This FW is seconds from bursting and going right thru the bellhousing and anything else in its way, including you.
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These are heat cracks and usually don't go deep enough to cause a problem
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This crack has propagated from the bolt hole.
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PH 00.181 prep (20).jpg
this crank flange is seriously 'bowed' and the fw likewise. In such extreme cases it is hard to know whether the FW or the crank wasn't flat in the first place. Probably it was the back of the FW. This crank needs a crank grinding op to true it up.
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PH 00.181 prep (15).jpg
If you have a cracked FW it is essential to check the crank runout at the center main journal. If it's bent it can be pressed, but that in itself can induce cracks, better to junk it really.
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PH 00.181 prep (7).jpg
Crack testing the region where a crank will crack with a FW out of true/balance. Dye penetrant has been applied and allowed to soak in for 1/2hr, rinsed off, the crank dried and then developer spray applied. It takes some experience to discern a crack and distinguish it from residual penetrant stains.
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dowels fitted on this unit were way too thick, they should be 10.0000 precision-made items. So the engine builder smashed them in with hammer, peening the tops over. Naturally his problem then was the FW would not fit so he 'radiused' the tops of the dowels, opened out the holes in the FW (including the bolt holes because they wouldn't fit either) and the rammed the fw on with a mallet.
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PH 00.181 prep (12).jpg
To get the dowels (above) out, I had to drill and tap them and use a slide hammer. Dowels MUST be a neat sliding fit in a crank, never an intereference (tight) fit. Dowels are there to take the shear load off the bolts (entirely) and then all the bolts do is generate holdiung friction between the flange and the crank. So dowels MUST be top quality, like Unbrako 10mm stainless steel which are (can I say this?) EXACTLY 10mm dia. The holes in the crank frequently need reaming and extensive polishing before the dowels will even go in.
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UNBRAKO DOWEL - aval from GCRE. You must make sure the dowel does not stand proud of the FW outer face because tightening the bolts will damage the spring plate under the bolts (that stops the bolts embedding into the FW and that should always be used on a FW). It is better to use a shorter dowel and pack it up in the hole in the crank till it just fully penentrates the FW.
PH 00.181 prep (22).jpg (116.5 KiB) Viewed 11065 times

Guy Croft
Site Admin
Posts: 5037
Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK

Re: Urgent technical note: Fiat/Lancia TC cast-iron Flywheels

Post by Guy Croft » April 8th, 2009, 1:02 pm

I just want to add, given some of the comments flying around the net about this post. It seems that every time I publish something there is a smart-alec out there how knows better and has to say so without delay.

1) I don't do aero engines and I don't operate under a CAA regime so I don't have to certificate my NDT. Thus I don't feel compelled to charge £120 an hour and I don't have to keep copious records.
2) I have NDT'd more engine parts than most will in their lifetime (in F1) and figure I have a sporting chance of discerning what is and what is not a crack.
3) Lightening a cast iron flywheel in the right places (ie: well outboard of the centre bolting region) makes the thing safer, not more likely to fail. Because there is less inertia and less load on the holding region. Of course if your go too thin and leave sharp corners everywhere you are asking for trouble. FWIW I have never seen a cast-iron FW exhibit cracks in the friction face area (apart from small heat cracks), only in the central region.

I remark on the lightening aspect because I have run cast-iron fw at unbelieveable speeds and never had a GC prepped one fail in 20 years and don't want people put off lightening simply because I publish an article about cracks.

Lightened Fiat 2 liter FW_01.jpg
This is the same lightening style used on every GC FW ever. Why? Because every GC cast FW is lightened by the same guy who balances for me (since I first met him in 1988, a world authority on balancing, former Foreman of balance shop at Laystalls and it don't get better than that. This FW is nowhere less than12mm thick and has radii in all the right places.
Lightened Fiat 2 liter FW_01.jpg (457.67 KiB) Viewed 10756 times


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