Flywheel bolts for lightweight aluminum flywheel

jwasbury · 06-27-2015, 12:46 PM

I'm in the process of reassembling a 3.0 liter SC race engine. The flywheel is an aluminum lightweight piece and I notice that the OE flywheel bolts that I purchased to replace the old ones are about 5mm shorter. It seems a non-OE bolt was used to compensate for the flywheel being thicker to achieve proper thread engagement with the crankshaft.

Have done some searching and found some discussion on the topic, including some info suggesting that the early 6 bolt cranks had 5mm longer bolts. However, I am not positive that the thread pitch is the same as the later 9 bolt cranks.

What should I be using for flywheel bolts in this case? Any help is appreciated.

Jon B · 06-27-2015, 09:51 PM

Quote:

Originally Posted by jwasbury

...including some info suggesting that the early 6 bolt cranks had 5mm longer bolts. However, I am not positive that the thread pitch is the same as the later 9 bolt cranks.

The earlier flywheel bolts were larger, M12 versus M10, so will not work with your crankshaft.

jwasbury · 06-28-2015, 04:51 AM

It took some time to find, but after several search sessions I found this thread:Flywheel Bolts

This part number is the one I need: 928-102-151-01-M260

Dave Kost · Dave Kost's Garage

If this was my application....Make sure you use a thru hardened flat washer under the sockethead capscrew head. Also, anti seize on the threads in order to have a controlled consistent toque. Use lubricated 10 mm shcs torque specification.

Just curious, what has been the failure mode of the aluminum flywheel?

jwasbury · 06-28-2015, 05:41 AM

Who said the aluminum flywheel failed? No issues for me with this flywheel.

This race engine is apart for fresh bearings, new cams, a bump in compression ratio, and cylinder head work. I just wanted to use fresh flywheel bolts. There is a steel ring (w/ 9 holes) that functions as washer/spacer presumably to distribute the load of the bolts onto the surface of the flywheel and prevent squashing the soft aluminum.

Henry Schmidt · Henry Schmidt's Garage

I've seen my fair share of aluminum flywheel failures. It doesn't seem to matter which brand. The issue is always the engineering behind the flywheel and the application. Generally if you must run an aluminum flywheel (not sure why) they seem to last better with a push style clutch as apposed to a pull style. Since no 911 engine with a 9 bolt flywheel came with a push style clutch, installing an aluminum flywheel for high performance applications becomes a challenge.

Walt Fricke · 07-01-2015, 02:38 PM

However, the 5.5" clutches are all, as far as I have seen, push type clutches. So they would work with aluminum flywheels (with steel wear surface inserts) as well as any. Of course, you have to change your linkage to have stepping on the clutch pedal end up with pushing on the TOB. Both cable and arm and hydraulic TOB are possible.

chris_seven · 07-01-2015, 11:44 PM

Basic Bursting Disk Calculations to allow for the safe design of a flywheel can be found in the 7th Edition of Roark’s Formulas for Stress and Strain on page 751/752. I would want to see around 2:1 bursting speed to running speed and this should cope with fatigue.

It should be quite straightforward to design 'safe' flywheels using alloys such as 7075-T6 which are much more homogenous and have better fatigue properties than many traditional flywheel materials.

Aftermarket steel flywheels will be more highly stressed than an aluminium component due to the greater inertia so a reasonably high strength steel would be needed to give a satisfactory result.