[Walt Fricke]

The 9 bolt 70.4 cranks will, indeed, shed the flywheel if reved about about 7,500 rpm without special measures being taken. Porsche had this problem with the 2.8 RSR, which used this crank (although it had a narrower bearing to allow a larger fillet radius).

The books say this was due to the wrong kind of 4th harmonic node (destructive?) unfortunately ending up right at the crank/flywheel mating surface. Caused the 6 bolts to loosen and back out. Happened to me before I had read a bit more than I have now. Per Frere, Porsche never solved this satisfactorily for that engine. I know they solved it pragmatically be using new bolts for every race. A friend knew of a team which tack welded the bolt heads in place, and they still broke loose!

US hot rodders solved it by using red Loctite and torquing to 150 pounds foot. That has worked for me for years, ever since Bruce Anderson told me to do it.

Would the fact that the bolts loosen suggest lateral vibration, overcoming the preload that the stock 110 pounds/ft produces? Me, I I couldn't tell a 4th order harmonic from a nunnery. But some of you guys know all about these effects and their causes.

The 917 takes its power off in the middle of the crank from a gear. Writers on the subject say that the harmonic node there is of the benign kind. I don't know if this was a happy accident or clever design, or some combination of both.

The 9 bolt 3.0 70.4 cranks don't seem to have this problem at all, at least not that one hears about as often as you hear about in its 6 bolt parent. Plenty of 3.0 race motors out there spinning over 8,000 rpm. Similarly, the 66mm cranks don't have this problem either, which was perhaps why it caught them out with the 2.8?

Next time I build a race motor I will send a 66mm crank with a damaged rod bearing to the grinders to have the journals turned down so I can use Clevite or the like. Will need new rods, too, of course. Guys like Bob have been praising this approach for a long time, and with the bad reputation Glyco has developed, spending this extra money is beginning to look like more of a necessity than a luxury.