While there are a few cases discussed on various message boards about lightweight flywheel crank bolts backing out, the simple truth is this: lightweight flywheels do not cause the problem, there are always other factors that result in the issue.  This issue also occurs with OE flywheels.

Why loose crank bolts? What happened? Three simple causes that are the most common:

1) Failure to apply loc-tite during installation.  The crank bolts should be secured with red loc-tite (which we provide with all kits). Some techs skip this. Or, if there's a mess of oil on the crank bolt holes, the loc-tite won't stick. You can't glue something to oil.  The fact is that even the best BMW techs make mistakes, and sometimes it's convenient to blame the part rather than the work.

2) Removal of the crank pulley for performance. The crank pulley (not to be confused with accessory pulleys) is a tuned damper that keeps the crankshaft from getting all sorts of deformations from second-order harmonics. When this happens, to quote one of the BMW Motorsport race engineers we've consulted with, the crank shaft "will shake like a wet dog and throw off the flywheel, piston rod bolts, and bearings." Lessons learned from high-end racing. The M5x family of engines (which covers all M50, M52, M54, S50, S52, S54) develop a fairly violent harmonic around 7K rpm, +/- a few hundred depending on specific configuration... track use with the motor in this range for extended periods, are where this perfect storm happens and damage results.

3) Certain race cams exacerbate this harmonic issue. With everything else done right, living at around 7400rpm with those specific cams does very bad things. One manufacturer that was particularly bad about that has since revised their design to fix that problem, and has an otherwise stellar reputation (and to be clear, of all the major brands, all that I know of have corrected this situation and no new product from them has that issue). While we used to see many instances of the loose crank bolt problem due to these cams a few years ago, the number has significantly dropped since that cam manufacturer changed their design.

Despite the few voices on the internet who like to brand-bash instead of understanding the physics and mechanics behind products and failures, this product has proven to be a solid and reliable performer with less-than-average failure rates. With over 15 years of offering the M5 clutch conversion flywheel design, we have a ton of data reinforcing this information.

BOTTOM LINE: Issues with crank bolts coming loose, with close forensic analysis, always turn out to be caused by something other than the flywheel itself.  When installed correctly and with proper understanding of what other engine modifications should not be done, the UUC Stage 2 / M5 clutch conversion flywheel is every bit as reliable as a standard OE flywheel.