i have 12.9 oem bolts but they are kinda short for the new flywheel i found 10.9's that are long enough. I know 10.9's aren't as strong,but are they strong enough ?

 

The factory engineers always design something and then they "overbuild" it until the cost cutter bean counters step in. The 12.9 oem bolts are actually bare minimum settled for after the bean counters got their hands on the vehicle. The 12.9's will do the job without excessive warranty recalls, anything less like the 10.9's would be asking for disaster. All of our engines torque goes right through those bolts - if they fail -there goes the transmission - and I can speak from experience - I had some flywheel bolts let go on a car traveling at 120+ miles an hour and the tranny dropped down punctured the fuel tank locked the rear wheels and I'm lucky to be alive today. The transmission did $4-5000 dollars damage to a bridge and the state said they might expect me to repay them for the damage - luckily they forgot.

 

x2. some bolts i never skimp on, and the flywheel, bellhousing bolts are some of them. head bolts/studs, and suspension stuff is another.