How much thread of a flywheel bolt should be engaged in the crank? I measured the depth of the hole in the crank face and its 15 mm deep. My flywheel bolts are 20mm long but only thread in 5mm into the crank when I bolt on the 15mm thick Clutchmasters flywheel. Weirdly, if I just run the bolts into the threaded holes without the flywheel they only go in 8mm, as if the hole tapers at that point.

Is 5mm of thread engagement sufficient to hold the flywheel to the crank, or do I need new bolts?

 

General rule of thumb is for proper engagement of a bolt the bolt should thread in to the threaded hole at least 1.5 times the diameter of bolt. If the bolt is an 8mm (0.315 of an inch) bolt then it should have 12mm (0.472 of an inch) of its threaded portion threaded into the hole.

Think the bolts are 9mm bolts if so then the bolt diameter is 0.354 inches and needs to thread in at least 13mm or 0.531 inches.

You want be sure you can thread the bolts in the hole deep enough that they will clearly clamp down and apply full holding pressure to the flywheel. A risk is with a longer bolt it bottoms in the hole. The torque wrench doesn't care and clicks but the bolts are not clamping (in this case) the flywheel to the crank with sufficient force. The bolts will fail and you will end up with a flywheel in your lap.