I have a mivec motor with about 70k on it but all timing components are OEM parts with 30k miles and my balance shaft belt broke at the top of first and luckily it looks like it only took out the crank sensor. I am going to dig in tonight to find out true extent of damage and fix it but i want to know what can cause such a premature failure?

I am on the fence about doing a balance shaft delete, and most of my decision would be driven by the ability to prevent a future failure.

i did some searching and didnt find much other than a few similar occurrences to mine but no good info on why it happened or how to prevent it.
thanks

I found my balance shaft belt sloppy loose 40K after a belt job I did so I know the belt was installed reasonably tight. My advice is to retension the belt 30K after installation. How or if a loose belt leads to breakage is something an engineer would have to address.

Does it still have cam gears cover? Any chance debris could get in there?

Balance shaft could have locked up, belt got too loose due to age/stretch, or it just plain broke do to age..

i'd say the most likely cause is improper balance shaft belt tensioner installation. if you put the ridge on the tension towards the block there is nothing stopping the b-belt from moving off center and into the crank position trigger disk. it will eventually chop up the belt or loosen the tensioner causing breakage.

I just left the belt off. Potential problem solved.
I was going to do the delete, but didn't want to deal with oil pressure issues, etc.
I do have a stubby on the oil pump, though.

i got it all apart and the crank sensor was destroyed, but the balance shaft still spins fine.

i guess the answer is new belts and cross my fingers?

Leave the belt off or risk another failure.

My old gvr4 was like that. I think that might be what i do, i just cant see ever trusting the car on a road trip with that belt in there.

what direction is the b-belt tensioner installed? belt alignment ridge on engine side or alignment ridge away from engine?

away from engine.