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Concepts: What damage exactly happens when a timing belt snaps?

We all know the party line: Timing belt snaps = major problems!

Yet, few go beyond this. What exactly happens?

I have heard that valves get bent.

Hmm, that makes sense:

  • Belt breaks
  • Cam stops
  • Valves left in position (maybe open position)
  • Piston still moves, comes back and hits the valve? (Where cam would have closed it)

Is this basically what happens?

yup .

Yup that’s it… You can also crack pistons, valves can break off and fly around the cylinders and mess the walls up badly. Mostly it’s the valve damage that costs the $$$$

This is usually only on interference engines. On some engines (non-interference designs) there is enough clearance between open valves & pistons so that they don’t crash into each other.

Even on non-interference engines there can be valve damage. A DOHC engine uses one cam for the intake valves and one for the exhaust valves. While the valves may not interfere with cylinder travel, the might interfere with each other once the valve timing is lost. That is, the exhaust valves strike the intake valves and damage each other.

It depends on the engine. Not all engines are designed equally. Some will have problems…some don’t.

The same problem can happen with a timing chain…but at much longer intervals…and timing chains usually start making noise before they get stretched enough to slip a tooth.

For those that DO have the problem, you’ve described it well. See the attached diagrams for illustration.
http://www.bing.com/images/search?q=interference+engine+diagram&qpvt=interference+engine+diagram&FORM=IGRE

I’d also argue for “damage” to your wallet, too, for the repairs.

A relative of mine has a 1998 Volvo that the timing belt recently broke on. She chose to have the engie repaired, and during the course of the repairs, the bill went from $1000 to around $3200 in the end. This on a car that’s maybe worth $500.

I would have taken the car to the junkyard, but folks have to live with their decisions.

And there is more than one way a timing belt can fail. Breakage is one way. Another way is, one or more teeth get sheared off. The belt circumference stays intact, but because of the lost teeth, the camshaft(s) stop turning properly, or get out of sync.

This is why it can be difficult to predict impending belt failure by inspection, because cracks at the base of the teeth can’t be seen unless you take the belt off and turn it inside out.