if timing belt is sereaded does it damage the engine

I'm going to assume you mean "shredded." Is it actually shredded, as in ripped into shreds, or is it stripped, as in some or all of the teeth are gone?

Either way, what kind of damage might have been done to the engine depends on what sort of engine it is.

In many engines, low compression ones, if valves are open and the engine continues to spin the piston tops will not hit the valves.

In other engines, high compression ones, if the valves are open when the piston comes up, the piston top will strike the valves. These are sometimes called "interference" engines because, if mistimed, the valves will "interfere" with the pistons. Damage to the valves, the piston tops, maybe the main shaft bearings, maybe the cam shaft, cam followers or valve guides. I've had timing belts strip (not shred) in three cars with high compression interference style engines. A 1977 Fiat X 1/9, where the strike did occur. Several valves damaged one piston punched through. The other two were 1987

16 valve DOHC VW Scirocco's. In both cases I lucked out in that the engine was at idle at the time, stopped near instantly, and the valves were in one of the two positions of the cam shafts where all sixteen valves are closed at the same time. After the belts were replace one ran for another 20,000 miles before I had to retire it for non-engine related problems. The other I'm still driving over 15,000 miles later.

Given that all you said was (I'm assuming) "shredded," and didn't specify what car or engine, it's hard to be sure why they assume it's necessary to replace the engine. I'm going to guess that it's an interference engine and you were running at speed when the belt went to pieces. They may have examined the valve stems and noted that some are stuck down even when the cam shaft is rotated or may have sounded the engine or other approaches to assessing the damage.

- Bill

I'm going to assume you mean "shredded." Is it actually shredded, as in ripped into shreds, or is it stripped, as in some or all of the teeth are gone?

Either way, what kind of damage might have been done to the engine depends on what sort of engine it is.

In many engines, low compression ones, if valves are open and the engine continues to spin the piston tops will not hit the valves.

In other engines, high compression ones, if the valves are open when the piston comes up, the piston top will strike the valves. These are sometimes called "interference" engines because, if mistimed, the valves will "interfere" with the pistons. Damage to the valves, the piston tops, maybe the main shaft bearings, maybe the cam shaft, cam followers or valve guides. I've had timing belts strip (not shred) in three cars with high compression interference style engines. A 1977 Fiat X 1/9, where the strike did occur. Several valves damaged one piston punched through. The other two were 1987

16 valve DOHC VW Scirocco's. In both cases I lucked out in that the engine was at idle at the time, stopped near instantly, and the valves were in one of the two positions of the cam shafts where all sixteen valves are closed at the same time. After the belts were replace one ran for another 20,000 miles before I had to retire it for non-engine related problems. The other I'm still driving over 15,000 miles later.

Given that all you said was (I'm assuming) "shredded," and didn't specify what car or engine, it's hard to be sure why they assume it's necessary to replace the engine. I'm going to guess that it's an interference engine and you were running at speed when the belt went to pieces. They may have examined the valve stems and noted that some are stuck down even when the cam shaft is rotated or may have sounded the engine or other approaches to assessing the damage.

- Bill

Most times a broken t-belt only requires a head rebuild. On occasion the pistons are severely damaged, but only they need replaced, not an engine. Good luck.

Yes. In the case of the X 1/9 I noted, it was a head rebuild and the one piston replaced.

I've also seen instances on non-interference engines were all that was needed was to replace the belt and retime the engine.

- Bill

One must consider the possibility that there is piston damage but the rest of the engine may also have problems too, hence the recommendation to replace it. The OP didn't really give any information such as engine or mileage or even engine condition prior to the failure but I suspect that if the timing belt failed as described that the engine was lacking maintenance and wasn't in good shape anyway.

One must consider the possibility that there is piston damage but the rest of the engine may also have problems too, hence the recommendation to replace it. The OP didn't really give any information such as engine or mileage or even engine condition prior to the failure but I suspect that if the timing belt failed as described that the engine was lacking maintenance and wasn't in good shape anyway.