I have a 2007 Mazda 3 2.3L with about 155k km on it.
I was attempting to replace the AC pulley bearing, and had to manually turn the crank pulley to get the belt back on the AC pulley. After making sure the AC pulley was tightened, I started the engine to see if the AC worked. When starting the car, I learn a bang/clank sound, and went back to see that I forgot to remove the torque wrench I had on the crank pulley. The starter would no longer start the engine, and it seemed like it was spinning faster than normal.
I was told that most likely the wrench caught when I started the car and stopped the crank pulley from turning, and subsequently caused the timing chain to jump, which is why it wouldn't start anymore. Also, I turned the crank pulley a couple more times and then it would no longer turn. I was told that means that the chain likely bent a piston when it jumped, which is a very costly repair.
Just wanted to see if this all makes sense, and if it's worth taking the engine apart to fix, or if I'm better off getting a used engine with equivalent mileage to swap in.
Thanks in advance.
Update: As per Howie's suggestion I took the valve cover off to check the timing chain, and it looks like it's still on. And now the crank pulley is turning again. I turned it for one full revolution of the chain, so I guess it's not a bent piston/rod etc. I'm not sure what could have caused it to get stuck before, but I guess it wasn't the chain. As I was turning it the chain got some slack on it at one point but then got tight again. I'm guessing that doesn't make a difference. Now I'm kind of confused as to what the cause of it not starting is, but I'm guessing since it's turning, the piston/valve isn't bent so it looks like I don't have to replace the engine just yet...
Update 2: As per Ben's suggestion, I got it at TDC (sticking a wooden dowel down the shaft, turning the crank pulley and stopping when it's at peak with the lobes pointing up) . The picture of the chain at TDC is below. There is some slack on it at that point but I'm guessing that's not an issue. There's also a close-up of what I believe is the balancer, and I can only see 19 teeth off the chain.
However, what I noticed when taking the spark plug out to try to get it at TDC was that one of the spark plugs was mangled. Seems like it was hit by the piston. Would that cause it to not start, or for the piston to be bent? At this point I'm thinking I should just replace the spark plug and give it another shot.
Update 3: I replaced the spark plug but am hesitant to start it just yet lest I also break the new spark plug. I shined a light down the shaft where I pulled the spark plug, and it looked like the piston had hit the spark plug from underneath, or something got in between the two and damaged both. There was a small gouge carved into the top, as shown below. I'm not sure if that helps, but it's just another clue to solve this mystery.