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...

head removed

timing chain

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.

enter image description here enter image description here

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.

enter image description here

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.

enter image description here

  • Since you can crank the engine over and have the covers off are your seeing the chain and valve train moving? If you are seeing the chain and vale train move pull the plugs and check compression this will let you know if you have a damaged piston or valve. If you do not see it move then as stated in answers look at starter teeth Aug 30, 2016 at 19:20
  • 1
    There's no way a piston could hit a spark plug unless there's something seriously wrong. Either the connecting rod failed and allowed the piston to travel to the top of the cylinder, or you've got some kind of foreign object (like a valve head) inside the cylinder. Either way, I don't think I'd try starting the engine without pulling the head first and finding out what hit that plug.
    – TMN
    Aug 31, 2016 at 15:58
  • @TMN I agree that piston and very likely the valve is trashed. If it is the only one. A leak down tester would tell if hole in piston or valve was snapped. Head teardown is needed to see if lower engine block and the heads are still able to be saved. Then cost of parts verses a rebuilt long block is a decision to make. Sep 1, 2016 at 0:02
  • I added a photo looking down the spark plug shaft, which shows that the top of the piston looks damaged. I don't see anything else in there, but perhaps it's hiding over on the side, which I can't see down that shaft.
    – Pedram
    Sep 1, 2016 at 0:30

8 Answers 8


Before jumping to conclusions, you could take the rocker cover off and see whether the chain is still connected. The fact that you were able to turn the engine over for a while quickly without it jamming may suggest that the chain snapped. The valve gear may be in a safe position and it could possibly just be the chain that is now jammed.

  • 2
    That would be my first check as well. If the chain broke the actual damage might be minimal. Depends on how expensive a new chain is, but it could be worth it to just replace the chain, re-align and see how the engine runs. Doing an (2nd hand) engine replacement will still be available as a last resort. (For a 155.000 km engine a rebuild just isn't worth it.)
    – Tonny
    Aug 29, 2016 at 12:28
  • I checked the chain (see my edit below). Looks like it's still OK. Thanks for the suggestion.
    – Pedram
    Aug 30, 2016 at 2:05
  • You should be able to check the timing marks are lined up now you have the rocker cover off (not the head).
    – HandyHowie
    Aug 30, 2016 at 4:49
  • A better name is a cam cover for your engine. I am used to playing with classic cars.
    – HandyHowie
    Aug 30, 2016 at 7:13
  • 1
    The only thing I can think of that damaged the plug is if the timing shifted and a valve head snapped off after being hit by a piston. The valve head must then have hit the plug. Can you see down the plug hole to see if there is anything floating around in there?
    – HandyHowie
    Aug 31, 2016 at 4:47

IIRC on the 2.3 there is no keyway to keep the crank gear and balancer aligned. If the engine is spinning and has compression you probably spun the balancer and it's out of time.

Set piston 1 at TDC and line the balancer up as indicated.

enter image description here

I think this is an M6x1.0 bolt.

This should be 20 teeth off the blank space on the balancer.

Here's an example using a scope, this came in after a head gasket replacement and you can see the balancer is slightly off.

Yellow: Cylinder 1 Compression
Blue: Crank Position Sensor
Green: Cam Position Sensor

enter image description here

To find TDC without special tools

Pull all the spark plugs and remove the cylinder head cover if it hasn't already been removed. Put a long screw driver or similar in the #1 spark plug hole and rotate the crank clockwise until the screw driver is at it's peak.

On the rear of the camshafts there will be slots that should line up with the cylinder head.

enter image description here

If you want to make sure it's dead on use these tools.

enter image description here


The pin goes in a slot on the left side of the engine and locks the crank shaft at TDC.

Note that on these engines the crank bolt holds the balancer with a diamond washer and needs to be replaced if removed.

Either way compression check the engine.

  • I'm not sure how to set piston 1 at top dead centre. Does something like this make sense for my car? clarks-garage.com/shop-manual/eng-13.htm
    – Pedram
    Aug 30, 2016 at 22:09
  • @Pedram #7 on that link is how you're going to have to do it I'll update the answer and post the scope capture showing how the balancer should bei n relation to the cam crank timing in a bit.
    – Ben
    Aug 30, 2016 at 22:17
  • I added a picture of the chain at what should be TDC, using the method you suggested from the link. However, I also found that one of the spark plugs was mangled. Perhaps that is the cause?
    – Pedram
    Aug 31, 2016 at 0:47
  • @Pedram get a picture of the back sides of the cams where the tool would go to lock them in place. if both sides aren't in line with the cylinder head try rotating the crank another 360 if they still dont line up you jumped cam timing. the plug being smashed like that isn't good. shine a light down the plug hole and see if the piston is damaged.
    – Ben
    Aug 31, 2016 at 0:51
  • I'll have to get the picture from behind a little later, but I shined a light down the shaft before I replaced the spark plug and it looks like the piston is scratched on the top, and it somehow either made contact with the spark plug, or something got in between the two and damaged both.
    – Pedram
    Aug 31, 2016 at 21:42

I don't see how the timing chain could have jumped or broken in this situation. If the wrench was on the crank pully, and this is what stopped the engine, there was little to no force on the timing chain because there's little resistance on the cams. I'm guessing you probably damaged your starter gear or broke some teeth off of the flywheel, so the faster spinning sound you're hearing might just be the starter spinning in place and not actually cranking the engine. First, make sure the engine is actually turning with the starter and go from there.

  • How can I check if the engine is spinning with the starter? I added an edit with a picture of the chain that I checked, which showed that it's still OK, so I'm wondering if it's the starter gear or flywheel as you mentioned.
    – Pedram
    Aug 30, 2016 at 4:19
  • Just have a friend look at the engine while you try to start it. The belts and and pullies should be turning while you crank the starter. Aug 30, 2016 at 13:05
  • @695Multimedia If you stop any moving part suddenly, there will always be a chance that it will break. If there was a weakness in the timing chain, there is a chance that the chain could have snapped.
    – HandyHowie
    Aug 30, 2016 at 13:24
  • Timing chains are very substantial items. Look at his photo. I doubt the sudden stop caused by the wrench caused much more load on the chain than the typical starting force of the engine. The whole point of helping the OP is to save him time and money. Starters are relatively easy to replace compared to all the other suggestions. The starter is also the first possible mechanical point of failure in the chain of events the user described. Especially on a 2007 car with 155k km on it. Aug 30, 2016 at 13:38
  • Looked at the engine while another person turned the key and the belt on the left (while looking at it from my angle with the hood up) was spinning, so I guess that means the starter is still OK.
    – Pedram
    Aug 30, 2016 at 22:08

The good news is that everything happened at low speed, because you never actually got the engine running. So whatever the damage is, it will probably be fairly limited.

The bad news is that with this sort of problem, you don't know what you are going to find until you take the head off at a minimum, and just doing that isn't cheap even if there you find there is no collateral damage. If the valves are obviously damaged you really need to check the pistons and bearings as well, and that's even more expense on taking things apart.

IMO taking a chance on just replacing the chain, retiming the engine, and performing your local customs to ensure good luck is not a good idea. If you have bent a valve stem or cracked a piston crown, you want to find out when the car is in a workshop, not when the engine spins up to 2000 RPM just after you started it, and most definitely not while you are driving!

If you can find a local car breaker with an old replacement engine that doesn't look obviously past its sell-by date, that would probably be your cheapest option.

  • 1
    Taking off the head should not be necessary to do the inspection for damage. You can get fairly inexpensive cameras that can be lowered through the spark plug sockets to inspect inside the combustion chamber. Between that and taking off the valve cover, I think you can reasonably assess the damage. Aug 29, 2016 at 16:48

If the chain jumped or broke while turning the engine with the starter, what could happen (if you are unlucky) is that a piston hits and bends some valve. The piston would probably survive. If so, it would not be quite as expensive to fix as you have been told.


If the engine is turning now I think there's a fairly good chance you avoided any serious damage. See my question and self-answer from when something similar happened to me: https://mechanics.stackexchange.com/a/4470/426

Try realigning the timing, using the procedure I did if possible: turn the engine until all pistons are centered or at least not at the top of their cylinders, then adjust the crankshaft to match, so that you don't have to worry about getting into an intermediate configuration where the pistons hit the valves. If you manage to get the car to start, it's probably okay, but it wouldn't hurt to take it to a professional to have them check it out. When I did this the timing was still off by a notch or two and the mechanic diagnosed and adjusted it for a very reasonable price: Low power, bad gas mileage, smelly exhaust after engine work


You guys really have no idea what yous are talking about get the head off and you will find the head of a valve gouged into the cylinder head, which is the only cause of the spark plug damage

  • Welcome to Motor Vehicle Maintenance & Repair! Considering this was already born out in the comments of Handy Howe's answer, I'd suggest someone on here knows what they are talking about. Apr 1, 2018 at 19:42

If the damage is what you think, most likely the repairs would cost more than a replacement engine. You might need some machining done in addition of replacement internals.

So I would go for a full replacement.

  • 1
    If the engine is actually damaged, this might or might not be true, but making that determination is a big part of the question. Just assuming it's ruined is not a good answer unless even trying to determine whether it is would cost more than a new engine, and I don't think that's the case here. Aug 29, 2016 at 14:43
  • Not exactly my intention to assume the engine is damaged, mainly stating that if the problem is as described, replacement makes more sense. Edited my answer to clarify what I meant.
    – Elias
    Aug 29, 2016 at 15:44
  • With what is showing now looks like it is time to make that cost comparison. Sep 1, 2016 at 0:04
  • Seems that giving direct answers to questions gives downvotes so I certainly can't be bothered to say anything further.
    – Elias
    Sep 1, 2016 at 7:36

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