The vehicle in question is a 2006 mazda 3 with the 2.3L DOHC. A few months back, the engine blew on my sister in the middle of the highway. The connecting rod on the #4 cylinder broke, sending bits of metal through the block & oil pan. The piston, hit & bent the #4 valves, no damage to the head (but still have 150,000+ miles on it). So we bought a new short block, replaced the bent vales & ground all the vales until sealed. Once reassembled, it started surprisingly quickly, ran very smoothly for about 30 seconds at which point I saw smoke & shut it down in a panic. As it turns out, the exhaust manifold had been soaked in oil when the thing originally blew up & that’s where the smoke was coming from. Then we ran into intermittent starting issues, eventually resolved my cleaning a very corroded terminal on the starter. Once fixed, we tried to start again & it just cranked & cranked to no avail. Eventually, I pulled the spark plugs to find ¼-3/8 “ of fuel was sitting on top of every piston—it was badly flooded. So I soaked up all the standing fuel, I got a lot of it, but they were no dry, used my inspection camera to ensure no bits debris was left in there, blew out the spark plugs & reassembled. When I cranked it, I had a major backfire that blew all the way back to my air filter. I very quickly pulled the plugs & attempted to crank it around manually & I only had 180 degrees of motion before a hard stop on either side. This persisted until I disconnected the crank from the cams and rotated the cams a bit. Now I could rotate the cams & the crank 360 degrees, and it felt good. Tried putting it back together & starting—would not start & now I had 0 compression and massive leakage on a leak down test. 8/8 intake valves were badly bent & stuck open. The markings on the valves made it clear they had come into contact with the pistons (though left surprisingly little damage to the pistons). So I pulled the head again, ordered new intake valves & ground them in. While I had it out, I rechecked the exhaust valves & it turns out 6/8 exhaust valves also contacted the pistons – but just barely, the marks were so tinny we initially missed them. The interference was so minimal that only the #4 valves were bent (0.0140” runout), the rest show 0.0001-0.0004” of runout.



I am sure people will suggest my problem is with timing, but I assure you, that was not the problem—they make it pretty idiot proof & I cycled the engine many times by hand to verify this. Even after the backfire, the timing was right.



So my real question is, how do 14 / 16 valves come into contacted with the pistons of a non-interference engine?!?!



Fyi – I am a machinist, but engines are NOT my thing, this is only the 2nd one I have rebuilt & the first was a 1980’s 3 cylinder diesel for my tractor & that was a very different beast. So turn to those of you will some real experience to try to prevent this from happening again. Thanks for lending my your ears!

some photos of the problems here: https://photos.app.goo.gl/JgXDfg6mF2J5eVRt2

 

Here's the thing, according to my resources, the Mazda MZR 2.3 used in the Mazda3 is in fact an interference engine. Mazda has a few other (older) 2.3 liter designs which were not interference engines.

 

what are you resources? Mine say otherwise, though they seem rather unofficial. I know from the serial number that it is in the MZR family, specifically, the 2.3L L3 [2006]. Is that the engine your resources indicate is an destructive head?

 

Did you replace the lock washers on the crankshaft & camshafts?

Did you use the special service tools to lock the camshafts & crankshaft before disassembly?

And yes these engines are 100% interference.

 

My understanding is the only Mazda 4-cylinder engines in the 2.0 to 2.5 liter range which are NOT interference engines are the ones with belt driven cams; the MZR and later engines are all chain driven as well as interference engines.