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why did the engine seize

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I have a 5 cylinder engine that my mechanic rebuild for me 4000 miles ago.

it run great. I took it back at 500 miles for a oil change.Still everything good.

3000 miles later I took it back for another oil change.

everything still good.1000 miles later I was driving on the highway and my temperature gauge started to climp.

I pulled over and got it towed to the garage.

The next day my mechanic said the engine was knocking.

the coolant was full in the radiator and the expansion tank.No signs of overheating. The oil level was full.

Today I went to the mechanic and he showed me the engine apart.

Connecting rod #2 #4 and #5 were dark like they got really hot and bearings were stuck on the crank.

My mechanic said is thermal break down of the oil.

My question is: Is it possible? and why?

Could be several contributing things, first, though, I would check the interweb for car clubs and/or engine forums that have the same engine as you do as it may be a known/common problem.

What were the other bearings like, mains as well as the big ends?

There are some things to bear in mind, like the type and viscosity of the oil, the ambient temperatures (it can get hot in that part of the world), how the vehicle was being used (driving at high speed and/or rpm?), was the engine using an oil to water heat exchanger or did it have a separate oil cooler and, if so, was it through a thermostat, for starters. Did you replace the oil filter when doing the oil change?

Discolouration is normally a sign of bearing(s) running hot and there can be several reasons for this. Were the clearances too tight, were the correct bearings being used; some engines have wide and narrow rod big ends and I know of instances where a wide bearing was used and while there was clearance, the edge of the bearing restricted oil flow through the bearing - this is very important as it is the oil passing through the bearing that is the cooling medium for them. If the bearing clearances were too big, there may not have been an adequate thickness of oil formed to support the loads from the con' rod. The other common reason for bearing failure is inadequate oil pressure - was that confirmed on initial build, if a guage was fitted to the vehicle did you notice any changes - some engine management systems, even some OEM, will log that if out of range?

To be blunt, it is primarily up to the engine builder to do the job correctly, including specifying whether an oil cooler was needed and the type and viscosity of the oil to be used and, if you had used the correct oil, it is going to be down to him to fix it.