Rebuilt engine it ran for about five minutes backfired and shut off

Yep in the oil pan, there are two ways to go either the rod is stuck somewhere up in the engine or it is laying in the oil pan which is fine and you can simply get another rod use a flashlight to see if you can see it and try to turn the engine over by hand to see if the rod is stuck somewhere. Let us know.

I agree, If you are lucky it got hung up in there. Unless you heard the thump when it hit the pan. If you did then you can drop the pan to get it or leave it in there and get another one. Unless you go 4 wheeling in this it should be okay to leave in there. Get a light and look down the hole and see if you can see it. If you can you may be able to get it out with a telescoping magnet. Its your option to drop the oil pan.

Rich

I would be wondering where a metal shaving came from in a new motor. Did this same guy do the motor too?

Rich

I was hoping you would say no. I am not saying this is your issue but it is a possibility. When you build a motor you need proper oil clearance on all the bearings. If the main bearing clearance is too tight on just one main bearing as the motor heats up everything expands from the heat. When a bearing is too tight the oil clearance becomes too small and the bearing over heats and starts dragging on the crank. If it is bad enough it can drag enough to stall the motor. The bad thing here is, it would take about five minutes of running for this to happen. That is the amount of time you have said it has run before shutting down. When the motor cools down the clearances open up again and it will start and run for another five minutes. The more this process happens the more bearing damage happens each time until eventually the bearing will melt down and weld its self to the crank, thus ruining the crank in the process. Eventually it will not start because there is so much load on the crank when turning over it cant get enough cranking speed to start.

Two questions.

Does it sound normal when cranking? Or does it sound slow and labored?

When it did run, did it run okay and at the end of the five minutes did the motor seem to lose RPM and just slowly shut off?

If you have a main bearing issue these are the only symptoms you will get other than a loss of power. They make no noise at all as they fail. They will also give you metal shavings in the oil pan as the bearing is torn apart.

Rich.