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Is there an abnormal clunking noise coming from your engine? One of the causes might be a blown big end bearing.
The big end bearings are the bearings between your crankshaft and conrods (connecting rods). The gap between the bearings and the crankshaft is measured in the thousandths of a single millimeter - an amount that should not be able to be felt were you to try to move one against the other.
Big end bearings don't actually touch the crankshaft in normal operation - or at least they shouldn't. There's a very thin oil film between the two surfaces, and if that breaks down and there is metal to metal contact the bearings can be eroded or destroyed fairly quickly - leaving a much larger gap. This kind of gap can sometimes be detected by using nothing more than a socket and screwdriver and can generally be done with the engine still in the frame/chassis, though how much hassle it is depends a lot on whether it's a bike or car and whether you have a helper or not...
First, locate where you can turn the motor over by hand. On our example motor - a Honda CX500, for ease of demonstrating solo - there's a small cover on the front of the motor that can be removed to reveal a bolt on the end of the crankshaft:
Find an appropriately sized socket - here it's 17mm - and put a breaker bar on the end of it. You can use a ratchet wrench but it's a lot easier moving the engine forward and backward with a breaker bar than moving it only forward with a ratchet wrench...
Now locate your engine head and spark plug well. On a motorbike engine that's out of the frame in our example it's rather easy...
Remove the spark plug - you may wish to remove any other spark plugs at the same time to make the engine easier to turn over by hand (you're not fighting compression then):
Now get your long rod or screwdriver and insert it into the spark plug hole to rest gently on the crown of the piston beneath:
Now, rotate the motor by hand (not sure which direction? Check your manual or Google in case there's the potential for damage by doing it in reverse for your specific engine) and observe the screwdriver move up and down with the piston. On a CX500 we rotate clockwise. You want to turn the motor until the piston rises to the top (TDC, top dead center) and then begins to fall slightly down the far side, then push the screwdriver in abruptly while listening for a clunk noise. If you have to wind the motor backwards make sure you go back to before TDC then go forward again to bring it up to TDC. A video to demonstrate:
There should be zero movement when you push down. If there is an audible clunk noise and/or the screwdriver moves enough that you can feel it (and presumably isn't moving the piston if your motor is a tiny motorcycle one) then there's a good chance that your big end bearings are damaged. No movement at all doesn't necessarily rule out bearing issues, but this test does help confirm a suspicion.
Repeat the process for each cylinder that your engine has - fingers crossed they're all OK!
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