your piston at bottom dead center
11-23-2006, 03:04 AM
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your piston at bottom dead center
what does this mean? i see people saying to do this after a run. ive only rebuilt one motor before, and it was a .15. took it all apart, new sleeve etc so im kinda of a novice on motors. that was pretty fun doing the rebuild though, just wondering what this term means
i assume you need to remove the head and just move the flywheel so the piston is in the middle, not all the way down or up?
it seems kinda strange to do, and i never see people doing it at our local track. but if it prolongs engine life i will do it!
i assume you need to remove the head and just move the flywheel so the piston is in the middle, not all the way down or up?
it seems kinda strange to do, and i never see people doing it at our local track. but if it prolongs engine life i will do it!
11-23-2006, 08:30 AM
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RE: your piston at bottom dead center
It means that the piston is at the exact bottom of the stroke. Just as tdc means the piston is at the exact top of the stroke.
You want to store the engine at bdc to help preserve the "pinch" of the engine. Once these engines run a bit they heat up and parts expand. That's why you want the engine to get to 200 degrees when you do the break-in, so that the parts can expand to their operating sizes. Now, we know that compression is achiehed by the p/s pinch. Pinch being nothing more than a slight taper of the sleeve and piston so that when the piston approaches tdc it seals off the combustion chamber. Pinch serves the same function as rings in a larger motor. If you were to shut a hot engine off and the piston was at or near tdc then when the engine cools down (and the parts contract) it would be possable for the piston and sleeve to sieze or become stuck togeather. A bad thing.
To locate bdc you can take the head off and look but there are easier ways. I just remove the glow plug and drop a 3' - 4' piece of clean wire into the glow plug hole and rotate the engine slowly until the wire quits going down - bdc. Then I use a sharpie and mark the flywheel . Mow I always know where bdc is.
You want to store the engine at bdc to help preserve the "pinch" of the engine. Once these engines run a bit they heat up and parts expand. That's why you want the engine to get to 200 degrees when you do the break-in, so that the parts can expand to their operating sizes. Now, we know that compression is achiehed by the p/s pinch. Pinch being nothing more than a slight taper of the sleeve and piston so that when the piston approaches tdc it seals off the combustion chamber. Pinch serves the same function as rings in a larger motor. If you were to shut a hot engine off and the piston was at or near tdc then when the engine cools down (and the parts contract) it would be possable for the piston and sleeve to sieze or become stuck togeather. A bad thing.
To locate bdc you can take the head off and look but there are easier ways. I just remove the glow plug and drop a 3' - 4' piece of clean wire into the glow plug hole and rotate the engine slowly until the wire quits going down - bdc. Then I use a sharpie and mark the flywheel . Mow I always know where bdc is.
