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Bdc / Tdc

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NJRC

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well I am gonna be a noob at this question since I can't figure a way to actually figure it out.
"how do you know when your at BDC or TDC?"
I know it's better to be at BDC when the engine shuts down, but what are you actually looking at when setting it to BDC?
Do you all take your glow plugs off just to peak in there and see if the piston is at the bottom or are you just assuming that when you spin the crank, it's at BDC?
Do you all have see through motors to actually see where the piston is set at, or are you taking the hot glow plugs out and actually looking, or is this just an assumption to where the piston is at?
 
What i did was take finger nail polish and put a line on the flywheel when the piston was at BDC and a diff color line for when it was at TDC so now i don't have any guess work of where the piston is at. And I don't believe there is such a thing as a see through motor.
 
yeah I know there is no such thing as a see through motor unless its just for display and non use, but I had to mention it since there is all talk about BDC and TDC when I couldn't even tell myself at where the piston lies unless I pulled the hot glow plug out which I won't do unless need be done.
But that nail polish tip seems like an idea for me to try.

Thanks
RJ
 
That nail polish trick is a good idea, i'll have to try that. Thanks.
 
Just grab hold of the flywheel and try to turn it. If it's easy to turn then the piston is somewhere near BDC because the sleeve is not pinching and/or the piston is not trying to compress the air under the head button.

If the flywheel is easy turn at any position then your sleeve/piston is shot and you probably won't do any more damage by leaving the engine to cool with the piston near TDC.
 
Yup... as long as the piston is not in the compression stroke, you are fine for the most part. You can tell because when there is a great deal of resistance and compression, you are near the top... when it is easy to move, that is because the exhaust and intake ports are wide open and there should be no resistance... All you do from there is see how much of a turn it takes from compression to compression when you rotate it either way, pick the middle... there's your BDC. The Nailpolish thing is a damn good idea too. It doesn't have to be perfect though... you just want it out of the top portion of the cylinder.
 

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