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Discussion and questions related to the course How to Degree a Cam
minutes 4 :12 mmmmm
the dial indicator is not in zoro point
i think if we use the dial indicator to looking for true TDC better than top dead center locater finder threads into a spark plug
Hello, true tdc is not done with a dial gauge, it is done using a positive stop method. we make a "stop" to stop the piston before tdc then measure the degrees on a protractor write this down example 30 degrees ( this number could be any number in 360 degrees ) we then rotate the engine in the opposite direction until it hits the positive stop again and recored the degrees again example 60 degrees, we then half the total degrees between the two 30 deg 60 deg the difference is 30 degrees 30/2=15 so the 15 deg mark is your actual true TDC. so now you can adjust the protector to line up the tdc to the correct point.
Ali, they're used in a similar way. With a DTI one brings the piston up to the same DTI value, which can be whatever one wants, in the clockwise and counter-clockwise directions, so the piston is at the same point in it's travel, then the degree wheel is used to establish the mid-point, which will be the TDC position.
With a piston stop it's just a lot easier as the piston is brought gently up to the stop and one doesn't have to mess around setting up the DTI (which can be a PITA), nor concentrate on reading it as the engine is rotated.
If I understand your query correctly, you're asking why one can't just use a TDI and make the highest point TDC - in theory you're right, but in practice there are issues with piston dwell and bearing clearances - not to mention actually reading it that precisely - that make the 'halving' process quicker and more accurate.
There are exceptions, of course, such as some older 2 stroke m/cycle engines which would have their ignition timing set at xx mm, or fraction of an inch, before TDC. This would mean using the DTI to establish TDC (or as close as possible) with the piston then being backed off and brought up to the timing point.