Find the TDC
- Thread starter pierodn
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If you can get a degree disc on the end of the crank, and can accurately replicate piston position then the easiest way is to determine degrees 'before' and 'after' that pre determined position (by rotating engine both ways )... Then just divide the two readings....
(Probably easier to do than explain)
(Probably easier to do than explain)
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Theoretically yes but practically no, the piston does not move enough close to TDC, as EstuaryBoy suggests a degree wheel and a stop is far more accurate as long as the 2 stops are some degrees apart.
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Depends on how accurate you want/need to be. There are all sorts of methods for determining TDC but the most accurate method involves using a degree wheel and piston stop. If you just need an approximate TDC - the eyeball method with a pencil (or whatever) in the spark plug hole with the eraser end sitting on the top of the piston will do just fine. Just watch the pencil move upward until it quits moving upward! 
Heck, for a very rough TDC you can put your thumb on the spark plug hole, turn the engine over manually and feel when the compression stops! I have used that many times to install a new ignition when you just need the timing to be "right enough" to start!
SO...it really depends on what you are working on as far as the necessary accuracy!
Heck, for a very rough TDC you can put your thumb on the spark plug hole, turn the engine over manually and feel when the compression stops! I have used that many times to install a new ignition when you just need the timing to be "right enough" to start!
SO...it really depends on what you are working on as far as the necessary accuracy!
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As the head is off, just a straight edge across the barrel and something non destructive on top of the piston will give identical positions before and after...
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If you are looking for maximum accuracy, whatever you put across the top of they cylinder barrel needs to STOP the piston before it reaches TDC - same process as with the head on. IOW, clamp something there that wont move when the piston meets it.
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With the pistons exposed and no compression or valve spring resistance to overcome, I would have thought a firm hand on the straight edge, and a cautious one on the crank nut would be sufficient to tell when the irresistible force meets the immovable object.. (Subject to any backlash being taken up as well)
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Sure, that will work reasonably well but it doesn't account for the fact that when the piston is stationary at TDC, the crankshaft continues to rotate before the piston starts moving back down. The piston stop/degree wheel method accounts for that. No other method can. HOWEVER, depending on what you are doing, knowing that the piston is at the top of its travel may be perfectly adequate. OTOH, If you are assembling an engine or installing a new camshaft, you'd want the precision of the piston stop/degree wheel to ensure proper cam timing. Same is true if you want to check the accuracy of the Norton ignition timing scale.
The important thing to remember is that although TDC is usually defined) as the point at which the piston is at the top of its travel, that isn't really correct as far as engine valve/ignition timing is concerned. Or maybe a better way to say it is, it isn't sufficiently precise to obtain optimum timing. Now, I will admit that on a stock Norton motor, it probably makes no difference at all. I suspect that watching the piston with a flashlight and saying "looks like it stopped," is precise enough!
The important thing to remember is that although TDC is usually defined) as the point at which the piston is at the top of its travel, that isn't really correct as far as engine valve/ignition timing is concerned. Or maybe a better way to say it is, it isn't sufficiently precise to obtain optimum timing. Now, I will admit that on a stock Norton motor, it probably makes no difference at all. I suspect that watching the piston with a flashlight and saying "looks like it stopped," is precise enough!
maylar
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If the crankshaft is moving and the rod bearings and piston pin have nominal clearances, how does the piston not move? How many degrees of rotation can there be without the piston moving?
Sorry if that's naive but my degrees are in Electrical Engineering and Computer Science not mechanical.
Lineslinger
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I have measured, using a degree wheel, and by "feel" for piston movement, about three degrees of imperceptible motion either side of TDC. A dial indicator would likely reduce that, but some "dead" range will persist, due to bearing clearance and the geometry of a crankshaft and rod system.
I split that dead range and call the mid point TDC.
Slick
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Same guy asking the same question!!
You appear not to have learned that there is only one way of getting TDC , that is put the rear wheel off the ground in gear and slowly bring up to the top in the one direction that the wheel revolves. If you go too far that the piston goes past TDC turn the wheel back till the piston is halfway down and start again. You have to take all the slack up everytime. Same applies whem turning the crankshaft. Using a long thin screwdriver or rod I can get TDC by "feel" everytime.
baz
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Bernhard You appear not to have learnt that texaslick was answering maylar about piston duration at tdc
Not "finding tdc" !!!!
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marshg246
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Agreed with the head on, but if I have the head off, I setup a dial indicator and choose a reading just before TDC and set the degree wheel to 0. Then turn the engine in the forward direction until I have that dial indictor reading again. I can then read the number of degrees exactly, divide by two and remember. Then turn the engine forward until I once again have my dial indicator reading and set the degree wheel to the the remembered degrees before zero. Finally, turn the engine forward to whatever I want before TDC and verify/correct the timing marks.
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or use the ‘tail’ of vernier calipers down the barrel, using barrel lip as baseline. very easy.
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I always use the soap bubble method with an old spark plug with the electrode removed. Bore clearance introduces an error if you use positive stop or a dial micrometer. As you rotate the crank, the pistons usually move slightly backwards and forwards in the bores while the rings do the sealing. It is the compression peak which is important. The soap bubble method is very sensitive. I usually try to rotate the crank forward, when finding TDC, so that any error ends up creating more advance. If I ever disturb the ignition timing, I always check my jetting. It does not matter if you get it slightly wrong as long as you jet to suit it.
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