Piston dead band

willy mac · Mar 27, 2015

Does anyone have the figure for the amount of Degrees that the crank turns after the piston reaches the top of it's travel and before it starts down again?
Just about to check ignition timing and want to get TDC bang on.

Fast Eddie · Mar 27, 2015

willy mac said:
Does anyone have the figure for the amount of Degrees that the crank turns after the piston reaches the top of it's travel and before it starts down again?
Just about to check ignition timing and want to get TDC bang on.

Willy, if you want it bang on, then the following method is the way:

Put a something to act as a hard stop thought the plug hole, slowly and carefully bring the piston up to it. Note the degrees BTDC. Now reverse the crank, slowly and carefully to the stop again. Note the degrees ATDC. Divide the two and that is your precise TDC point.

Personally, I have done the above before, but find I can get it pretty cock on by inserting something that the piston can push up and down, and rocking the crank at TDC and visually splitting the difference.

texasSlick · Mar 27, 2015

Theoretically, it is zero. As a practical matter, it takes about 2 - 3 degrees crank rotation before the piston movement becomes perceptible.

One has to rock the crank back and forth to estimate the mid point of the imperceptible part of the piston travel, and call that mid point TDC. If done carefully and repeatably, you will not be off by more than a degree or so.

Slick

concours · Mar 27, 2015

As mentioned, simple note degrees at .005" BTDC, then at .005" ATDC, split the difference, that is TDC.

toppy · Mar 27, 2015

if you are using a piston stop that fits into the plug hole take care it does not contact the valves as you turn the engine over. Depending upon what cam and valve sizes your engine has it is best to remove the stop turn the engine then carefully refit the stop and turn the engine the last few degrees until piston lightly touches the stop. Then remove stop turn engine over TDC and repeat the process.

I did my engine the other week (no head on) and using a dial gauge i found 0.300" before or after TDC was 30 derdegrees on the timing disc so your piston stop doesn't need to be a great long thing. Ebay has many centre finding tools that may help as the plug size is very common i have one that holds a dial gauge an works very well even tho it cost less than the price of a cheap dial gauge on its own.

dennisgb · Mar 27, 2015

"Piston Dead Band"

Awesome name for a rock band...I call dibs.

Hobot · Mar 27, 2015

A tight-new clearance crank will balance at TDC on pressure bleed down tests but when mine moved about 1' off TDC It became almost impossible to hold the torque hit. Measurements and settings before first start is only to get started so ya can fine tune by ear- vacuum guage-trial-error then put a light on to record where it lands. hobot dumb trick is force pistons to BDC set degree wheel then turn to TDC and have at it. Btw probes stuck in plug hole encounters piston at a angle not straight on so can snap or bend the probe if not careful. I suppose a pressure blast in crank case would force pistons up to perfect TDC as valves open would prevent sucking pistons to TDC.

Dances with Shrapnel · Mar 27, 2015

concours said:
As mentioned, simple note degrees at .005" BTDC, then at .005" ATDC, split the difference, that is TDC.

I would go significantly more than 0.005" as there is more "gain" in the measurement and I would also use a hard stop (as elaborated on by Fast Eddie) rather than a measurement.

concours · Mar 28, 2015

dennisgb said:
"Piston Dead Band"

Awesome name for a rock band...I call dibs.

Have to be retro music... no one under 30 knows what a piston is :shock:

willy mac · Mar 30, 2015

The answer to my question is 30 Degrees of crank rotation from piston stop to piston start.
TDC now marked onto Alternator stator along with the 30 BTDC for the ignition, the scale on the primary case is so far out, but then we all know that don't we.
Thanks to all who replied with your own various ways of finding TDC on your own beastie's I had just purchased a DTI with a screw in the plughole adapter so seeing the smallest amount of movement was the easiest thing to do.
All sorted now and going well.

johnm · Mar 30, 2015

willy mac said:
The answer to my question is 30 Degrees of crank rotation from piston stop to piston start.

This cannot be correct unless you have totally worn big ends / little ends. The piston cannot stay at TDC for 30 degrees of crank rotation unless something is very serously worn.

Texas Slick gave the correct answer to your question. Theoretically it is 0 but practically with a bit of play here and there 1 to 3 degrees.

Hobot · Mar 30, 2015

hehe two guys talking about the same thing but different interpetations, so sitting back to see how ya resolve the missunderstanding with both being correct.

johnm · Mar 30, 2015

willy mac said:
Does anyone have the figure for the amount of Degrees that the crank turns after the piston reaches the top of it's travel and before it starts down again?

That is the question to which I was refering. The original question.

TexasSlick gave the correct answer and 20 degrees cannot be correct.

However if you refering to the number of degrees which should be split when using a piston stop to find TDC then that is something else again. The answer then could be 20 degrees, 21 degrees or almost anything else.

Others have explained clearly how to use a piston stop to find TDC.

Egholt · Mar 31, 2015

http://www.wikihow.com/Find-Your-Engine ... -%28TDC%29

willy mac · Apr 1, 2015

Just read through what I posted about crank movement against piston movement and now realise that I missed out the bit about using the DTI to catch the bit of piston movement before and after TDC, hence the 30 Deg of crank movement "DOH"

Anyway thanks again to all concerned.

johnm · Apr 1, 2015

Piston dead band

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