[67cruiser]

Kinda been wondering , why do you have to put a piston stop across a cyclinder bore when degreeing in a camshaft to find TDC??? Why can,t you just use a.250 travel dial indicator and when piston comes up to highest point just adj. indicator face to 0 and be done with it??? Also ok,so then you turn crankshaft counterclockwise to bring piston back up in oposite direction to the stop why??? You have TDC located already on the first move????
Sorry for the dumb questions but i like to know why???

[Matt Meaney]

piston dwell. the crank moves a few degrees without the piston moving up or down.

[Singleton]

A dial indicator is not a very accurate way of finding Top Dead Center, because there are several degrees of crankshaft rotation while the piston stays at TDC. Since there are a certain number of degrees that the piston will dwell at TDC you need to find the exact center of that dwell at TDC to time accurately in DEGREES.

[paul s.]

you can use a dial indicator, but you have not located tdc just by rotating the engine to the highest point because of dwell. I never quantified the amount of error in trying to do it that way, but you are DEFINITELY getting yourself into trouble. No use in looking for a shortcut if you are going to do it right. You still need to go past tdc and average the readings on the degree wheel.

[67cruiser]

Hum, I did not know there was a dwell point at TDC when i installed my cam i just used a dial indicator and when piston stopped i called it TDC. But wouldn,t a indicator be more accurate than some sort of stop??

[paul s.]

Ultimately, no. Because it is a physical hard stop reached in both rotational directions. Then you simply split the difference. You could easily be 4 or more degrees off.

[67cruiser]

Great!!! Thank You so much, can,t wait to get into this a little more!

[Matt Meaney]

have you ever seen it done? if not here's a link to ONE way of doing it.

http://www.youtube.com/watch?v=7cNoVRLb73c

[Old Man Taylor]

If you do it with a dial indicator then you need to do it like you do when degreeing the cam. Find the point 0.250" before TDC and 0.250" after TDC. Record the degree wheel readings at each point, split the difference, and you should have found DC. Make all readings while turning the crank shaft the same direction.

[slowbird]

Originally Posted by Old Man Taylor

If you do it with a dial indicator then you need to do it like you do when degreeing the cam. Find the point 0.250" before TDC and 0.250" after TDC. Record the degree wheel readings at each point, split the difference, and you should have found DC. Make all readings while turning the crank shaft the same direction.

X2 I personally like this method the best, I only use a stop when heads are on. Also be care when using a stop dont go spinning the engine fast & slamming the piston into the stop.

[cfmcnc]

Piston stop is the most accurate way of finding Mechanical Top Dead Center,all other methods fail.Bill C.

[cgeise]

Originally Posted by cfmcnc

Piston stop is the most accurate way of finding Mechanical Top Dead Center,all other methods fail.Bill C.

So - your saying that measuring the piston location down to the .001 is a FAIL -- must be a lot of guys failing around the country.

[67cruiser]

I really thinking about pulling off my heads, but for a second, is one actually better than the other for acuracy???? Heads on or off????

[Scarebird]

Originally Posted by 67cruiser

I really thinking about pulling off my heads, but for a second, is one actually better than the other for acuracy???? Heads on or off????

I certainly would not pull them just the degree the cam.

I degreed my new cam the other day. One thought that occured to me was with chain slop. Finding TDC of course can be done with a piston stop or as OMT says, but I wonder if I should have found out how many degrees timing gear slop I have and divided it by 2, then factored that into true TDC?

postscript: OMT's method of finding TDC would negate chain error due to rotation being in the same direction (as long as it was in the same direction as the engine turns). This may account for the 2° retardation discrepancy of the cam I installed last week using a piston stop.

[67cruiser]

Scarebird, heads really never been touched except for new valves and springs. Love to have a Good Pontiac man go over them for extra flow. I noticed rear soft plugs starting to look bad also i need to get them off now or later.

[cfmcnc]

Originally Posted by cgeise

So - your saying that measuring the piston location down to the .001 is a FAIL -- must be a lot of guys failing around the country.

Failing to make the power they could-should by not paying attention to details.Bill C.

[60man]

Correct me if I'm being a DA...
But chain slop should not matter when finding TDC of piston using stop method.. Chain has no influence on piston travel??!! Just my
Would also think it would be best way...

[gearbanger]

Dont know if this was mentioned or not but I made a piston stop out of a spark plug by gutting it and welding in a peice of 1/4 rod. I made it long enough that I knew it would stick down 1/2" or more below TDC. Thats one way to do it with the heads on.

Something else to check while doing this is the relationship of TDC on the balancer with actual TDC. I have seen them off a couple of degrees. If you have a spun balancer, its a good time to find that as well.

[Skip Fix]

So what is the different using a piston stop that is say 0.050 down from deck going both directions to split the difference vs a dial indicator measuring 0.050 doing the same? Not talking about the dial at exactly TDC to find TDC due to piston dwell. Many of the cam makers us a 0.050 off peak lift either direction to get the lobe centerline seems like the same thing to me.

Heck I bet you can wedge the piston 0.001 variance at least against a stop if you smack it hard against it.

[gearbanger]

Well, in my mind with a stop you are taking all the slop out of the rod bearing, wrist pin, and main bearings in the same manner on each side. A dial indicator doesnt provide any resistance so things may not be exactly the same. My thinking you would be a few thousandths difference especially if you were traveling up to .050 before TDC and continuing down to .050 after because of the slack in the clearances.

Also on that .050 before and after peak lift, I think it is pretty important to always rotate the crank clockwise when taking readings on the cam. If you pass it up then you just have to continue around to it again, not back up to it. That keeps the slack pulled out of the timing chain and everything else the whole time. If you back up, then you introduce a few degrees of error to your mesurements.