How to use a piston stop for TDC?

[R.J.]

It seems that Comp Cams and Crane Cams directions are a little different and I would like to know which one is correct. Both procedures are listed below and the major difference is highlighted. It seems that Comp has you turn the wheel to the average of your two points. Crane wants you to adjust the piston stop until the two stop points are exactly the same and that is your TDC? I can see how they would both work but I think Crane's is more exact. Who has done this before? Thanks


CRANE CAMS
Begin the procedure by first mounting the degree wheel on the end of the crankshaft securely, and rotating the engine to approximately T.D.C. Mount the pointer and line it up at zero on the degree wheel. Now rotate the engine to move the piston down into the cylinder. Install your positive stop device into the spark plug hole and extend the bolt. Now hand turn the engine (do not use the starter motor or you will put a hole through the piston), rotating until the piston comes up and stops against the bolt. Look at the degree wheel and write down the number of degrees shown by the pointer.



Hand turn the engine in the opposite direction until the piston comes up and stops on the bolt again. Go back to the degree wheel and write down the degrees it now reads. Add these two readings together and divide the answer by two. Now either move your pointer by this many degrees, or carefully loosen the degree wheel (without disturbing the position of the crankshaft) and move the wheel this required amount. Re-tighten the bolts, and rotate the engine again making sure that the readings on each side of T.D.C. are equal degrees away from zero. If they are, the zero on the degree wheel will now be the true T.D.C. point. Remove the positive stop device from the spark plug hole, as this procedure is complete.



COMP CAMS
: Position the #1 piston at Top Dead Center (TDC). Attach the degree wheel to the balancer or crank

socket, and install the assembly on the crankshaft. The crank may be rotated from either the front or from the

flywheel end. Obviously, if the engine is in the car, you must rotate from the front. Remember, the greater the

leverage, the smoother the crank rotation, and the more accurately you can rotate the crankshaft. Install degree

wheel and pointer, and set the pointer to zero on the degree wheel. Note: Never use the starter to turn the

engine while degreeing a cam.

Step 3: Rotate the crankshaft opposite the engine rotation direction about 15-20 degrees. This will lower the

piston enough to permit the installation of the piston stop in the #1 spark plug hole. Rotate the crankshaft

clockwise until the piston hits the stop, and record the number indicated on the degree wheel. Next, rotate the

crankshaft counterclockwise until the piston hits the stop from the other direction, and again record the number

on the degree wheel.

Step 4:

Remove the piston stop after marking the two points on the degree wheel. Rotate the crankshaft to the

midpoint of the two marks (this is determined by adding the two points on the degree wheel together, then

dividing by 2). This point is TDC for cylinder #1.Without rotating the crankshaft adjust the degree wheel to read

0 degrees at the pointer. You are now ready to locate the intake lobe center line relative to TDC. If you are not

absolutely sure that your zero degree mark is set at TDC, repeat this procedure. This step is critical to proper

camshaft alignment.

 

[Stan Weiss]

Other than when the positive piston stop is removed what is the difference in the two methods? Once you have the two reads and set the pointer or degree wheel to zero for TDC, I normally go through the procedure again to make sure I have it zeroed correctly.

 

[BobHyatt]

I agree. But rather than using a stop, I just use a dial indicator. Bring the piston to TDC and zero the dial indicator. Turn the motor counter-clockwise until the dial indicator reads some easy-to-repeat value (say .05 or whatever). Mark the degree wheel at the TDC mark. Now rotate the motor clockwise past TDC until the dial indicator reads the same value. Again mark the degree wheel at the TDC mark. Now take those two marks, and read the degree markings they are on. Add them together, divide by two, and true TDC is at this point on the degree wheel. Rotate engine to bring that point to the tdc marker, which puts the engine at TDC. You can then loosen and move the degree wheel if you want to set the 0degree point to align with the TDC mark.

Using the positive stop does exactly the same thing.... all methods for finding TDC rely on finding the point before and after TDC where the piston is at the same depth in the cylinder. TDC is then exactly 1/2 way between those two points...

You can't use the indicator to actually try to detect TDC, because for a few degrees of crankshaft rotation the piston will not move down in the hole due to the geometry of the crankshaft/rod...

 

[R.J.]

Sorry I did not highlight the difference like I said I did. Here are the two steps, one from each procedure, after you get two degrees on the stop and divide by two.

-Now either move your pointer by this many degrees, or carefully loosen the degree wheel (without disturbing the position of the crankshaft) and move the wheel this required amount. Re-tighten the bolts, and rotate the engine again making sure that the readings on each side of T.D.C. are equal degrees away from zero. If they are, the zero on the degree wheel will now be the true T.D.C. point.

-Rotate the crankshaft to the midpoint of the two marks (this is determined by adding the two points on the degree wheel together, then dividing by 2). This point is TDC for cylinder #1

 

[Stan Weiss]

Maybe because I have done this many time, I do not read the above the same as you. To me they are saying the same thing. here is an example. If when I do the above procedure i have 35 degrees on one side and 45 degrees. I would move the pointer from zero to 5 degrees on the side that had 45 degrees. When I do the procedure a second time I should get 40 degrees on both sides.

 

[R.J.]

Stan Weiss: That is a great explanation, thank you. The next step after your example would be to remove the piston stop and turn the crank until the degree wheel read 20 deg. This mid point would be perfect TDC, Correct?

 

[Stan Weiss]

When you get done you should have 40 and 40 with 0 in-between being TDC.

40  0   40
\   |   /
 \  |  /
  \ | /
   \|/

 

[d98gt]

at the drawing

I just went through installing timing chains/guides on my car and once you get everything torn apart, it really isn't difficult to figure out.

 

[R.J.]

I figure that it will all be clear when I get the car apart but I like to understand what i am doing before I do it then it is so much more clear. It almost seems that Comp has left some information out of their procedure and Crane is much more exact. The way that I read Comp's is that I would add my two readings together and divide by two then turn to that number and it will be TDC. Crane says zero will be TDC when you get both readings to be equal. That means according to Comp that a reading of 40 and 40 would have TDC at 40 on the wheel and Crane says that a reading of 40 and 40 would have TDC at zero on the wheel?!?!

 

[BobHyatt]

Sorry I did not highlight the difference like I said I did. Here are the two steps, one from each procedure, after you get two degrees on the stop and divide by two.

-Now either move your pointer by this many degrees, or carefully loosen the degree wheel (without disturbing the position of the crankshaft) and move the wheel this required amount. Re-tighten the bolts, and rotate the engine again making sure that the readings on each side of T.D.C. are equal degrees away from zero. If they are, the zero on the degree wheel will now be the true T.D.C. point.

-Rotate the crankshaft to the midpoint of the two marks (this is determined by adding the two points on the degree wheel together, then dividing by 2). This point is TDC for cylinder #1

It doesn't matter whether you move the pointer or the wheel. All you want to end up with is a degree reading of zero for TDC on #1 cylinder. Doesn't matter whether the pointer is straight up, straight down, or on either side. Just so it points to zero on the wheel...

 

[Stan Weiss]

REMOVE this line from Comp cams
(this is determined by adding the two points on the degree wheel together, then dividing by 2)

Step 4:

Remove the piston stop after marking the two points on the degree wheel. Rotate the crankshaft to the midpoint of the two marks. Added by me: the midpoint between 40 and 40 would be zero This point is TDC for cylinder #1.Without rotating the crankshaft adjust the degree wheel to read 0 degrees at the pointer. You are now ready to locate the intake lobe center line relative to TDC. If you are not absolutely sure that your zero degree mark is set at TDC, repeat this procedure. This step is critical to proper camshaft alignment.

 

[R.J.]

Stan Weiss: Is that all that you would change in the procedure that I PM'd you? The PM was my wording of the procedure from start to finish.

 

[Stan Weiss]

If by PM you mean Personal Message on this Forum? I have not received any.

 

[R.J.]

Sorry, PM sent

 

[SonicStangGT]

Off subject, but was wondering where you can get a piston stop for the 4.6, as I need one to degree my cams. Thanks

 

[R.J.]

Summitracing.com
PISTON STOP (COMP Cams® Part #4795)

 

[SonicStangGT]

Thanks for the info.