[mgarblik]

If you run lots and lots of piston to wall clearance, over .010-.012 and using a dial indicator rather than a spark plug stop or bridge stop with a solid stop there could be a slight variation in your zero readings with a dial. Try it yourself even with the piston down 1/2 inch. As you rock the piston in the bore with the indicator on the top of the piston it will vary a tiny bit, .001-.003. I think we are really splitting hairs here, but putting the cam exactly where you want it should be as perfect as you can get it. How much HP ? Retarding our cam 5 degrees gained us 30 peak HP, 17 ft. lbs. torque. So there is meaningful power there if you get each cam in it's optimum position. With the heads off, I use a bridge tool with a dial indicator but I wiggle the piston and take the measurements multiple times to make sure they are consistent. I agree it is also a good idea to check TDC with your balancer and the pointer. They can vary several degrees as mentioned .

[gearbanger]

I always try to make the stop right over the wrist pin to take the piston rocking out of the equation as much as possible. That is another thing that is harder to do with a spark plug style piston stop. Making the rod a little longer puts the end pretty close to the center of the piston.

[Tom Vaught]

Originally Posted by cfmcnc

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

Bill, in simple terms, you have Artists and you have House Painters (and some are not very good at painting houses either).

Expert Engine Builders do stuff for reasons.

Tom Vaught

[Skip Fix]

If a solid stop because of the head bolt spacing or angles between bolts for the stop it is not dead center it would cause more piston rock I would think if you do have larger piston to bore clearance.

[Probird]

How much dwell is there at TDC? From what I remember when I've done it with a dial indicator it's no more than one degree on the degree wheel.

[Scarebird]

Originally Posted by Tom Vaught

Bill, in simple terms, you have Artists and you have House Painters (and some are not very good at painting houses either).

Expert Engine Builders do stuff for reasons.

Tom Vaught

Tom, I do not buy his theory that piston stops are the only way to find an accurate TDC. OMT's method would be more accurate, especially if the dial tip was closer to mid-stroke where the piston speed/angular ratio is much higher. As an engineer you must have had some surveying or astronomy classes and realize that turning the crank in one direction eliminates the chain slop error where the piston stop method does not.

[cfmcnc]

What does the chain have to do with finding Absolute Mechanical Top Dead Center of piston travel.Bill C.

[Scarebird]

It doesn't - until you are degreeing in the cam for reason described above. Your statement was piston stops are the only way to determine TDC, this is not true.

[XSIVPWR]

Question, My bottom end is together and my balancer is already on, So just for the heck of it I put a dial indicator on the block put the point dead center of the piston rotated the crank to 240 before set it to 50 then went to 20 after and it was at 50 would that be close to the same as a degree wheel? I have one just haven't put it on since Butler already degreed the cam. I was mostly checking if my new ATI balancer was right on 0

[Tom Vaught]

Originally Posted by Scarebird

Tom, I do not buy his theory that piston stops are the only way to find an accurate TDC. OMT's method would be more accurate, especially if the dial tip was closer to mid-stroke where the piston speed/angular ratio is much higher. As an engineer you must have had some surveying or astronomy classes and realize that turning the crank in one direction eliminates the chain slop error where the piston stop method does not.

Think you are looking at two different operations on the process Scarebird.

The first operation involves making sure that you have an accurate reference point for top dead center (of a given cylinder) vs a degree wheel (either a degreed Balancer or a add-on adjustable location degree wheel.

If you have a degreed balancer, you first need to know that the degreed balancer actually reads true Top Dead center (0 Degrees on the balancer) vs when the Piston and Rod are at True TDC of the rotation of the crankshaft.

You do easily do this by the "Piston Stop" method with the degreed balancer, if the head is off the Driver's side bank of the engine by the previously mentioned process submitted in the other posts. (Rotate the crankshaft until the piston hits the piston (ideally in the centerline of the rod beam) in one direction, take a reading, then rotate in the opposite direction until you contact the stop again and take a second reading. Do the math and then you have a number which should be exactly the same as the TDC mark on the degreed balancer.)

If you have an adjustable degree wheel, you might need to tweek the degree wheel's position slightly to get the Bisector of the two degree angles. (Definition The bisector of an angle is a ray whose end point is the vertex of the angle and which divides the angle into two equal angles).

IN EITHER CASE, NO WHERE IS THE TIMING CHAIN WHICH IS DRIVING THE CAMSHAFT INVOLVED IN DETERMINING TDC OF THE SELECTED PISTON VS THE TIMING MARK ON THE BALANCER OR THE DEGREE WHEEL.

No if you want to talk about degreeing in the camshaft accurately THEN the timing chain comes into the process as does the rotation of the crank to reduce errors from chain slop, etc.

Tom Vaught.

It does no good to "Degree in the Camshaft" by whatever method IF the reference device (Degreed Balancer or Degree Wheel) is not set properly in the first place. The original Sin will be that you did not check that your initial reference point was correct before you started the camshaft timing verification process.

[67cruiser]

I just went to Pontiac Street Performance, a Mr. Boyles writes there, Why not use a dial indicator when bringing up TDC thats what i was asking earlier??

[slowbird]

Im having are really hard time believing that either method of finding TDC is going to be off enough to make a difference. Lets say one is off by a 1/4 of degree big deal (heck just reading where the pointer is pointing on the degree wheel is going to have a bigger error factor than either of these two methods). Most engine won't even notice if the cam is off by a .5 degrees (unless its a big effort deal then it will notice). Just my opinion but I think we are splitting hairs here.

[darbikrash]

Another vote for the dial indicator method. While it may be correct that the chain does not influence any backlash during setting piston TDC, there is backlash in the system (however small) which is eliminated by using the dial indicator and always turning the crank in the same direction (preferably in the direction the engine actually runs).

Placing the indicator tip directly over the piston pin centerline eliminates the rocking of the piston in the bore as a factor. While the outer edges of the piston perpendicular to the pin centerline will see significant rocking due to wall clearance, the area directly over the pin centerline will not.

Reversing the direction of crank rotation to hit a hard stop is a contrivance that does not occur naturally on the engine- it never (hopefully) runs backwards. This may or may not create an inaccuracy (wrist pin offsets, asymmetric dwell, etc) but why go there if you don’t have to?

The indicator method allows you to establish a clearly measured pair of opposing points in one direction of rotation (ex .050” before TDC and .050” after TDC) which you cannot do with a hard stop, as with the indicator you can just rotate on through without stopping. This to me is a significant advantage, and coupled with the elimination of any possibility of banging the soft aluminum piston top into a hard piece of metal, as long as the heads are off, why do it any other way?

[Tom Vaught]

Originally Posted by darbikrash

The indicator method allows you to establish a clearly measured pair of opposing points in one direction of rotation (ex .050” before TDC and .050” after TDC) which you cannot do with a hard stop, as with the indicator you can just rotate on through without stopping. This to me is a significant advantage, and coupled with the elimination of any possibility of banging the soft aluminum piston top into a hard piece of metal, as long as the heads are off, why do it any other way?

1) Easy to make an aluminum 1" x 1" Aluminum Bar Hard Stop that has the Piston Stop .050" below the deck surface (or .100" below, or any other number you wish to machine off one surface of the hard stop on ether side of the bore dimension. You bolt the ALUMINUM hard stop on to the block and the piston now stops exactly .050" below the block deck WITHOUT banging a hard piece of T-6 aluminum into that "Soft Piston Material". Idiot Proof. I have seen people who could not mount a dial indicator TRUE to the centerline of the bore to save their life. So really which method is really more accurate?

2) Back to the Chain Slop comment. If you have Chain Slop, why are you worried about where your camshaft phasing is" You are basically saying that no matter how accurately you measured your camshaft, you are now willing to have the chain slop move the camshaft phasing all over the place. The fact that you keep the chain in tension during your measurements means squat in the running engine. Fix the Chain Slop.

3) If you take your measurements with the head on, it might be fast (vs removing the head) but you really do not know where the tip of the stop is contacting the piston top, if it is moving slightly from the lever arm effect of the stop on the spark plug threads, etc, etc.

Looks like most guys, Bill, want to do the "quick and dirty, I feel good" method vs actually knowing where their stuff is.

Tom Vaught

[Scarebird]

Tom, I was mulling this over and Bill is correct on that finding TDC has nothing to do with chain slop. He is incorrect that the piston stop is the only accurate method. As stated by slowbird I doubt any difference in accuracy between the two methods will make a tangible difference. To me as an infrequent builder using a tool I have (dial indicator) vs. a highly specialised tool (piston stop, though made one) will color the decision.

Unfortunately, I have had the mispleasure of dealing with "housepainter" machinists (looking at you Jay Barnes).

[slowbird]

Tom V or Bill C please explain why the dial indicator method is no good? We are assuming both are setup correctly so remove human error. Mechanically why is it a "house painter" way of doing it?

[pastry_chef]

A picture can be worth a thousand words, a video can be priceless.

Tom knows this guy knows a thing or two

showing us how it's done in his shop.

https://www.youtube.com/watch?v=S7ZzYaMVi00

[cfmcnc]

When we do our video we will use a Piston Stop.Bill C.

[Tom Vaught]

Originally Posted by slowbird

Tom V or Bill C please explain why the dial indicator method is no good? We are assuming both are setup correctly so remove human error. Mechanically why is it a "house painter" way of doing it?

Never said that the dial indicator method was no good. An artist can paint a picture with feather tip or a 3" paint brush (though the canvas might be larger in the second case). He will get the same result, a beautiful picture at the end of the day.

A House Painter gets paid to paint the house, some slop the paint on and some do a nice careful job. The difference is in the effort involved.

So I will talk a bit about INSTRUMENT ERROR.

With a Piston Stop, whether the stop is at .049" below the deck or .051" really doesn't matter one bit. If you find the bisector of an degree wheel angles, you will be at TDC.
A very simple fool-proof process.

With a dial indicator, every revolution of the dial indicator may have actually a different actual number of travel vs the assumed .001" number on the dial. Unless you follow this attached process for calibrating a dial indicator: http://www.usbr.gov/pmts/geotech/roc...2/USBR1007.pdf you are assuming that every time the dial makes a sweep the reading is the same reading. Without calibrated gage blocks and a knowledge of Metrology (Metrology is the science of measurement) you are assuming a lot of things.

With a Piston Stop, you easily eliminated a lot of the bs with making sure that the degree wheel was at least at True TDC of the crankshaft (Zero Degrees) before you started the camshaft measurement process.

That being said, you can easily check true TDC (for the other cylinders) in 90 degree increments by just moving the piston stop (I can check all 8 holes accurately in less than 30 minutes, with the dial indicator you might have 16 different sweeps on the dial indicator. Instrument Error.

I did not bring into play any of the set-up errors on getting the dial indicator EXACTLY true each time for a different cylinder.

So I personally will stick with my simple custom T-6 Piston Stop that will never lie to me and will also allow me to check things like: were all of the crank journals ground 90 degrees to each other, etc.

I explained why I like the Piston Stop method. That was the original question. "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???"

It is all in the details. "Failing to make the power they could-should by not paying attention to details.Bill C."

Tom Vaught

[Torment]

I like to set it so the intake valve is just proud of the exhaust in overlap... and call it a day! lol Any of you guys that have had to 'degree' a cam without the benefit of a wheel due to 'racing', probably get the humor