[darbikrash]

Not trying to be argumentative on this, but I think my point may have been lost, and I do have a question around presenting a method using a physical stop administrated by a breaker bar (rotating the crank/piston assy) as being more accurate than a precision measuring instrument.

I would be surprised if this was true.

So here are the concerns, for whatever it is worth:

1.) If we are going to start bringing in metrology and calibration standards for the measuring instrument, than we have to start talking about a GR&R study (method repeatability) for the hard stop method. I don’t see the hard stop method winning that battle.

2.) To list some specifics, what I tried to say in my earlier post is that the hard stop REQUIRES one to reverse the direction of crank rotation to find TDC, the indicator method does not. You cannot find TDC without REVERSING the direction of rotation during set up using a hard stop. This is significant, the indicator method has a clear advantage.

3.) Now to the topic of offset wrist pins. Let us examine some assumptions that are implicit in the hard stop method- that the dwell of the piston at TDC is symmetric, meaning, that it starts and stops at the same point as measured in crankshaft degrees irrespective of direction of rotation. If the wrist pin is offset in the piston, is this assumption correct? Is this not an intrinsic error? Note that none of this matters if the indicator method is used, as both points before and after TDC are determined in the same direction of rotation- the direction the engine actually runs in.

Is the hard stop method really better?

[Tom Vaught]

Originally Posted by darbikrash

A) Not trying to be argumentative on this, but I think my point may have been lost, and I do have a question around presenting a method using a physical stop administrated by a breaker bar (rotating the crank/piston assy) as being more accurate than a precision measuring instrument.

I would be surprised if this was true.

So here are the concerns, for whatever it is worth:

1.) If we are going to start bringing in metrology and calibration standards for the measuring instrument, than we have to start talking about a GR&R study (method repeatability) for the hard stop method. I don’t see the hard stop method winning that battle.

2.) To list some specifics, what I tried to say in my earlier post is that the hard stop REQUIRES one to reverse the direction of crank rotation to find TDC, the indicator method does not. You cannot find TDC without REVERSING the direction of rotation during set up using a hard stop. This is significant, the indicator method has a clear advantage.

3.) Now to the topic of offset wrist pins. Let us examine some assumptions that are implicit in the hard stop method- that the dwell of the piston at TDC is symmetric, meaning, that it starts and stops at the same point as measured in crankshaft degrees irrespective of direction of rotation. If the wrist pin is offset in the piston, is this assumption correct? Is this not an intrinsic error? Note that none of this matters if the indicator method is used, as both points before and after TDC are determined in the same direction of rotation- the direction the engine actually runs in.

Is the hard stop method really better?

a) I would not call a dial indicator (Dial Caliper or the plunger type Dial Indicator a "Precision Measuring Instrument" unless you also owned a complete set of certified gage blocks to go with the "Precision Measuring Instrument" and actually used the things correctly. Right or wrong?

2) If I bring the piston up to near the Piston Stop, install a inch lb torque wrench on the damper bolt, touch the stop, and then go to 12 lb/inch on the damper bolt I will get a number. I can then either rotate the crankshaft in the opposite direction until I get near the Piston Stop, install the Torque Wrench, touch the Piston Stop, now apply 12lb/in of force on the Damper bolt IN THE OPPOSITE DIRECTION and get my number. My Piston stop is actually not .050" down in the bore, it is .200" down in the bore. I guarantee that when the piston is .200" down in the bore on the Expansion Stroke, the piston, pin, rod, and bearing/crank journal will all be locked tightly together. Just like they are locked tightly together on the Compression stroke. In your case you are pulling the components apart after you cross over TDC (no firing pressure in your method so more chances of errors due to the slop in each component as you try to stop at exactly the same dial indicator reading). So your indicator method is filled with error potential if you do not just happen to stop at exactly the correct moment.

3) If the piston is locked at precisely .250" from the top of the block deck, the crankshaft is loaded the same as it would be under the Expansion Stroke, and the degree wheel is read at that point, how will there be an error from Piston offset. The Piston Offset bias is long gone by .250" down in the hole.

So we can discuss this till the cows come home but I think that a Piston Stop, designed correctly, will always contact the piston at the correct location, will have no deflection if you follow the lb/inch Torque Wrench Method, and will provide consistent readings every time the checks are made. I cannot agree that a Dial Indicator can do that. I cannot agree that two Dial Indicators tested one right after the other will always give the same reading.

Tom Vaught

[Old Man Taylor]

Just a point of clarification. I was only responding to how to do it with a dial indicator. When I do my own engines I do it with a piston stop right in the center of the piston (assuming the piston contour allows it there).

[Tom Vaught]

Originally Posted by Old Man Taylor

Just a point of clarification. I was only responding to how to do it with a dial indicator. When I do my own engines I do it with a piston stop right in the center of the piston (assuming the piston contour allows it there).

I rest my case!

Tom Vaught

One clarification, My personal Piston Stop is .200" down in the hole. Some of the old fabricated piston stops used the head of a bolt, below the crossbar, so maybe it was .250" down in the hole. The cross bar had a threaded hole in it and the bolt could be adjusted for domed pistons. Yes, with the domed piston was now maybe .500 down in the hole but it did not matter, it was a solid piston stop doing exactly the same thing that a machined .200" depth stop would do. Help you find True TDC on the Degree Wheel device.

[67cruiser]

Not to jump on anyone but a dail indicator is a very precise measuring instr. At least i think so, used one for 30 yrs. never a issue.

[Tom Vaught]

In those 30 Years did you ever check it against a gage block set?

Tom Vaught

[gearbanger]

Why is it so hard to understand that there is some slack in the piston rod assembly that will cause the dial indicator method to vary a couple of thousandths depending on whether the piston is being pushed up the cylinder or pulled down the cylinder?

If you are pushing it up against a stop both directions, then it takes that out of the equation. Sure you can do it with the dial indicator, it is just slightly more accurate to do it with the stop.

[67cruiser]

yep, usually Starrett makes a great indicator. May i ask a question to help me out here please, I noticed that when a degree is mounted on the crankshaft and pointer is installed the 0 point that i have seen never comes up once on tdc. Can i watch the piston come up against the piston mechanical stop and pre set my degree wheel to 0 point and then go counterclockwise motion???? I also noticed somehow all pointers are usually before TDC point like 10 or 15 degree,s. Just wondering???

[Tom Vaught]

Factory "Pointers" on early GTOs or V-8 LeMans & Tempest looked like the sharpened end of a pencil. When the Timing Cover was installed correctly with the "location sleeves" in the lower timing cover mounting positions, the Pointer cast on the timing cover was supposed to line up with the left most mark on the factory balancer at TDC.
The second mark (a few degrees to the right of the TDC mark) was scribed at 6 degrees before TDC. If you put the piston stop on the number 1 cylinder and go thru the process, the pencil pointer should line up with the left mark on the harmonic balancer IF the balancer ring has not slipped. If it has slipped then your timing setting was always in error. It might have even been off from day 1 if the balancer was assembled slightly off. That is why you check stuff vs ASSUME stuff.

Tom Vaught

ps I have had people tell me that they precisely torqued their heads to some value and they know that they did it right, same deal with setting their timing with their dial back adjustable timing lights. Most did not know that dial back timing lights do have accuracy issues so if you use a "bad one" your performance will be off. Or if your Sears Torque Wrench is not calibrated correctly, your head bolt torque clamp load will be off. There are Tools made to calibrate Torque Wrenches. Most OEMS have them in their Engine Build-up Rooms. Some NASCAR Pro Engine Build Shops have them too.

So my point is, If you don't check stuff, then you do not know for sure and you ASSUME that your stuff is right. If the guy doing your engine build does not have the equipment to do it right then you wasted your money or you "got lucky".

[Skip Fix]

"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."

Since we are talking about TDC only in regards to the crank the main reason this alone would be needed would be one for a reference for your cam timing(but then the chain slop potential variable comes in) or your ignitions timing.

I would be willing to bet seeing even 0.5 degree of ignition timing difference(Tom's timing light variable and the readers parallax of reading at the exact same angle on the balancer mark and timing cover mark) or 0.5 degrees of cam timing difference would be within even a dyno's variables of measurement, not to mention is the cam timing on every cylinder ground right on the exact degree split from Comp, Lunati etc.

[65 GTO Convert]

On a stock Pontiac 400. Piston Travel from 0.001" BTDC to 0.001" ATDC is around 3.3 degrees of crankshaft rotation. Ignoring any clearance issues due to reversing piston / crank rotational direction (Up vs down).

[Tom Vaught]

Originally Posted by Skip Fix

"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."

Since we are talking about TDC only in regards to the crank the main reason this alone would be needed would be one for a reference for your cam timing(but then the chain slop potential variable comes in) or your ignitions timing.

I would be willing to bet seeing even 0.5 degree of ignition timing difference(Tom's timing light variable and the readers parallax of reading at the exact same angle on the balancer mark and timing cover mark) or 0.5 degrees of cam timing difference would be within even a dyno's variables of measurement, not to mention is the cam timing on every cylinder ground right on the exact degree split from Comp, Lunati etc.

Yes, I agree Skip that for most it will be "splitting hairs" but this is "the race section" where guys are supposed to be trying to get the most out of their combinations (it is all in the details), lol.

So if you use the piston stop deal on every throw of your Chinese crank and find that the throws are off by "x-amount" do you say 'It is what it is" or do you look for a better crankshaft that is "indexed" properly.

So if you use the piston stop deal on every cylinder and check every camshaft lobe (like it was #1 cylinder) and find the camshaft was ground wrong, do you say 'It is what it is" or do you look for a better camshaft that is "phased and ground" properly.

If close is good enough, go for it. If you want to get the last bit out of your competition engine then you do what it takes or you hire someone to do whatever it takes.

JMO

Yes, Skip, the average guy might just throw the camshaft into the engine. The average racer might try to check everything to get an idea where the stuff really is, the serious racer will know where his stuff is. (Langer or Rex come to mind). The Pro Racer pays someone who knows how to do it right to build the engine correctly checking all of those processes.

It is just where you fit into the scheme of things. I have seen a lot of guys with little money who were "Detail" guys who always had the fastest engines. They may not have been Engineers but they knew what to check, how to check it, and how to fix it if it was not right.

It is all in the Details.

Tom Vaught

[67cruiser]

Not really sure if my question was answered?? Thank You, maybe i just did not ask it the right way???

[gearbanger]

67 cruiser,

Yes you can bring the piston up against the stop, install the pointer and then installe the degree wheel at 0, then rotate back around to the stop again. Take that reading on the degree wheel and divide by two. For example if the wheel read 50.5 degrees. Divide that by two and that is 25.25 degrees. Remove the stop and rotate the crank to that number. Then loosen the degree wheel carefully without moving the crank and retighten at 0 or TDC. Now your degree wheel is installed correctly to coninue with you camshaft degreeing process.

You may think it sounds stupid to talk about .25 degrees but that is exactly why max performance engine builders use really large degree wheels. They want to be able to deal with fractions of a degree and not just say close is good enough.

[Skip Fix]

Tom I guess when you start checking every cylinder and find crank is X off from #1 to #8 you have to decide how much off what difference in power will it make to use it vs a another $$ part that you HOPE will have better tolerances. The same with a cam with different specs slightly form lobe to lobe. Will the next one be any better? Are we really tuning each individual cylinder anyway?

Let's say we are using a single plane intake the outer 4 cylnders are going to have a shorted intake length that I would bet would cause more difference cylinder to cylinder power than 1 degree difference in IC on that cylider's cam lobe since we are not grinding a different lobe for each cylinder(although the new 4 pattern Comps are trying to get closer to that) and if I have let's say a high end motor to squeeze every HP out of it how much cost/ HP benefit a custom cam ground with let's say 2-4 different intake lobes and the same for exhaust lobes(maybe a little easier for an exact equal length header for a chassis car) going to make us. Cost/HP benefit is the game we all play just on different budget ranges

Then some folks feel having different cylinders with different rpm peaks based on intake runner,exhaust runner are acutally better as they make a broader more balanced TQ curve.

So how much increase/decrease for a 1 degree difference in IC on a high end motor? Anyone dyno and change that only 1 degree at a time? Bill you guys do any 1 degree at a time?

FWIW my machinist finds even high end rods,cranks off also, maybe not as often as the Chinese stuff, but even Oliver sends out rods that aren;t right.

[slowbird]

With all this talk about accuracy getting the cam degreed in just right, anyone think about how much the cam twist due to spinning a 60-80lb oil pump off the back of it? What does that due to the cam timing events? Like I said earlier we're splitting hairs here. The hole process is filled with accuracy issues once that engine fires up in my opinion.

[mike leech]

I want my 13 minutes back

[gearbanger]

All these things are the difference between a person building their first engine and Hendricks motorsports trying to win another championship. If you think its "good enough" then it probably is, for you.

[cfmcnc]

Originally Posted by slowbird

With all this talk about accuracy getting the cam degreed in just right, anyone think about how much the cam twist due to spinning a 60-80lb oil pump off the back of it? What does that due to the cam timing events? Like I said earlier we're splitting hairs here. The hole process is filled with accuracy issues once that engine fires up in my opinion.

Thats why we use the biggest cam barrell that will fit and external oil pumps!.Bill C.

[67cruiser]

Gearbanger!!! Thank You , i guess you understood what i was asking & maybe i did not ask the right way. Appreciate your input and help, makes things a little clearer now.