Accurate TDC finding

[zeeshan]

I have a lq9 that I'm installing a cam in and I'd like to verify its operation using a degree wheel. I want to find TDC accurately using a dial indicator and do not want to use a piston stop cause it is not as accurate. I was planning to make a mount for either my 1"/2"/3" dial indicator to mount at the same angle as the spark plug to find TDC. How come no one seems to do this? Am i missing something obvious? I know the dial indicator will be at an angle (huge parallax error), but I am only looking for where the needle switches direction.

[digitalsolo]

Why would a piston stop not be accurate, exactly?

The best way is to pull the head and use a bridge with a dial indicator, for what its worth.

[spacevomit]

That's how they taught us to find TDC at school (using magnet mounts).

[Speedfab]

... do not want to use a piston stop cause it is not as accurate.

100% bullshit.  Piston stop is the ABSOLUTE most accurate way to find TDC.

[zeeshan]

Why would a piston stop not be accurate, exactly?

The best way is to pull the head and use a bridge with a dial indicator, for what its worth.

i think there's two errors that effect its accuracy:
1. assumption that the piston stop is in contact with the piston (anything that you do by feel has an inherit human error to it)
2. divide by 2 on the degree wheel and mark -- there will be human error in marking

i know piston stops get you pretty damn close, but i'm a bit OCD after doing lab grade experiments for my thesis

[zeeshan]

so do you guys forsee any problems with mounting a dial indicator with the way i described? i really don't want to waste my time CNC turning an adapter if it's going to be a fail

[zeeshan]

i think it will work, instead of doing some bench fabricating, i decided to go try it. also used my attachment for indicating internal bores to get rid of the parallax error. just need a way to mount hold the attachment using the sparkplug hole..

[Speedfab]

so do you guys forsee any problems with mounting a dial indicator with the way i described? i really don't want to waste my time CNC turning an adapter if it's going to be a fail

Yes, fail.  You need a positive stop, not a dial indicator.  A dial indicator in the spark plug hole introduces a horrible angularity issue because of the fact that factory LS heads have plug holes approximately 40 degrees from vertical/perpendicular to the piston face.  Your dial indicator should be used to measure motion directly in line with its linear travel, period.  If you do what you are proposing the extension will flex laterally and have a friction component introduced both at the piston crown and in the bore of the gauge itself.  The plug hole centerline also does not contact the piston crown directly above the wrist pin, so there is the issue of error from skirt clearance/piston rock.  A positive piston stop eliminates all of this, and also takes out bearing tolerance errors too.  A dial indicator is a great TDC tool... in a 4 valve engine with centered vertical plug holes or with cylinder heads removed.

The errors here are pretty small, but if you want absolute OCD TDC, A piston stop really is the end all answer.  Sometimes simplest is best.

[Speedfab]

used my attachment for indicating internal bores to get rid of the parallax error.

You didn't get rid of it, you added ~10 (varying through the travel) degrees of angularity to it.

[zeeshan]

so do you guys forsee any problems with mounting a dial indicator with the way i described? i really don't want to waste my time CNC turning an adapter if it's going to be a fail

Yes, fail.  You need a positive stop, not a dial indicator.  A dial indicator in the spark plug hole introduces a horrible angularity issue because of the fact that factory LS heads have plug holes approximately 40 degrees from vertical/perpendicular to the piston face.  Your dial indicator should be used to measure motion directly in line with its linear travel, period.  If you do what you are proposing the extension will flex laterally and have a friction component introduced both at the piston crown and in the bore of the gauge itself.  The plug hole centerline also does not contact the piston crown directly above the wrist pin, so there is the issue of error from skirt clearance/piston rock.  A positive piston stop eliminates all of this, and also takes out bearing tolerance errors too.  A dial indicator is a great TDC tool... in a 4 valve engine with centered vertical plug holes or with cylinder heads removed.

The errors here are pretty small, but if you want absolute OCD TDC, A piston stop really is the end all answer.  Sometimes simplest is best.

How do you ensure you don't gouge your piston when using a piston stop?

[Speedfab]

Cut a piece of 3/8 steel rod about 1.25" long (that length will hit the piston near centered in an LS) and round off one end to a convex hemispherical shape, then take an old spark plug, knock/grind the center and side electrodes off, and weld the flat end of your piece of rod to it.  Take the exhaust rocker off the cylinder you are using, because it will open into the piston stop and either break it off or bend the valve if you dont.

[zeeshan]

Thank you! will try this out.

[digitalsolo]

FWIW, I have a brass piston stop that threads into the spark plug hole.   It's nice and round on the end and about 3/8" thick (just small enough to fit into the spark plug hole).   It won't let the piston move and it won't hurt anything if you don't go Hulk smash on finding the stops.  I think it was 10 dollars on eBay.    It has a hole drilled through the length of it to let the air out of the cylinder so that you can pull both rockers also.

Tracy's way is just as effective and free, too.

edit:  Looks like this: 

[JapCrap]

I guess the old screwdriver trick ain't good anymore with the LS?

[zbrown]

I'd see a positive stop as the only real way to do it accurately

There is so many degrees of dwell up by tdc I don't see an indicator being very accurate