Positive Stop Tool

[Anonymous]

For degreeing my cams I will need a positive stop tool to find TDC. What does everybody use? Just a long bolt screwed into the spark plug hole? If so, what is the plug thread size?

Thanks,
Mark

[pano]

I drilled out an old spark plug for my engine. I tapped the center hole to 8mm because that was the tap I had that fit. I just need to get an 8mm bolt and locktite it in place.

I ordered a degree whell last night and plan to do mine in the coming weeks.

[Paul]

Use a dial gauge so that you can see exactly when it is at TDC, the lock your degree plate in place aligning the 0 degrees to a pointer.
Rotate the engine backwards & forwards to ensure you are at TDC.

[rosco15]

I also just used my dial indicator down the piston hole right on top of the piston.

[SqDancerLynn1]

Make your own degree wheel

http://www.grimp.net/ryan/degree_wheel.pdf

[Einar J Skog]

Hi

You must think that the piston is almost standing still a few degrees at the wery top.

Using a positive stop is 100% accurate.

I go for the plug version.


Einar

[biker66]

I have a picture of a modified plug and description on my site, at modifications/piston stop.
Cam timing instructions are also available.

[RacingJake]

Hi

You must think that the piston is almost standing still a few degrees at the wery top.

Using a positive stop is 100% accurate.

I go for the plug version.


Einar

That is a very good statement, thats why I use the biggest degree wheel.

[Anonymous]

Make your own degree wheel
http://www.grimp.net/ryan/degree_wheel.pdf

Thanks, Lynn. I was just about to start looking for that link again.

You must think that the piston is almost standing still a few degrees at the wery top.

Using a positive stop is 100% accurate.

I go for the plug version.

I don't trust a dial gauge because it is very hard to see exactly where TDC is and you can be off a degree or two quite easily. I trust the positive stop method because it uses absolute geometry and, as an engineer, I trust in that more than my ability to find TDC perfectly with a dial gauge.

I have a picture of a modified plug and description on my site, at modifications/piston stop.
Cam timing instructions are also available.

Very helpful, thank you. Good instructions and useful pics, too. You should change your lobe center formulas to include some brackets for the correct order of operations, though. It should really read:

Intake: [R1 (BTDC) + R2 (ABDC)]/2 - R1

Exhaust: [R1 (BBDC) + R2 (ATDC)]/2 - R2

Thanks for the help, guys.


Mark

[rosco15]

Somebody help me here. I don't see how a dial indicator isn't accurate.

If I check the piston at say .200 before tdc and get say 20 deg. Then I spin the motor back and at .200 after tdc I get 20 deg. I would say I've found true TDC. I started out using the positive stop on an old kaw I had, but hit the valves. After that I went to the dial indicator and never thought it wasn't accurate.

[Anonymous]

Somebody help me here. I don't see how a dial indicator isn't accurate.

I was going to mention your method as a reasonable way to use the indicator, but I got in a rush...

That works just as well as the stop method and is really the same thing in end. It's using the indicator to try and find where the piston changes direction that is not accurate

Mark

[oldschoolGS]

Modified spark plug for me-simply remove the core and weld in a blunt edge stop.

[Paul]

If I check the piston at say .200 before tdc and get say 20 deg. Then I spin the motor back and at .200 after tdc I get 20 deg. I would say I've found true TDC. .

That is the best & most accurate way of finding TDC.
Rereading my earlier post I see that I didn't explain it very well.

I started out using the positive stop on an old kaw I had, but hit the valves.

This is why using a dial gauge is better.

[Anonymous]

Marcus....

Positive stop is most accurate, in my opinion. Using a stop with a little length to it puts you on the steeper slope of the curve for piston rise and fall - in other words - the steep slope of the sine wave that describes the displacement of the piston with crank rotation.

I will also say that either method is sufficiently accurate that the difference does not matter.

I took a plug, broke the electrode center and brazed in a 8 or 10mm bolt. It stuck out about quarter of an inch I believe. I rounded the edge of the bolt a little so it didn't scratch the piston. Don't get brazing on the threads of the plug.

You can measure the depth from the top of the piston at TDC to the top lip of the spark plug hole to get an idea how far the bolt should stick out of the end of the "stop plug". A vernier caliper works well for this - use the depth gage end of the caliper. Good luck. - Dieter

[RJ]

www.aperaceparts.com and look under tools.