Finding True TDC

[Nditiz1]

So I have a dial gauge, I have a degree wheel. Let me run this logic to find true TDC.

If I line up Z1 with the split in the case and set my digital degree wheel to 0 and have my dial gauge on number 1 retainer I can rotate the engine 360 back to Z1 to get my cam timing.

If I then rotate until I hit 4.0" of lift and record the degree and then rotate, passing over max lift, to the other side of that lobe and record the degree at 4.0" again I should be able to effectively find how far off my digital degree wheel and Z1 are so that I can correctly match Z1 to 0 degrees.

This all stems from my cam timing being .4mm on the left side and .45mm on the right. Maybe I am just splitting hairs and I'm within spec so, good enough? I realize when getting back to Z1 a slight mm before or after the split in the case will show on the dial gauge and can make the difference between being with in spec or not.

[cgarr]

So you are trying to split the "dead" area at TDC? You need the indicator right on the piston center line to avoid any piston rock, To check or set the degree wheel it should really be done with the crank only in the case and dial it in on a rod journal then you can check all 6 AKA splitting hairs lol

[Nditiz1]

So why do people even use a digital degree wheel if Z1 is good enough?

[cgarr]

To time the engine, I have one where the magnetos are set at 28 degrees BTDC

[Nditiz1]

Assuming Z1 is lined up with the case split, the timing can be achieved with just a Z-block and a dial gauge. Am I missing something?

[cgarr]

1975 2.7L CIS Cam Timing Question

[911pcars]

Added: While the factory TDC notch is fairly accurate, engine builders should double check anyway - just in case (manufacturing error stack and all). On other cars, there's a damper attached to the pulley OD that can shift or be replaced. At that point, the factory notch may not exist or no longer be accurate.

To find true TDC, you need a mechanical stop. Reason why; at TDC the crank can move X degrees with no/little measureable movement of the piston.

The mechanical TDC procedure briefly works like this:
1. Adjust the stop to block #1 piston somewhere near TDC. At that point, mark the crank pulley opposite the reference mark on the block.
2. Rotate the crank in the opposite direction until the piston again stops at the stop. Mark the crank pulley again.
3. Measure the radial distance between your two marks. Midpoint is TDC.
4. Ideally it should match the existing TDC mark (notch) on the pulley/damper. If not, re-notch and ID it with some white paint (or equivalent).

Use that corrected TDC mark as a reference for cam and ignition timing.

TDC stops can be homebuilt or purchased over-the-counter.



Sherwood

[winders]

You can do this with the engine in the car using something like this:

https://www.amazon.com/DEWHEL-Center-Locater-Screws-Spark/dp/B07TC2CTYB/ref=sr_1_6?dchild=1&keywords=TDC+tool&qid=1618692176&sr=8-6

[911pcars]

If it clears the valves as you rotate the engine, fine. That would be very convenient, but does it clear? The Stomski tool obviously clears. If you don't use the Stomski tool, make note of the OD of their stop bolt probe.

https://youtu.be/2CxwYtjEjGI

[Emo993]

As Scott Winders said. This is what I used. Will send if you want, just send back.Mark

[winders]

Originally Posted by 911pcars

If it clears the valves as you rotate the engine, fine. That would be very convenient, but does it clear? The Stomski tool obviously clears. If you don't use the Stomski tool, make note of the OD of their stop bolt probe.

https://youtu.be/2CxwYtjEjGI

How could it not clear??? It can't interfere with the valves....not with the location and angles of the spark plug holes....

[boyt911sc]

Originally Posted by Nditiz1

Assuming Z1 is lined up with the case split, the timing can be achieved with just a Z-block and a dial gauge. Am I missing something?

Nick,

Yes. You can set the cam timing accurately with just the Z-block and dial indicator. This is on the assumption that the crank pulley is good and Z1 is perfectly line up at top dead center. But it would be more convenient and easier to do a cam timing using a degree wheel with a TDC tool.

If you take 5 readings using just the crankcase split method and another 5 readings using a degree wheel with SR-022, you will find that the measurements you collected using the degree wheel is more reproducible or closer to its other than the case split method.

The cam timing using the case split method is good and accurate but requires more concentration or focus to get reproducible numbers. The degree wheel method is easier and convenient to use with greater precision. This is like shooting in a target range with a rifle using a scope and you hit the bull’s eye almost all the time compared to a non-scope rifle. Once you tried a degree wheel for your cam timing, you will be reluctant to go back using the case split method. Both methods works and a choice you have to decide.

Tony

[BURN-BROS]

I have seen a stock crank pulley off 1.5 degrees. That’s a significant deviation. Use a piston stop and calculate tdc if you want to do it right.

[OSC911]

Originally Posted by winders

How could it not clear??? It can't interfere with the valves....not with the location and angles of the spark plug holes....

Yeah, i was thinking the same thing

[911pcars]

Originally Posted by winders

How could it not clear??? It can't interfere with the valves....not with the location and angles of the spark plug holes....

I didn't say it wouldn't work, but to be careful. I haven't tried it. LMK.

[winders]

Originally Posted by 911pcars

I didn't say it wouldn't work, but to be careful. I haven't tried it. LMK.

You are raising alarm for no reason. If you don't know if it works, ask....

I would not have suggested the item I did if I did not know if it would work. I recently used a part like that on my 3.6L race engine to verify TDC on a custom crank pulley.

If I did not know if the part worked on a 911 engine, I would have included the caveat that "This might work...maybe someone here can say for sure".

[Emo993]

Worked fine on my 2.2E with S pistons and Mod soles cams. Best, Mark

[dannobee]

Originally Posted by BURN-BROS

I have seen a stock crank pulley off 1.5 degrees. That’s a significant deviation. Use a piston stop and calculate tdc if you want to do it right.

I've seen crankshafts off too. They're not always indexed correctly, and pistons often have offset pins. Use the positive stop method to verify what you're working with.

[Nditiz1]

Thanks for the responses everyone. I think I was getting caught up on setting my degree wheel to true TDC. If I assume that the mark of Z1 is True TDC for number 1 cyl and I 0 out my degree wheel then I am good to go, but if I am ever so slightly off because the split in the case and I 0 out the degree wheel I will be that same amount off 360 out. Splitting hairs with this, but its the first time working with the inside of this (flat 6) engine.

I did come up with .47 mm of overlap on the left side, but .33 mm on the right so I'm going to adjust the cam on the right side to get it closer within spec of the book.

Now to acquire some cam tools

[boyt911sc]

Originally Posted by Nditiz1

Thanks for the responses everyone. I think I was getting caught up on setting my degree wheel to true TDC. If I assume that the mark of Z1 is True TDC for number 1 cyl and I 0 out my degree wheel then I am good to go, but if I am ever so slightly off because the split in the case and I 0 out the degree wheel I will be that same amount off 360 out. Splitting hairs with this, but its the first time working with the inside of this (flat 6) engine.

I did come up with .47 mm of overlap on the left side, but .33 mm on the right so I'm going to adjust the cam on the right side to get it closer within spec of the book.

Now to acquire some cam tools

Nick,

Use your degree wheel + TDC tool + dial indicator to do your cam timing work. You can NOT use the degree wheel without a TDC tool because you are relying on the assumption that everything is OK. What if the crank pulley if off a few degree? Use your tools correctly.

Tony