Maybe I can add something here no one else has.

You are correct about zero'ing in the degree wheel with true TDC. Using a dead stop has typically some level of inaccuracy. It often has some piston rock included.

The best time to find true TDC is before you fit heads. But often that is not the case, so an accurate piston gage with dial indictor fitted is the next best way.

With the degree wheel fitted with a pointer, as close to what you think TDC is, insert the dial indictor tool and screw it down into the spark plug hole. You need to be carful here for two reasons. Make sure the dial indictor does not bottom out when the piston reaches top and make sure the pointer does not skip across the face of the piston. The plugs are a compound angle to the pistons face. For this reason, its better to do with the head off as you can be at right angles to the piston face.

Bring the piston up to its top position until the needle in the dial stops and turns in the opposite direction. This will be the end travel of the piston. Zero out the degree wheel. This will bring you close. Its always a good idea to repeat this a couple of times. Now. either turn the engine backwards carefully or turn it a complete revolution and stop when the dial indicator reaches, say, 10 units from 0. I have no idea if you are metric or imperial. Read the degree wheel number. it may be 8°. I think from memory, if you are using an imperial indicator, and you stop at 0.020" before zero, the degree wheel will be at 8°. Now turn the engine past TDC until the dial indictor read the same amount other other side of zero. Read the degree wheel. They should be the same. if 0.020" it is, and 8° is correct, then each side of zero, the degree wheel should read 8°. If not adjust the pointer until you read the same each side of zero on the degree wheel. This is splitting the TDC so that you remove any rock. This is true TDC on that piston.

You use the same process when degree'ing in the camshafts. Instead of the pointer on the piston face, you can set it on the spring retainer. If you want the Intake centerline to be 110° for example, you would have full lift on the valve and the degree wheel would show 110° ATDC. To find this, you go either side of zero on the dial and read the degree wheel. Add both readings together and divide by 2. Then adjust accordingly until you are a max of 0.25° from the number you want.

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

To me, the phrase, "Something like this", leaves some room for interpretation. Would a long bolt the same size as a spark plug (14mm) clear as well? Will it work on all 911 engines? 2 and 4 valve? If so, fine. Thanks for the heads up, but confirmation is best. A bent valve as a result of an assumption is expensive confirmation.

With the heads on the engine, a positive stop and a degree wheel should be just as accurate as using a dial indicator and a degree wheel. Just make sure you put the same pressure (enough to get the same piston rock if any) on the piston at each positive stop.

In fact, since you have a positive stop which should have the same piston rock at positive stop, it may be more accurate than the dial indicator method which assumes the piston is going have the same rock but may not. Since it is not a positive stop, who knows?

I'm closer to the end of my life than the beginning and have been doing this sort of work all my life, BUT you learn something every day, it appears.

Amazing, huh?

Yes, indeed it is amazing, but maybe not what you think????

Nothing more to say.

Take your compression tester flexible hose, remove the schrader valve form the end.
Screw into the plug hole, get the engine very close to top TDC then apply some detergent to the end and watch the small bubble grow and reduce in size. You will be shocked at how sensitive it is!

I never did it on a Porsche, but on other DOHC motors I did as Neil suggested but not rotating backwards, always forward. Check the dial indicator on the up stroke, note the marking on the degree wheel, continue rotating until it reads the same number, note the degree wheel again.

Purpose of all this is to take the inaccuracy of TDC out of the dwell at the top.

I think with the Porsche motors sometimes people overthink things. You are probably losing more HP from fuel inconsistencies than you are from piston rock. YRMV

This post may be a little dry but funny as hell.SmileWavy

Oh, no, I understand what you are trying to say...

You two really love to play up the idea you are the only ones that actually know anything. That is what is hilarious......

Of course positive stops have to be bad inaccurate since Neil says they are. Yet they are standard fare in the industry. The industry must be full of nothing but stupid people that are not enlightened like Neil.

I do it the Old Fashion way! Find the mark and take out the 1st plug and turn the motor and I stick me finger in the hole and when I feel the 1st force( May the Force be with You) of the piston I stop. There it is TDC! Had many ALFA's and doing this this for decades and has never failed me. No meters or junk to assist.

I'm a positive stop guy, probably because I was raised on small block chevys and you couldn't trust the balancers, or the cranks, or the machining on the block.

Funny!

Or... maybe not.

It will be fine, just send it! (as the kids say) Do some skids! :)

I'll check true TDC on my boosted 924 engine I'm building now, for fun. Not sure what I will do with it, I don't have a clue what the cam likes in timing. But the engine will have more than enough power to brake everything behind it anyway.

I do feel that I miss a "like button" on this thread...