Finding True TDC
 

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.

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

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

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

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?

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.

http://forums.pelicanparts.com/uploa...1618686427.jpg

Sherwood

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=16186921 76&sr=8-6

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

As Scott Winders said. This is what I used. Will send if you want, just send back.Markhttp://forums.pelicanparts.com/uploa...1618696361.jpg
http://forums.pelicanparts.com/uploa...1618696378.jpg

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

Cam Timing Made Easy...........
 

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

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.

Yeah, i was thinking the same thing

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".

Worked fine on my 2.2E with S pistons and Mod soles cams. Best, Markhttp://forums.pelicanparts.com/uploa...1618745650.jpg

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.

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

Degree wheel + TDC tool...........
 

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

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...