Methods to find TDC?

It's just all clearances lumped into one. I just put in a value to provide a example of the behavior. But I do agree that while the clearance will close up and squeeze out oil, there will always be a residual film of oil. Surface tension, molecular attraction, surface roughness will all contribute to holding some film.

Edit: To get about 1 degree of dwell would mean closing the clearance up by about 0.005 mm.

 

This is why the Ferrari way exists.

 

Umm, for all of this crazy trying to be super accurate, are you checking TDC and cam timing on each individual cylinder? I can tell you that cranks and most cams are not ground to such precision that you need to be concerned with more than 1 degree of accuracy. We had a car in the shop with a Lampredi motor that had reproduction cylinder heads with rocker stands that were so bad that we had to have cams ground with different timing for each cylinder.

 

Nice large degree wheel. DTI through spark plug hole. 0.200" before zero, read degree, turn until 0.200" after zero, read degree wheel, split the difference ai absolute TDC

 

5

No, it would not be similar o the dead stop method. With the dead stop method the piston is coming up the bore form both sides. That is, the clearance issue is the same and the con rod in in compression for both measurements, thus symmetry. If you approach TDC for one side with the piston coming up (con rod in compression, minimum clearance case) and mark the angle at some position X, then continue to rotate past TDC with the piston coming down (con rod in tension, maximum clearance case) you will have to rotate a slightly greater angle to reach the same value of X. See my post https://www.ferrarichat.com/forum/posts/145740159/ The Pink line represents this case. Notice that when clearances is considered the 175.5mm position is less that 2 degrees coming up and more than 2 degrees going down. But as the position in the bore, X, gets further from TDC the error would become less.

 

I would not call it similar but.... But Brian's method would be like finding the start of the dwell and the end of the dwell and saying TDC is 1/2 between.
How would it tell you where the piston first cones to a stop?

Brian's method will give you TDC somewhere in the dwell. The bigger X (he said 0.2") the closer to TDC, but never exact. If you like, I can modes the error assuming a 1 degree dwell. (Something to do today, for 15 minutes.)

 

Just did a quick check. If you use 0.2" (well I checked 5 mm) Brian's method would be pretty accurate. As good as the piston stop method. The biggest advantage of the piston stop would be that it removes any possible error in setting the piston position by reading a dial gage. Both are ultimately limited by the accuracy in reading the degree wheel. The advantage of both is that you get away from trying to read angles where the piston position is insensitive to angular displacement, like the with Ferrari method.

{edit} Hum, 14 minutes.

 

You still have to read it though. But here is a really simple extension to what he said.

1) bring the piston up the bore anywhere near TDC. Doesn't have to be particularly close.
2)Set the degree wheel to zero.
3) Set the dial indicator to zero at that position.
4) rotate the engine backward to where the dial gage reads 0.2"
5) read the degree wheel. It would be X degrees before the zero reading, say 15 before the zero point for example.
6) rotate the engine forward, past the zero degree point and continue until the dial gage reads 0.2 again.
7) read the degree wheel again. It may be Y degrees after the zero point, say for example 23.
8) subtract, 23-15 = 8.
9) divide by 2, 8/2 = 4

TDC is 4 degrees past the zero on the degree wheel.

Confused? Need a picture?

 

No, starting point is completely irrelevant.

 

Actually no need to set the degree wheel to zero. That just makes changes the math. . Just measure the two angle where the piston is at the same position and TDC is 1/2 way in between to within measurement accuracy plus a little error due to clearance changes. Brian's suggestion is the simplest. I was just over thinking it.

About clearances. If the clearance variation is like 0.005mm, and the stroke is 77mm then the amount the crank has to rotate to take up the clearance is
2 x clearance/stroke = cos(a1) - cos (a2). If a1 is TDC, 0 degrees, a2 = 0.923 degrees. But if a1 = 20 degrees a2 = 20.02 degrees. So the effect of the clearance becomes insignificant on crank angle when you move away form TDC. Again, making Brian's suggestion and the dead stop method basically equivalent, and IMO, this just makes the "Ferrari" method stupid!

 

Well, you see Dave, it's thinking out loud. Bouncing things off one another. That how progress is made. It's necessary to defocus form time to time. I knew the clearance issue was less important as you move away from TDC, but I hadn't taken a look at how insignificant it became.

 

That's be cause what you do out loud isn't thinking.

Ouch!

AIA

 

So when are you pulling a piston out of the 95? I know you want to. It's killing you not to know.

 

Depends on how deep the piston is, whether it's a stop of Brian's approach, when you measure the angles. But it looks like the GM offset applied to the 355 would make it about 1/4 degree difference.