Determine TDC from rocker arm position(s)? Options

[sfrenck]

Was doing the Cap'n (RIP) valve adjustment method the other day and started wondering - is there a way to tell when a cylinder is at TDC by rocker arm position(s)? I certainly can feel when one of the cylinders is compressing air/air leaves the combustion chamber based on how hard it is to turn the non-blocked wheel/tire.

Thought this may be a good way to mark the fan with TDC points for doing a leakdown test soon (can't figure out how to get TDCs on a hot engine for the leakdown).

[r_towle]

Stop over thinking it.
Rotate motor till both valves are closed and adjust.

Or, rotate motor till valve is all the way open (easier to see) and adjust the same valve on the opposite side of the engine.

[sfrenck]

Wasn't overthinking valve adjustment...did that in 15 minutes. I'm asking about tdc for future leak down testing

[Dave_Darling]

You can tell that it's somewhere near TDC if both rocker arms have slack in them; they will both wiggle against the spring between them and they will both rock slightly. That means the valves are both closed, which should be good enough for a leak-down, for instance.

Hydro lifters change that behavior, though.

--DD

[pilothyer]

https://www.youtube.com/watch?v=QtPd5qzyTk8

Look at the flywheel mark lined up with the engine case seam and the dizzy rotor pointed at tower #1 That will be TDC for cylinder #1

[McMark]

Depends on the pressure you're using. Most of the time I find you need to be at true TDC so the piston won't be forced down the bore by the air pressure.

[Mark Henry]

You can use the rocker to determine what stroke you are on, but you still need the line to be truly on TDC.
Spin the engine CW, when the #1 intake valve closes the next time you see the line is TDC of the ignition stroke.

I have to do this on my T4 powered bug, crankfire so no little line on the dizzy to line the rotor onto, because I don't have a dizzy.

I prefer the old adjustment method on aftermarket performance cams.
I've found the base circle tolerances can have a larger deviation than OE stock cams.

[stugray]

I prefer the old adjustment method on aftermarket performance cams.
I've found the base circle tolerances can have a larger deviation than OE stock cams.

Hey! Someone finally agreed with me.
I recall this argument from a few years ago when I said that very thing and no-one believed me...
TY

That is why I prefer using the TDC marks when I set the valves.
I have a white mark for TDC 1,3 and a yellow mark for 2,4 and I can see both from under the car.
So when the white mark is on TDC #1 at the top (like in the video posted above), the yellow mark will be aligned at the bottom.

[Dave_Darling]

I believe that I agreed with you, Stu.

--DD

[Mark Henry]

I believe that I agreed with you, Stu.

--DD

if that was what it was all about then sorry, I thought it was a tangent about rocker arms.

That said to me it's not a big deal, never has caused an issue, other than I do the valve adjustment the factory way.

[injunmort]

just my 2 cents, i think it is possible to determine tdc by rocker movement, not really rocker position. you could measure in degrees crank rotation from when int rocker fully closes and both are closed to when the exh rocker begins to open and then divide by 2 and that will put you at tdc .the point is there are so many degrees of crank rotation/piston travel both before and after tdc where the rockers are fully closed that you would not be able to accurately determine tdc sole by the rocker position.