Am I correct that flat tappets, as in a Model T or A, are in constant contact with the cam lobe. In other words, they are always "riding" the cam lobe?
That's the way it's supposed to work. If the valve springs are too weak, the tappet can periodically lose contact with the lobe and then you get the dreaded "valve float".
Thanks Mark, sounds like I may need new springs.
Tappets (or cam followers) on the T are just like any engine. The valve spring holds the the valve against the cam lobe. On a T the valve springs are relatively light weight compared to more modern high speed engines. They work just fine at the speeds a T engine runs.
If you feel that the tappets are not in contact with the cam lobes, how would or do you know that?
When the valve is completely closed the spring is not holding the cam follower against the cam lobe. Not if you have any valve clearance at all.
When running fast the lobe may be touching the valve but not the cam, so the clearance is between the cam and cam follower
Maybe Mike is asking if there is a time when the valve is open and the lifter is not on the cam.??
If that is the case there is valve float.
There should never be valve float.
I agree with Aaron.
With the engine in the running position, I can manually pull the follower or tappet down against the heel of the lobe. The valve is fully closed. I just thought the tappet was always in contact or "riding" the cam; not "stuck" in the "up" position. Does this makes any sense?
Sounds like the clearance is bit tight on the cam follower, you want to make sure it is allowing the valve to close fully. Is this a recent rebuild?
What you're describing is normal. I suspect that when the valve & tappet interface conform closely to one another after many miles of use, the oil film between them causes them to stick together. The tappet does not have to ride on the cam constantly. When the valve is closed the tappet can cling to the valve, or lightly ride the cam heal, or even something in between. The important part is that you have the correct clearance.
To further clarify my statement above, once you have the proper clearance set, the tappet can float anywhere within that clearance that the forces of nature send it.
It sounds like you have a sticking valve. It the valve is stuck in the open position then the tappets will be loose most of the time.