Testing quality of valve seatin

[Trevorc]

Hello one and all.
What is the considered opinion as to the best way to test the quality of the seating on inlet and exhaust
valves on i/c engine models.
I read that many prefer a vacuum test applied at the appropriate manifold inlet. Why?
What is wrong with a pressure test by pressurising the cylinder, this is more readily arranged.
Thank you for your comments.
Merry Christmas.
Trevorc

[Roger B]

Both will work, as will applying pressure to the manifold. All will detect small leaks across the valve seat. It depends which is easiest to set up for your particular engine. If you pressurize the manifold the valves will open at some pressure.

[cnr6400]

Fundamentally, I suggest pressurizing the cylinder (as it will be pressurized when running) and detecting valve leakage is a better test than using a vacuum on the manifold or manifolds. If the cylinder with valves closed does hold pressure and has low leaks or no leaks, it and the valves are in a run-ready condition. Mechanics on full size engines have used this test, in the form of the compression tester, since the turn of the 20th century, or earlier. Well proven.

I think a vacuum test on the manifolds could detect lots of other leaks that may or may not affect the engine running, but also may detect a valve leaking. Vacuum in this case could give false or misleading results. (a perfect vacuum by definition is a whole lot of nuthin')   

Cylinder pressure test detects what is happening in the cylinder, where the action is. 

[Admiral_dk]

Cylinder pressure test also includes the piston ring sealing .... this can be good or bad - depending on your conclusions .... Ie. the pressure is dropping too soon -> my valves must be leaking, when it actually is the rings or vice versa.

A third not yet mentioned solution could be filling the inlet tract with fuel, alcohol etc. to see if it runs past the valve and likewise with the exhaust. NO - I'm not claiming this to be a better test, just another option.

Per

[Mike R]

Fundamentally it comes down to this: 

• are you trying to prove that it all seals, or
• are you interested in how an individual part seals?

I would suggest cylinder pressurization or leak down testing could be done on a "built" engine as a form of trouble shooting (i.e. is there a leak?).  Pressurizing the cylinder means that all the sealing areas are being tested simultaneously, however if there is an issue it can be harder to track down the source (piston to cylinder fit or piston rings, intake valve or exhaust valve).
Individual valve testing with vacuum applied to the individual valve manifold is more inline with new engine construction and verification of a good fit and mate of parts.  Ensures that as you go your engine should not have valve issues when you go to run it.  Especially useful on multi cylinder engines to not have to chase a "bad" one later on.

Just my take on it


Mike

[Art K]

I just thought I'd throw this out since Per mentioned a compression test. This assumes an assembled engine. One of the tests I've seen is to run the compression test, if it is low squirt some oil in the cylinder test it again. If it then raises the rings are bad, if it doesn't change its the valves.
Art

[Zephyrin]

a vacuum test during the making is useful, vacuum is easily perceptible, if not, go further with the running-in process.
the compression test with the whole cylinder head is the final test, no noticeable compression in this case, a valve cage leaks badly!!!!

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