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I've lapped my valves and want to check if they are sealing properly. Everywhere that I've read talks about using stuff like, acetone, gasoline, paint thinner... to fill the exhaust/intake ports and see if they come out on the other side.

I'm curious why people don't just use water?

And is a little leak like 1 drop every few minutes acceptable? Or should it have no visible leak for x amount of time?

Thanks

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If I were to suggest to use thick honey, you would quickly respond that honey will not find very small leaks. That is the answer really, since we are trying to find a gap where exhaust gasses under pressure could leak from, we need to use something with very low viscosity and is visible to us, to find the leak for us. Look at this chart, you will see that acetone has less than half the viscosity if water, so it will find the leaks a lot easier than water will.

http://www.engineeringtoolbox.com/absolute-viscosity-liquids-d_1259.html

As far as how long you wait and how many drops you get, I would have thought a drop of acetone every few minutes would equate to a lot of exhaust gas leaking.

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I'm curious why people don't just use water?

People do use water, as Jafro shows in this epic video, but he also qualifies his statement in the video description:

I'm using tap water for the test because both cylinder heads I'm testing will receive extensive machine work and cleaning before being re-used. If you were to do this test on a freshly-machined head, you'd want to use deionized water as it contains none of the salts (sodium, chlorine, etc...) that would leave deposits and corrode metal parts.

So water can leave behind deposits or result in corrosion. Given you have just lapped the valves, I'd avoid using straight water for peace of mind.

And is a little leak like 1 drop every few minutes acceptable?

Or should it have no visible leak for x amount of time?

The video explains this quite well. You're best off waiting for something like 30-40 minutes in order to see if the meniscus has dropped. You should also inspect the inlet and exhaust ports for droplets to help pinpoint the source of the leak(s).

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