Cylinder head vacuum test sufficient? - Honda Prelude Forum

b16b18 · 10-08-2007, 12:09 PM

Just got my head back from port/polish and valve job. I'm a little concerned about the valve seals even though all of the chambers passed the vacuum test. If I shine a flashlight from under the head around each valve, I can see cracks of light when looking inside the chambers. If I put a few drops of water inside the chamber, the water very slowly starts to leak through after a few minutes (see pic). If I shoot compressed air around the edges of the valve, I can get the drops of water to bubble (see vid). Is this normal? I expected the valve seals to be air tight. I never performed these tests on the head prior to getting the work done.

So I lapped one of the valves with fine lapping compound and re-tested. No water leak now and the bubbling has reduced. I took the head back to the shop (which by the way deals with a lot of high profile Hondas) where I saw them vacuum test the head with my own eyes. So why do my tests fail? I realize the compressed air test is trying to force a lot more psi through. Should I lap all of the valves? The shop said the head material is hard enough that lapping wouldn't do much. They suggested to run it as is and don't think I should be concerned.

I just don't want to sacrifice any compression, it's an all motor setup, I need all the compression I can get!! Any advice would be helpful..

Pic
http://dmostudio.com/Temp/waterleak.jpg

Video
http://dmostudio.com/Temp/airwatertest.mpg

marcucci · 10-09-2007, 07:08 AM

While I don't think in the end it will amount to much, this is definitely unacceptable work. I wouldn't bolt it on without at least lapping, but they should be doing the work, not you.

Also, if they had properly machined the valves and seats, there shouldn't be a problem. this tells me that they either didn't machine one or the other or did something wrong.

b16b18 · 10-09-2007, 09:54 AM

The valves have been machined as well as the seats. The seats are very thin now. I'd say about half the width of the stock seats. Also, the valves and seats are very smooth after the machining. Lapping roughened up the seat a bit which eliminated the light shining through and the water leak test. Am I just being paranoid?

71dsp · 10-09-2007, 12:06 PM

What do you mean "the seats are very thin now"?

You want the machined surfaces to be perfectly smooth, not rough.

Did the shop vacuum test one of the ports for you? If so, what were the results?

Have you tested the contact pattern between each seat and each valve with some Dykem?

Some builders purposely create a very thin margin of contact between the valve and the seat with the anticipation that the valve will conform to the seat when the engine is run (I'm talking a difference of valve seat angle and valve face angle of around 1 degree, so not a huge difference).

Be very very careful when lapping. You can do more harm than good if you're not careful.

b16b18 · 10-09-2007, 03:35 PM

The width of the newly cut seats on the head are thinner than stock widths were. I am very hesitant to lap any more valves. I've only done one very lightly to see what affect it would have on my tests. I brought the head back to the shop over the weekend for them to look at it. They vacuum tested all of the ports in front of me. They all checked out good. I can't remember the exact vacuum #'s but the machine measures for something around -.9 bar?

71dsp, what do you mean by testing with a Dykem [marker]? You mean, testing if the valve touches the seat all the way around?

71dsp · 10-09-2007, 04:44 PM

Yes, you put Dykem on both the valve face and the valve seat, then use a little compound and very lightly lap a valve just enough to wear away the Dykem to see the contact pattern.