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Old 11-13-2011, 11:26 AM
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Valve seating; air tight?

Had a valve job done on my LT-5 DOHC engine, assembled the motor and pressurized the cylinder, there is some leaking around the valves. I have a very experience machinist who is well know, I brought them back to him...had him check them again......he said that he checked them with machinist ink and all show a good seal.

Thoughts? I am a newbie to engine building

(There are no cams in the engines, I orginaly did this test to see if the cams was holding the valves open)

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Old 11-13-2011, 12:45 PM
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Air leakage on a removed/dirty cylinder head can be caused by carbon/crud deposits on the seat. Also if you blast the air at the valve/seat with shop air and an air nozzle. You will get air leakage.. Spring seat pressure is 60-80 lbs. Shop air is 100-150 psi.. will raise valve from seat..

If the head/valve seat will hold solvent after assembly. It is sealing.
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Old 11-13-2011, 06:07 PM
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leaking valves

turn head on backside and pour some gas into chamber and watch for leakage -should not leak - your trying to retain combustion pressures eh
not just comp pressure . if you can make thos valves leak with shop ait you
got a problem going on !!!!!!!!
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Old 11-13-2011, 06:42 PM
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i imagine what he calls machinest ink is probably the same way i checked for sealing on my valves. The trick my gpa and myself do ( because he taut me ) after you've ground the valve seats take the valve and mark all the way around it with a magic marker and then put the valve into the cylinder head and press tight and turn just a hair then take it out and see if marker is rubbed all the way around. ive never had a problem with valve seatting after doing this.


i'm just throwing in what i imagine he did to test it.
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Old 11-13-2011, 06:59 PM
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Likely used prussian blue, if the dye is showing a clean, even, mark all the way around the face, with basically the same transfer to the seat, you are good to go. Remember, these are sealing combustion pressure , so forces are going to be trying to hold the valve closed, even more force than just spring pressure.

If, after the blueing test, you can put solvent in the port, and none leaks past the valve (spring installed), you are good to go.

EDIT:

I see you are putting air into the cylinder, so it would be forcing the valve shut. One thing to remember, though, is that these valves are not "run in" yet. they will seat more positively after 1000 miles or so. You could do a light lap, if you are concerned.
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Old 11-14-2011, 11:06 AM
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I use a device from Sioux that pulls a vacuum on each port to test sealing of the valves, without any valve spring installed. The tiniest imperfection shows up with this test.
If you are leaking on 100psi air, you have a problem.
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Old 11-14-2011, 11:18 AM
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thanks guys doing some testing tonight, will let you know
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Old 11-14-2011, 12:32 PM
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Quote:
Originally Posted by lgaff
Had a valve job done on my LT-5 DOHC engine, assembled the motor and pressurized the cylinder, there is some leaking around the valves. I have a very experience machinist who is well know, I brought them back to him...had him check them again......he said that he checked them with machinist ink and all show a good seal.

Thoughts? I am a newbie to engine building

(There are no cams in the engines, I orginaly did this test to see if the cams was holding the valves open)
Normal, valves don't make an absolutely tight seal till they've been run a little. The valve spring just isn't enough force to pull the seats tight, it takes running cylinder pressures and a few heating/cooling cycles to finally bring it in.

Bogie
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Old 11-14-2011, 02:50 PM
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How did you test this on an engine? Are you positive it was air leaking past the valve? Which valve?
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Old 11-14-2011, 03:38 PM
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Ran an air hose with 50 PSI into the spark plug hole, there are no cams in, just applies air and some air leaks past the valves.....
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Old 11-14-2011, 03:43 PM
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Quote:
Originally Posted by lgaff
Ran an air hose with 50 PSI into the spark plug hole, there are no cams in, just applies air and some air leaks past the valves.....
Yeah, pretty normal, as I said. The minimum to seal them up would be to go around hitting the stems of the offenders with a hammer to snap them open and shut, that might let the spring pull the valve shut with enough force to make a seal. But like I said earlier it usually takes a running cycle to get 'em leak tight.

Bogie
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Old 11-14-2011, 03:56 PM
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There are some engines that are more prone to leak when the head is torqued down so they must be machined with a torque plate to achieve 100% seal.
I dont know if an LT5 might be affected in that manner.
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Old 11-14-2011, 08:08 PM
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Leakdown test:

Cyl-%
1-22%(Exhaust valve leak)
2-8%
3-12%
4-14%
5-31%(piston rings)
6-10%
7-35%(exhaust valve seat/piston rings not sealing)
8-5%

These head bolts are torqued to 120ftlbs....wonder if that plays into the valve seal. The rings are new and liners/pistons are usedand matched sets(nikasil)
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Old 11-14-2011, 09:05 PM
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I would be concerned with some ring leakage with new rings.
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Old 11-14-2011, 09:40 PM
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Valve seal

Dudes I am new to this site and hate to put a damper on your thoughts on valve sealing but after a decent valve job there should be ZERO leakage. If that is the case find a new machinist. Valve seats do not get rounder or flatter after they run because leakage causes burned seats from combustion blow by. If my vacuum tester does not pull 25in. I go back and recur the seats. It is also critical that all of your valves are checked on a good seat and confirmed to full pull on vacuum tester.
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