Valve Job Question, leak and light testing
 

Hi Guys,
I sent away my heads to be re-worked by a Porsche centric machine shop in Alliston Ont several months ago. What is still bothering me is when I shine a flashlight up through the ports, the exhaust seat to valves show light (springs on). They also pass air bye when blowing air from my shop vac through the ports. I know the valves are ok, they are new and when I rotate them through the cylinder head, the gap always stays in the same location, also swapping valves has the same effect. A 0.001 inch feeler gauge still catches but doesn't pull as much in the same areas so clearly the gap is less than that. I took it to a local old school mopar type guy here and he pulled 18 inches of vacuum, if I recall.

Please, does this look reasonable? It may not be perfect but what do the experienced out there think? I'm ready to re-install but decided to ask.

Regards,
Phil

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They should seal better than that. I’d lap those valves in before I reinstalled the heads.
Tony

Thanks Tony,
I bought some Permatex compound, but the grit looks too course (particles range from 120 to 220 and grinds down to 220, according to the package), locating 400Grit lapping compound is not easy here, for some unknown reason.
Phil

Absolutely unacceptable. They must seal completely or they will burn in short order.

-Andy

Yup. A LOT of machine shops will cut new seats on the heads but won't reface the valves. I have seen this many times. It takes two minutes to reface the valve once the spring is off, unless they cut the seat incorrectly as well.

Phil,

Something is very odd here. There is no way you would be able to pull any vacuum with a gap of that size. Do not fit those heads as they are.

Give the heads back to whom ever did the work for you and ask them to do the work correctly.

If they ask why, ask them to vacuum test them in front of you. If they have no way of testing the sealing, they shouldn't be doing this work. Then you have to decide to give them another chance or ask for your $$$ back and go elsewhere.

The problem could be guides too, but regardless where the problem lies, its just poor poor workmanship. Tell tale to me is just how low they cut the seats. There is a limit, then the insert should be replaced.

Its not just about obtaining a good seal, its also about how much less the seat will flow and lower the engines performance. For example if you lost 10 CFM per seat, that's 60 CFM less in total and that will make a difference in how the engine performs. So you do not have to mess up the seat work to hurt the engines performance a lot. These heads suffer from flow when the seat is too low. They perform well when they are high. You can cheat this by cutting away the chamber some, but now you are increasing the chamber volume and decreasing the CR number. Also you will see the rocker adjuster further in, in the rocker arm.

This is typical of a lot of production machine shops. They work on the principle that the customer will not check their work. This is why you should use one of the well known Porsche machine shops that frequent this forum.

Valve seat sealing....
 

A simple test to check if the valves are sealing is to pour some liquid over the cylinder dome and observe very closely for any sign of leak. Install an old spark plug and pour sufficient liquid like gasoline or rubbing alcohol to test for seepage.

There’s no way you could produce a vacuum with that gap. Send them back to the rebuilder. This is unacceptable workmanship. You are lucky you found this problem before assembling the engine.

Tony

Hi Guys,
Thanks for your input. So, first thing is I ordered a "2 in 1 Brake Bleeder Kit & Handheld Vacuum Pump Tester, Vacuum Pressure Range: 0-30inHg, 0-760mmHg" from amazon, be here tomorrow. I can also use it to test decel and aux air valve. For 35$ bucks, it's about time. I want to see the vacuum it can hold for myself. As a note, the machine shop guys who checked the vacuum are a different company, local, but they didn't spend much time on it as they are not Porsche machine shop specialists like the one I sent this to and they knew they weren't going to get my business, just a friendly helping hand. I thought they said 18in and 20in for intake, but it was months ago.


Edit: Guys, to clarify, I have a 1thousand inch feeler gauge cut to a point. I removed the springs and feed it between the valves and seat, then press the valve down by hand a little, to see if the gap is less than 1thou, my thinest feeler. It is but in the areas where light shines through, the feeler is definitely encountering less resistance, so the gap is only gauranteed to be less than .001in. That's what I meant in my original post, should have been more clear. But again, the light and the bubble tests were with the springs on.
Phil

I would NOT send them back to the same company. Heads are valuable and better go to someone else ! Don't let them touch your heads again. Forget about sunk cost

A seal is a seal. A leak is not a seal. Forget about feeler gauges, though, fine pointed or not. There is no plus or minus on a seal.

Since you think your valve grinding compound is too coarse (maybe someone here can tell you it is fine, though), use Prussian blue. Coat a seat, install valve, and rotate valve head round and round. Remove and look for anywhere it wasn't worn off. If any, seat not true.

Repeat with blue on valve. Same deal. If it isn't worn evenly, the valve wasn't ground true.

I made a plate I can bolt to the ports, with a gasket to seal it, and a tube in it to pump air in or suck it out, using the same tool you bought. Did I see photos of soap bubbles, showing how air passed the seated valve into the combustion chamber? You don't need any significant pressure to find a leak - just a small differential will blow bubbles. You could blow through a rubber or other tube, long enough to bend around so you could see the valve head while blowing. And there shouldn't be any at all with you just holding the valve head in place with a thumb, must less with the valve springs.

Seeing light is kind of huge.

I suspect some of us might like to know which shop did this. Trond wouldn't - he's in Norway. The guys like Neil don't care, as they do their own work. Others might.

On the other hand, it is appropriate to start with whoever did this. Discussion will give you some more insight into how they do business. Me, I can't imagine not lapping the valves in at least a little bit, just to see. Or at least using blueing to see where the contact is, and how wide. Plus doing a vacuum or pressure test before the work goes out the door.

I once bought some reconditioned EFI injectors from a shop which sells them, or reconditions yours. Then I wondered why my engine wouldn't start (first go around with EFI). Turns out they wouldn't open. Called the company. Oh, blast. My son must have forgotten the final step, which was to run some lubricant through each injector before shipping. By then I had purchased other (much more expensive) injectors, so he sent my money back without a fuss. I'd do business with him again. They did know what they were doing. I now know I should test them, though.

If new valve seat was installed, it might not be parallel with the valve guide.

As a company owner, I would appreciate knowing if I did something wrong before the world found out. We all have to feed the family, our employees do the same. We all can make a mistake. Its about how we fix it that counts at this stage.

FYI, if a new insert was fitted the seat should be concentric to the guide as the guide should be used to center the pilot. If the guide is way out of spec and EX guides typically are, this could the cause. If the seats were ground (stones) or cut on a Sunnen seat and guide machine, for example, a tapered 'dead" pilot is used to locate the pilot. But once the seat is cut, the valve can "flop" around in a worn guide as the stems are straight. One of the reasons why I like "live" pilots.

You can fit the valve and push down on it and it should deflect enough to seal. This will give an idea the seat is not correct as well. If you decide to lap, ( good luck with that), it probably will take a while to get it right. Bounce the valve hard back on the seat and it give of a "tink" sound. If its like a "thud", its not sealed. You will never hit the seat as hard as the engine does when the engine is at RPM.

That will not “ lap out.”

Guys,
No new seats, guides all replaced and measure in spec using a split ball gauge. I also don't think lapping will help on some of these so I'm not trying yet. First I'll see how much vacuum they hold, then I'll try the liquid test but wouldn't vacuum be a better judge of this? I put rubbing alcohol around the valve last night without the springs in, just to see what happens, nothing came pouring out, that's for sure, but by the morning either it had evaporated or dripped in, which I think happened because I marked the seat with permanent marker and the alcohol disolves it as it runs across, so I could see where the leaks were, but I need to repeat this with the springs plus spark plugs in and more liquid, just as I did before they were sent out, and they did not leak for a whole week with degreaser in them before being sent out, but I don't think that means too much as the seat can be gummed up and prevent leakage anyway. I was just getting warmed up.

I'm kinda on my own with this, with your help maybe some sense will come of this.
Phil

They are absolutely not acceptable. Sadly, I had the exact same issue a couple of years ago and the job was by a well regarded shop. When I called the shop owner out on it, he showed me they held vacuum. Indeed they did, that is with the pump on continuously, as soon as the pump was turned off, vacuum vanished. It's sort of like saying a boat doesn't leak if you have the bilge pump running all the time. They need to make it right without cutting the seats too deeply or other injustices. Good luck.

You're "seeing the light" with the springs off? Or is the head assembled? Since I see the valve spring compressor in the pic, I'm assuming that you took the springs off.

Of course this isn't something that we like to see. The seat "concentricity" is less than ideal. Every cylinder head guy that I know checks concentricity on every valve seat for just this reason.

If the springs are off, it actually makes it look worse than it is. The Porsche factory spec for concentricity is 0.002". It could be that everything is within spec and you still see light with the spring off. And if your 0.001" feeler gauge drags, it could be within spec (although a seat concentricity gauge is the proper way to measure). Granted, ideally we all would want to see 0.0000" concentricity and no light even as we spin the valve around with the spring off. But if we're using the factory specs, it could be that the guy followed the Factory Workshop manual to the letter and ended up with this.

Now, in practice, what does this all mean? As Neil alluded to, once the engine lights off and the valve pounds the seat, it'll seal. You might find a slightly tighter valve lash on that cylinder the next time you check. If you just installed the spring and rechecked, the valve will deflect enough to make it seal. And if you're using vacuum to check the seal, it is, in effect, acting as a small spring to help it seal. Serdi's, IIRC, have a vacuum check built in, with a leakage indicator.

Now, how to fix it. Go back to the shop and see what they say. Give them a chance to fix it. All it would take (assuming the guide clearance was within spec), by your feeler gauge check and the amount of light, is a VERY light touch with the cutter or a fine stone. Then recheck the seat concentricity with the gauge.

FWIW, I've seen 911 heads with concentricity off a LOT more than that with leakdown and compression checks that were perfectly acceptable and only came in for oil leaks or oil consumption.

Dannobee,

Thank you for the response. Springs are on in all the light and bubble test pictures. The springs are removed Only to clean the surfaces or rotate the valves or to try the feeler gauge. Funny thing is, with the spring on or off, it hardly makes a difference to the light test? (It's definitely less, but not by much)

I'm going to double check but awhile ago, I tried the original valves that were returned to me and checked the gap between valve and seat and if I recall .0025 inch roughly in the worst spots (to be re-confirmed) due to the old valves not being true. So I figured that it has to be better than before. I wonder if the old valves beat themselves into the seats and no longer rotated therebye achieving a good seal. Before I sent them out, I did the exact same bubble test and light test, Nothing came through except a little on cylinder 4, if I recall. Now it bubbles significantly where it didn't before on all cylinders, but only on the exhausts ports.

I wish I could send it back , but that's not an option and the transportation issues arose on the parts return (broken fins...) so I'll work with what's possible.

Phil

Its easy to sit on the side lines and speculate what the problem is. However, they need to be inspected by someone who knows etc and has the ability to see the fault.
The problem will get discussed here but not fixed.

Be careful about fixing just the one head. If the seat needs a retouch, they all should get retouched to the same height.

If you do have to ship them again, go to H Depot or Lowes and buy a 17 gal plastic Tote. They cost approx $ 11.00. Go buy some cardboard boxes that the heads will fit in and pack those boxes in the plastic tote with some paper padding. Zip-ty the top and ship it. This will protect the heads. We tell our customers to do the same when shipping their cases.

Not to self promote PD, but often in cases like this we offer assistance to those that have gotten caught in issues like this due to no fault of theirs. We will help you out. It will not be free, I have to cover some time. But it will be at a huge discounted and reduced cost. You disassemble the heads completely and ship us all the parts. The more you do and less we have to do the less the discounted cost will be.

We will investigate the fault and tell you what we see. Hopefully its nothing to major, we would fix the sealing issue, vacuum test each valve, measure the retainer heights, and recheck the head heights. You need to tell us the camshaft used. We would test the springs and calculate the seat pressures. Send you a full spring and shimming report, what shims are required under each spring base. You then do the reassembly.

Thanks Neil. In my case, the return packaging was created by the shop, it made it there just fine.

I'm going to study this some more with the armchair pro's first. We'll see how that goes, with all the pain in the ass shipping and delays I've had lately, let's first see if it's as bad as it might seem.

Vacuum checker is coming today. Con: people on rebuild forums have a consensus that vacuum checks are a placebo and mean little unless the defect is very large.

Liquid test, Con: first there is surface tension. There should be no leak, ideally. But should also check contact area.

Marker on valve, rotate the valve on seat to find contact area, done that will show images after I repeat.

Shining light test with spring on: As shown in picture, but can it be deceiving? Stuff is shiny, but most boards about rebuilds say if you see light, uh ohh.

Pressure into ports: Shop vac, what pressure? I'd need to insert a pressure gauge, but before I sent them out there were no leaks... but carbon would have sealed things up, so...

Neil, I'm in Ottawa Can, if you/Craig/Olllies or anyone lived within a 4 hour drive, man I'd be there in 5 hours 4 months ago. This is a specialists game, and in Canada the population is so low that it's tough to find the expertise.

Phil

heads
 

A lot of the time when the valve seats are touched up after new guides there can be a bit of chatter from the grinding method and you always need to lap them in with a valve grinding compound.I bought my last tube on Amazon.Like Walt says after lapping them in use a little Prussian Blue ink to check the seat width.You will feel better to to do this yourself.Fred

Thank you everyone so far. Part 1, vacuum testing at home setup and results.

Setup:
1/4 inch plexi, thermostat o-ring to seal port lubed in dow111, guide bushing for oring to seat properly (white thing), hand pump, Dow111 thick silicone.
Hand pump goes to 25inches max. All stems lubed up, added more oil on valve stem/guide end junction at spring side during testing, no change in results, therefore guides not leaking for the purpose of this test.

Test Try to prove in setup:
No spring, DOW111 under Valve lip and seat: 20.5 inHg. (Second Image)
No spring, cleaned valve seat interface: 15 inHg if lucky.

REAL Test:
Spring On, clean valve seat interface: ~17-18 inHg (EDITED). That's what the local guy I took it to said as well, for the same cylinder. The vacuum is hard to keep constant at that leakage rate so I take the near max vacuum reading as it jumps up.

Does this sit well with you guys? Please let me know, next I'll try the liquid test.

EDITED: I have added a T connection with a one way valve to a shopvac to assist in vacuum in parallel to the hand held vac pump. Reading 19 inHg and possibly 20.

Phil

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I use a very similar arrangement with a brake bleeder, only I use a closed cell foam pad with a center attachment for the brake bleeder hose. Hold against the hole then pump bleeder. Does the job just fine.

this:
https://goodson.com/products/dvc-plates?variant=29264024390

Ivporschepilot,
That looks a lot simpler and would work for the intake as well.

An update:
I've improved the setup by inserting a T splitter and a one way valve attached to my shop vac in parallel with the hand held pump, this gave 5 inHg on it's own. Then I used them together, the result was less choppy (hand pump) and looks like ~19 and possibly ~20 inHg, the hand pump is not the best and has it's limits.

Phil

Hello, I've just done the liquid test results as requested before moving on to the next test.

Summary so far:
All exhaust valve/seat contact areas show bubbling and leak light.
Exhaust valve/seat holds ~19 inHg, but the quality of the handheld pump is questionable, so +/- 1 inHg, say.

Liquid test Results: Springs on, contact area clean, spark plug in.
Exhaust Port: Pooling of liquid, ~15min. Image Included.
Intake Port: No liquid ~1/2 hour so far.

What to make of this so far? I'm off to do the Prussian blue test as requested.

Phil

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Hi, next test done, prussion blue.
Did as recommended, added blue to seat, dropped in valve and rotated several times around. First 5 images show that result.

Then, cleaned off seat, placed valve back in the head and rotated a few times. Last two images of seat. The light test that help start this thread in that spot there is little/no blue, it requires very minute amounts of prussion blue for that test, but it shows up as no contact. Not shown in image but can provide, it's a test with faint marks and I use a jewler's mag glass.

Does the contact area look ok? To my eyes it looks good, at least where the overall contact position is, but I really don't know for sure, should I repeat without rotating the valve as much, say 1/4 turn?

Phil

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Final operation for the day.
Light lapping with the permatex. I don't think there is any chance this will lap out.

A .001inch feeler gauge can be slid into 1/4 of the circumference for the worst exhaust valve/seat (which is the one I've been working on) without even needing to lift the valve. If I press the valve down a little, then it won't enter freely. Always the same area as the light shining through of course and no springs on.

Summary:
Light bleeds through all exhaust six valve/seats with springs on, intakes do not. (Image in first post)
Liquid in chamber bubbles when port fed with shopvac pressure for all six exhaust valve/seats, intakes do not bubble. (Image in first post)
Exhaust valve that was tested showed around 19 inHg vacuum. (Setup image shown)
Exhaust valve that was tested doesn't hold rubbing alcohol as shown in image, takes about 15 min to puddle. Springs on. (Image shown)
Prussian blue shows the exhaust valve/seat contact, but when applied sparingly not in area of gap. (Images shown)
Exhaust valve that was tested accepts a 0.001 inch feeler gauge (cut to a point) between seat and valve for about 1/4 of the circumference without the springs on, but only if you don't push the valve down by hand.
Light lapping has no beneficial or other effect so far.

That's all the info and tests recommended I believe.

Do I lap more? Send it to be re-worked? Put it in as is? What sort of issue would it run into putting them in as is?

Phil

Why are you wasting your time on this? Get the exhaust valve seats cut correctly and then put it together.

Sorry ahh911 but those valves are way off. Something went wrong during the seat cutting.

If I were you, I'd send them to Neil /PD as he has bent over backwards to help here.

I just finished a DIY valve job on my 912 heads for fun and achieved very good results. I know because I devised a bench leak down tester and got 0-1% leak down on the combustion chambers.

Don't even bother with lapping compound, it will make this leak even worse when it gets up to operating temperature.

PM me if you want more DIY techniques, but I'd suggest you send them to a pro.

This really pains me that shops get away this stuff.
Tom

I may very well seek Neil's kind offer of help.

By the way, the last thing I did tonight was test vacuum with the springs on, I get vacuum of 19 inHg as stated earlier. By just adding thick silicone paste around the valve seat to help block any air leakage a vacuum of 26 inHg is achieved. As I added the thick silicone to the area where the light shone through increased the result to 23, but it needed the silicone paste all around the valve perimeter to achieve the 26 inHg. 26 inHg is the maximum the hand pump can achieve on it's own, therefore, in my mind the vacuum test setup is solid and the 19 inHg result is valid. For whatever that means.

Phil

I'm with Eagledriver , why are you wasting time ? Send them back or to Neil .

Sage advises.......
 

Phil,

Nobody could stop you from what you are doing. You could do whatever you want and be satisfied. It is your engine but you are going against common wisdom. Those exhaust valves are leaking and need work. What are you thinking? Anyone in his right mind would realize the condition of these valves are unacceptable. Find someone who could fix those valves correctly.

Tony

Ha, yeah for sure a proper re-exam and re-work would help considerably, especially the experience I am lacking. I have to be careful of the churn though, I need to be sure that transporting six heads across north america and borders is justified in time, risk and cost. Shipping is the risk and time factor (things sit here at UPS for weeks lately in the local depot, another time two months ago I had stuff sit at the border for many weeks and a member can attest to that (rockers), I've also had terrific shipping, but at a cost, another member can attest to that (Cams) , all great work by the way guys.) Also, these heads themselves were dropped I presume on re-delivery to our house, broken fin+damaged fins, though the repacking was not up to my standard. So there is risk in this that I'd like to balance out with potential reward. This is a 81 small port sc, and I can always dissasemble very quickly later in time at little expense to fix things up, I just don't want things grossly wrong or valves head breaking off or valve stem guides wearing down to the point where all hell breaks loose.

I don't mind spending a few days exploring, if you don't mind as everyday I learn something new.
For instance, this morning when the springs were compressed to remove the valve spring retainer clips, the vacuum goes up again to 22 inHg, so it's moving in the right direction when there is pressure applied in the combustion chamber.

One more test I think I'll try, that is leakdown of the head. Plexi, 1/4 inch glued to 1/4 inch thick and mounted to the cylinder head with a gasket cut to the right diameter, or an o-ring of the right size. Clamp them down (just as I did for the port as shown in the earlier images) then go to a shop and have them do a leak down test through the spark plug hole as usual. I think at 100psi, it may be anyone's guess, but since the intakes are very tight, I'm thinking around a percent or two may leak through the exhausts, maybe I'm wrong, that's for sure, but we've come this far. But really, I didn't want to have to do all this, remember I sent these to a Porsche centric machine shop that does work for air cooled race teams here in Canada, so I did commit to what I thought was the correct shop, so now before sending them through our highly delayed and drop prone delivery systems, I'd prefer to see this to a best at home decision based on a battery of tests, the final test being a leak down test of the head. This may take a day to setup, hopefully the plexi doesn't explode! Trust me, I wanted to send them out...

Phil

Update:
Didn't bother with leak down test.

Instead, lapped with 320 Grit Wheeler lapping compound for a couple of minutes (light oil of very minute quantity dapped onto grit on face), light seepage is all but gone (without springs) so I tried the vacuum test, now it pulls to 26 inHG (without springs, the max the handpump can achieve), the handheld vacuum dial is nice and smooth, it no longer jumps around and the needle returns to zero nice and slowly, around 35 seconds (with cleaned valve/seat). A very big change from before where the needle might make it to 19inHg and jump around as you pump and drop very rapidly after you stopped pumping. No spring for any of these latest tests, also, I rotated the valve around and tried a new non-lapped valve as well, same results, 26inHG and good vacuum. I don' t know if it's the rougher surface (EDIT: surface chatter from minimal machine cut was lapped out, you could see the light and dark banding with a very light lapping in areas of light penetration, after some lapping the vertical banding on the seat would diminish, light would not pass through and vacuum jumped and would hold) but obviously it holds vacuum really well and the contact area between valve and seat hasn't noticeably changed and it still sits nicely for road car performance (I hope because that's what I'd asked for, I wanted it similar in performance to when it left the factory) as the pictures above indicate. Also, the valve stem tip to valve spring seat distance are all at nominal (measuring with new uncut valves), so one more seat cut available (that's what I was hoping for as well, but a little more could have been taken off). This was all for the worst head, I'll try the others.

Thank you for your help, it looks like these are fine and I'm going to use them and see how they perform.

Phil

Sounds like you are finding a way. Just make sure that they don’t leak. Intake valves are more forgiving because they are cooled by the intake air and fuel.

-Andy

Lapping valves to seat them isn't advisable. When the valve face and vave seat are lapped at ambient temps, an abrasive slurry between the two helps form a conforming sealing pattern on each part. If the lapped pattern remains constant, the seal will be good.

However, as the engine comes up to operating temperature, engine parts expand. In the case of the aforementioned valve face and valve seat, the sealing ring area migrates as the valve expands from heat.

When both valve face and valve seat angles are identical or if cut with an interference angle, the valve can expand all it wants. The contact area merely shifts due to heat expansion.

Sherwood

Sherwood,
I'm not sure I follow. The seat/valve were cut to within .001 inches both with 45degrees. An additional .001 inch was removed from the face and valve simultaneously. The seal became fluid/vacuum tight when I inserted brand new untouched valves (non-lapped), given the 45 degree angles haven't been disturbed, the valve simply rides up or down the 45 degree seat slope as it heat and cools. So in summary: basic cut was close, a little more was pruned off, brand new valves are inserted in the lapped heads and it seals, so no special seal formed between a particular valve and seat, therefore a generic seal. I expect this to work well. Perhaps you are thinking that .01 inch or greater has been lapped in, I don't know what to expect then, but in this case 320 Grit is used for several minutes, it's really not removing much and shows the precision needed for a really good job, which is in my opinion not what I received, unfortunately.

Phil

Understood. Before there were valve grinding machine tools, valve seating was accomplished by lapping the valve face>valve seat surface. Perfectly adequate for the level of engine performance in thos vintage times. Even when valve facing machines were in use, there were old school mechanics that did it by lapping. Even today, many techs still rely on some level of lapping to ensure a tight seal. Things is, if overdone, the lapping process can create a lapping rut that causes the valve face and seat to be uneven (I.e. no longer a flat, machined surface).

Will the customer know the difference? Probably not, maybe a loss of compression 50k miles down the road. More demanding engine performance goals might result in some premature power loss or power not realized due to some level of combustion chamber leakage.

Can someone quantify the power difference between a leak down difference of 2% vs 4%?

Sherwood

Yup. We tried the gapless rings on a few race engines. Leakdown was zero. So was the difference on the dyno. As in ZERO difference in hp or torque. As long as the valves and rings somewhat seal, there's very little if any difference in power output.

Way back when, when I started racing, before Serdi and Rottler machines, the final test before checking on the flow bench was to put on the VERY lightweight flow bench valve springs and fill the ports up with solvent at the solvent tank. Flush any leftover grinding dust out and see how well they hold. If the solvent held tight on the chamber side, it was off to the flow bench before final cleaning and assembly.

I do my own heads ... I just hold the valve in with finger pressure from the chamber side, hold the head with the appropriate port pointed up, and fill the port with water. If nothing shows at the valve edge after 15 - 20 seconds, I call it good. The valves are stainless - they won't rust...

Valves on cylinders that had 2-3% leakdown (measured while the engine was still assembled) have shown slight weeping with this quick check, so I feel it is sufficient.