Hand Valve Lapping Needed?
Hand Valve Lapping Needed?
(OP)
I am having leakage on the intake valves of my new Dart heads and valves on my small chevy when shop air is applied into the compustion chamber threw the spark plug hole.
I believe that if I had of lapped the valves there would of been no leakage.
Is it recomended that valves are hand lapped like the old days?
I have seen the same amount of leak on the exhaust side of high milage engines with no problems, but will a leaky intake valve cause back fires?
When cranked over with the starter with no intake mainfold and a little wd 40 down the intake port I can see a puff of mist when cranked back out of the intake port.
This worries me, but there is no cranking pressure diference between leaky valve cylinders and non leaky cylinders.
I believe that if I had of lapped the valves there would of been no leakage.
Is it recomended that valves are hand lapped like the old days?
I have seen the same amount of leak on the exhaust side of high milage engines with no problems, but will a leaky intake valve cause back fires?
When cranked over with the starter with no intake mainfold and a little wd 40 down the intake port I can see a puff of mist when cranked back out of the intake port.
This worries me, but there is no cranking pressure diference between leaky valve cylinders and non leaky cylinders.





RE: Hand Valve Lapping Needed?
As to lapping valves...The current state of the art in high performance racing valve jobs say NO. If I have taken great pains to cut the seat angles in the heads and valves to very precise angles and widths, then I am not going about sticking a great gob of grinding compound and whacking about with the valve and simply destroy hours of tedious work.
Having said that, it has on occasion been necessary, usually in the field or as a temporary 'fix' to lap the valves, <ever so lightly>, to get by for a race. On the dyno, lapping the valves by hand as opposed to precisely cutting them, shows a distinct drop in torgue and hp!!!
Not to say I haven't lapped a bunch of valves in my day, just saying that the machines that do the job are a lot more precise today. Heck, I used to seat the valves in my flatheads with a big mallet...worked too. I don't think I would do that on one of my Lotus engines, though.
Rod
RE: Hand Valve Lapping Needed?
RE: Hand Valve Lapping Needed?
I am going to try something before I pull the heads, the head gaskets are about $65 cnd. a peice.
If I apply air, remove the spring and chuck the stem with a drill motor and spin the valve against it's seat with wd 40 to keep it from getting hot maybe I can lapp them in without compound or removal of the head. If a hammer works I would asume this might work.
I have had no reply from Dart yet, I feel I will get more help from this site then the manufactur.
As far as leak down what is the allowable limit on leaking valves? Maybe I am over reacting and they will take care of themselves in a few hours of operation.
RE: Hand Valve Lapping Needed?
A well known hydraulic component manufacturer assembled some complicated special purpose manifolds in a clean room. The manifolds included a number of check valves of unusual design, comprising an acute angled hard steel pintle (think christmas tree) spring loaded into a 'seat' comprising the sharp- edged intersection of a ground flat surface and a round lapped hole in a steel valve block.
Gently questioned, the design engineer explained to me that the theoretical zero area seat would be brinelled into a perfect fitting finite area seat by the pressure of the spring at assembly.
He didn't know that the assembly techs had been lapping the seats for years with valve grinding compound ( because unlapped valves had _never_ worked right ) ... in a quiet corner of the clean room.
He was so sure of himself that he wouldn't have believed the truth, so I didn't bother him with it.
</off topic>
Mike Halloran
Pembroke Pines, FL, USA
RE: Hand Valve Lapping Needed?
sreid.
That's the way we did it when I started building engines.
That's the way it is still done in some shops for street.
That's the way it is done at 'shade tree' racing, for sure.
That's NOT the way it is done in the top level pro shops.
That's NOT the way I do it on my Mini or my Lotus, I'll let my log books and finishing record speak for itself.
Rod
RE: Hand Valve Lapping Needed?
I've done one valve job, without lapping, just like Chevrolet said. I think the trick is to let the valve and seat grinders 'spark out', so the generated surfaces are circular. It takes a little extra time. It worked fine for ~200,000 miles.
The strange little valve in my example was unguided. Given the eccentric load applied by a coil spring, and the clearances in the assembly, it could _never_ have worked as designed without lapping.
Mike Halloran
Pembroke Pines, FL, USA
RE: Hand Valve Lapping Needed?
Make no mistake, I ment that I do not systematically lap the valves on my race engines, at least that's the initial setup. "S*** happens" so there are no 'absolutes' in racing.
I have, on many occasions needed to use that little green can of lapping compound to effectively seal valves in machinery, air compressors and, yes, even an old engine or two. All that, however, does not change the reasoning behind 'not' lapping a cylinder head that I have spent two days setting up properly. It's all about time and money, bottom line, is it not?
Rod
RE: Hand Valve Lapping Needed?
RE: Hand Valve Lapping Needed?
As a practical matter, however, I seem to be ok with less than 5% on a fresh engine. Most usually come in at well below 5% after one warm up. As a general rule, the leak down remains fairly low for the first couple of sprint race weekends then I see some degradation in the #4 exhaust valve for this particular engine. I'm working on getting better coolant flow to that end of the cylinder head. Better, but not good enough, yet. Somewhere, post race, >10%/<20% on any one cylinder and I will freshen the valve job...that's probably too soon, I see my competitors at ~20%, but I'm to anal for that much.
I did run a dyno test pre and post race, VARA enduro of June 2004 at Willow Springs...approx 5 hours engine time...the leak down was 3% pre and 5%-5%->5%->10% post race. Comp ck was same pre and post but the dyno showed a loss of 3hp at 7600 rpm. That's not a biggie, but this is my hobby these days and if there is 3hp to be had, (to paraphrase Jessica Simpson) "I want it!"
Rod
RE: Hand Valve Lapping Needed?
Dart got back to me and said that the valves will self lap in themselves and not to worry.
RE: Hand Valve Lapping Needed?
I never know what I'll find until I look and measure it myself.
It is usually much easier to discuss and resolve manufacturing defects when the parts are new and have not been run
RE: Hand Valve Lapping Needed?
RE: Hand Valve Lapping Needed?
I would hate to think of the mess that the OEM's would create with a policy of 'hand lapping valves' in all their new, high tech engines! We would be back to the days where 'valve jobs' were needed every twenty to fifty thousand miles and it was extremely rare to see an engine go 100,000 without major repair!!!
Rod
RE: Hand Valve Lapping Needed?
If you are a machinist, yes the same old technology still applies. Believe me aerospace machining still involves "lapping". And if you were to purchase special gauges and standards I'm sure you would not want them unless they were lapped.
So gentlemen lapping is the way to go if you want closeness.
http://en.wikipedia.org/wiki/Gauge_block
RE: Hand Valve Lapping Needed?
For gauge blocks, ok. I'm not a professional machinist.
For automotive valves (to which this thread pertains), I must respectfully disagree. As I previously posted, a 'simple' light lapping in of a low time engine cost three hp. No biggie, but why waste three hp? I don't mind the extra hours it takes to make the valve seats perfect. I've been present at some pro NASCAR engine shops and I can guarantee the valves on those engines were not lapped in prior to dyno testing.
Rod
RE: Hand Valve Lapping Needed?
Hi Evelrod,
Would you expect carefully prepared valves to leak air as described in the original post?
Thanks,
Dan T
RE: Hand Valve Lapping Needed?
Gauge blocks ordinarily are used for an absolute dimension at a fixed temperature, e.g., 25 oC.
Lapping perhaps improves room temperature sealing but has an adverse effect when things heat up.
RE: Hand Valve Lapping Needed?
And with a machined finish that wouldn't also happen?
Actually I somewhat agree with what you say kenvlach, but I would take it a bit farther and say no matter what the method of finishing the seats and faces there will be inconsistances due to heat stresses. Lapping with the correct media, would assure a better cold fit than without.
RE: Hand Valve Lapping Needed?
Think about the shapes.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Hand Valve Lapping Needed?
No. I think all this comes down to 'workmanship'. All theory aside, the man on the 'mike' is the determining factor, IMO.
Greg
A little logic is always helpful. It is an uphill battle to convince (why I try, I do not know) folks that just because lapping the valves in has always been done that it is the correct way and not 'just a crutch'. Thanks.
Rod
RE: Hand Valve Lapping Needed?
RE: Hand Valve Lapping Needed?
This makes me extra suspicious that the manufacturing workmanship of AlkyUser's heads is in serious question. I'd take them apart (keeping parts identified for identical reassembly) and check out the valvestem-to-guide clearance, seat runouts and concentricities, including putting some bluing or magic marker on the seats and rotate the valves with hand pressure to test for 360 degree seat contact. That also can be used to test seat width and if the seating occurs in the outer third of the valve seat, both of which can be pretty profoundly effect set performance and life, and have little or no effect on sealing when brand new.
RE: Hand Valve Lapping Needed?
RE: Hand Valve Lapping Needed?
I'm just a hobby builder these days and retired to boot. I have lots of time to waste it seems. You don't need mega bucks or high tech machines to turn out quality valve jobs. Talent, experience, skill...qualities gained from doing this for 50 years will help. As I said, I lap the valves in street engines or when a problem arises in a race head that needs fast attention...no problems. However, what we are talking about here is a high dollar set of RACING heads and in that context, I just do not accept anything less than perfect if I am given a choice.
Tmoose
I usually "test" a valve (if something is suspect after a leak down ck) , not so much when it's fresh, but later, after run in with a vacuum cleaner and a stethoscope. I'm not sure what you mean by compressed air. I use engineers 'white' marker to ck the seats, less messy.
Evelrod...(appropriate handle, hey?)
RE: Hand Valve Lapping Needed?
RE: Hand Valve Lapping Needed?
Mike Halloran
Pembroke Pines, FL, USA
RE: Hand Valve Lapping Needed?
I used to have the job of hand lapping the valves on those engines which had failed their compression tests on the dyno.
I imagine these days that everything is cut to size and no lapping is done, but must admit, have never wandered over the road to check.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Hand Valve Lapping Needed?