Hand Valve Lapping Needed? 1

[AlkyUser]

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.

 

[evelrod]

To start, you need to pick up a simple leak down checker. You don't need a high dollar unit, Harbor Freight has a nice little cheapy that works just fine.

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

 

[sreid]

After grinding valves and valve seats, the sealing will not be perfect and any leaks will result in hot combustion gasses quickly eroding the valves. Laping the valves is required to guarantee a seal. Alternately, the valves can be given a hard hit on the head (big brass drift and a big hammer).

 

[AlkyUser]

Thankyou for the info,

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.

 

[MikeHalloran]

<off topic>
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

 

[evelrod]

<off topic> Mike, have you ever had to work under/for an architect that thought he was an engineer?

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

 

[MikeHalloran]

The trouble with being a mechanical engineer is that _everybody_ thinks they understand what you do, and can do it better.



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

 

[evelrod]

Mike.
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

 

[BXRStang]

Evelrod, any comments on what you allow as a maximum leakdown figure? Street or race, from your experiences?

 

[evelrod]

Obviously, I shoot for zero!
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

 

[AlkyUser]

Thanks for the input.

Dart got back to me and said that the valves will self lap in themselves and not to worry.

 

[Tmoose]

I'd put some bluing or magic marker on the seats and rotate the valves with hand pressure to test for 360 degree seat contact.

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

 

[ebola]

There are hundred of theories "how to" lap valves is best way, but lapping is not "old days" stuff.Especialy if You change guides.Proper seat angle cut makes 90% of job in best way.

 

[evelrod]

Sorry, but if your changing guides and 'lapping the valves in', your are absolutely doing it the "old days" way. All your missing is perhaps a 'large hammer' to seat the valves!

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

 

[dicer]

Funny stuff. Well it is the year 2007. And guess what?
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

 

[evelrod]

dicer
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

 

[Tmoose]

"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."

Hi Evelrod,
Would you expect carefully prepared valves to leak air as described in the original post?

Thanks,

Dan T

 

[kenvlach]

Um, a big difference between gauge blocks & automotive valves is that the latter work over a wide range of temperature, even with exhaust valves much hotter than the valve seat, due to the angular geometry.

Gauge blocks ordinarily are used for an absolute dimension at a fixed temperature, e.g., 25 [sup]o[/sup]C.

Lapping perhaps improves room temperature sealing but has an adverse effect when things heat up.

 

[dicer]

"Lapping perhaps improves room temperature sealing but has an adverse effect when things heat up."


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.

 

[GregLocock]

Lapping would make the sealing of a conical pair of faces /worse/ if there is thermal expansion.

Think about the shapes.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.