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]

Dan T
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

 

[dicer]

Good point Gregg.

 

[Tmoose]

So, carefully and nicely machined valves and seats are likely NOT to leak during a compressed air pressure test.

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.

 

[ebola]

waste of time evelrod? Its one my favorite jobs - angle cut and valve lapping.Im not a billion dollar industry shrick to apply to high tech machines to do elementary bussines.I'd treat lapping as a higiene.

 

[evelrod]

Ebola, (interesting handle, hey?)

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?)

 

[sreid]

Does anyone know how valves are seated in brand new engines? Do manufactures rely on accurate grinding of valves and seats?

 

[MikeHalloran]

For decades, Chevrolet has recommended ground valves against ground seats with a 1 degree difference in angles, so they nominally seal only at the outer diameter of the seat. That's what's in their service manuals. I assume they do something similar in production.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Does anyone know how valves are seated in brand new engines? Do manufactures rely on accurate grinding of valves and seats?


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.

 

[chrisborrell]

It seems that this thread is over but I have one thin to add that seemed was missing. Each valve seat needs to be ground to ensure the guide and seat are concentric to one another. In my experience in Industrial Engines lapping w/ fine compound was acceptable but not to correct a major problem. I was also taught to vacuum test each valve seat after assembly was completed. Do manufactures rely on accurate grinding? Yes, but they calibrate at specified times and they also use cutters and not stones so they do not have any grinding dust.