valve lapping

[scooby]

Still working on putting together howell v4.. Valve cages are phosphor bronze, valves are stainless steel.

Made valves and seat cutter without changing compound rest angle so they both angles should match.

I have a few questions:

1) What compound should I use for lapping? I have really fine diamond paste (yellow&green), and some timesavers lapping compound.
2)Does the lapping compound need to be cleaned off the valve cages/valves after lapping?
3)What is the procedure for lapping valves? Do them by hand, half turn cw, half turn ccw, lift off seat, rotate, then repeat?

Please keep answers simple cuz I'm stupid..

Thanks

 

[kf2qd]

Lapping compound is an abrasive, and if left in place will migrate and cause all kinds of wear. As small as those valves are you want to use some very fine compound.

 

[Brian Rupnow]

I machine my valve seats to have a total included angle of 90 degrees. I machine the valves to have an included angle of 92 degrees. That means that I get a line contact between the valve and it's seat at the top part of the valve seat. I use 600 grit aluminum oxide lapping paste, and after coating the contact surface of the valve I pull the stem of the valve with hand pressure against the seat and spin it back and forth between my finger and thumb. I generally spin it back and forth about ten times, then releasing pressure I turn the valve about 45 degrees and repeat the back and forth spin under pressure another ten times. I go thru that routine about ten times. Then I remove the valve, and scrub the surface with liquid dish detergent under a running tap of warm water. I clean any remaining grit from the valve seat with a cotton q-tip. My valves are turned from cold rolled steel. My valve seats are brass.

 

[David Shealey]

I was taught, decades ago, that one should NEVER lap valves to correct ANY misalignment! I have only used fine lapping compound lightly to insure the seating is even. never to remove any appreciable material. The reason I was taught this is that if you use lapping to correct bad seating, then when the exhaust valve head gets hot and expands, then it will seat on the tiny amount of unlapped area, and that on the inside of the valve seating area, exposing the cold seating area to the exhaust gas and promote seat burning.

 

[bluejets]

Many times on small engines a wack with a hammer and a brass drift is enough, forget the lapping.

 

[simonbirt]

Many times on small engines a wack with a hammer and a brass drift is enough, forget the lapping.

Interesting, this is the way we seat balls in non return valves.

I found that the valve seal on my Farm Boy was much better after its first run- same principal I imagine.

 

[Steamchick]

Fundamentals: Proper machining and grinding are processes that remove metal to get "the perfect shape" we need.
LAPPING the surface (as described by Brian) is removing he "peeks of machining" from the metal - thus improving the surface finish of the MACHINED surface.... You cannot LAP a surface to be "in a better place" - and using a coarse grinding paste under a valve to try and correct an "OUT-of-TRUE" seat will never work properly. You will get an "Uncontrolled" and "Worn" seat that isn't good from an Engineering perspective, even if you can get it to seat "well enough" for running.
It is a BODGE - by innocent amateur mechanics who have not been taught the "proper way" to get a narrow, controlled circular seat that will be wear resistant (due to the nature of the material surfaces and precision of mating concentric to the axis of the valve), good for heat transfer (cooling of the valve) and good from day 1 (Already correctly made so does NOT need the seat to be "run-in").
The WHOLE aim of "good manufacture" is GOOD MANUFACTURE.
Enough rant... Shoot me if you disagree... (It is Human Frailty that causes the masses to rise up and destroy knowledge, or for the "Top Bullies" to keep knowledge from the masses... KNOWLEDGE IS POWER! - Let's use it wisely.).
K2

 

[simonbirt]

Fundamentals: Proper machining and grinding are processes that remove metal to get "the perfect shape" we need.
LAPPING the surface (as described by Brian) is removing he "peeks of machining" from the metal - thus improving the surface finish of the MACHINED surface.... You cannot LAP a surface to be "in a better place" - and using a coarse grinding paste under a valve to try and correct an "OUT-of-TRUE" seat will never work properly. You will get an "Uncontrolled" and "Worn" seat that isn't good from an Engineering perspective, even if you can get it to seat "well enough" for running.
It is a BODGE - by innocent amateur mechanics who have not been taught the "proper way" to get a narrow, controlled circular seat that will be wear resistant (due to the nature of the material surfaces and precision of mating concentric to the axis of the valve), good for heat transfer (cooling of the valve) and good from day 1 (Already correctly made so does NOT need the seat to be "run-in").
The WHOLE aim of "good manufacture" is GOOD MANUFACTURE.
Enough rant... Shoot me if you disagree... (It is Human Frailty that causes the masses to rise up and destroy knowledge, or for the "Top Bullies" to keep knowledge from the masses... KNOWLEDGE IS POWER! - Let's use it wisely.).
K2

Having revisited this, I wonder if what is actually happening when things improve after a run is the valve stem is bedding in, thus allowing the valve to fully close. I found when testing on my Farm boy that just pulling the valve stem made for better compression. It is now very good and will hold compression for a minute or more.

 

[Steamchick]

I suggest that the valves should be lapped so they properly seal "as new" . I have never done tiny valves on models, so only speak from my apprenticeship (over 50 years ago) and motorcycle and car engine "hobby" work since then. All valves are lapped so no petrol leaks when the intake or exhaust port is filled - and any leakage checked on the combustion chamber side after 5 to 15 minutes.
When cutting seats on "refurbished" cylinder heads, we had 2 angles of cutter or grindstone, 45 plus/minus 1/2 degrees. Seats were cut to have an equal width of each, then when the 45 degree valve was lapped you always got a fine line of lapped-polished surface. About 20 to 40 thou width was good. Lapped surfaces were checked/examined by eye with a jewellers loupe.
Perhaps models are simply too small to have the double- angled seat cutting process? Brian's post above is the best advice.
If your valves don't seat perfectly as new, but settle down after a short run, then I suggest the surface is a bit rough and able to be smoothed by running. Valves do 2 things. They hammer seats, and they rate and wear the seats truly circular.
Lucky you for holding pre sure as well as you say.
Enjoy modelling!
K2

 

[kwoodhands]

Still working on putting together howell v4.. Valve cages are phosphor bronze, valves are stainless steel.

Made valves and seat cutter without changing compound rest angle so they both angles should match.

I have a few questions:

1) What compound should I use for lapping? I have really fine diamond paste (yellow&green), and some timesavers lapping compound.
2)Does the lapping compound need to be cleaned off the valve cages/valves after lapping?
3)What is the procedure for lapping valves? Do them by hand, half turn cw, half turn ccw, lift off seat, rotate, then repeat?

Please keep answers simple cuz I'm stupid..

Thanks

I use tooth paste as a lapping compound. Do not know how it compares to manufactered lapping compounds.
I clean the tooth paste off completely whenthe lapping is done.
mike

 

[Brian Rupnow]

The engines I build all require at least "some" compression to start and run. I find that if I can get the valves close enough that they mostly seal, the engine will start. Once the engine starts and runs for 10 or 15 minutes there is a remarkable rise in compression. I use cold rolled steel to make my valves and brass for my valve cages. My theory is that once the engine runs, the steel valves will actually "form" the softer brass seats until they seal tightly.

 

[Steamchick]

Brian, I think that there may be 2 elements to increasing compression, piston blow-by the main reason, with a much lesser degree of valve seat leaks. But the brass and steel combination will have some bedding-in, by "hammer" if the springs do not keep the valve train "connected" so the cams lower valves onto seats. But that will be noisy! And some bedding-in of valves takes place as gases "clean the surfaces", and when they rotate. Are your valves "petrol tight" when assembled? - If so I expect "no blow-by compression loss" through the valves.

Interesting, this is the way we seat balls in non return valves.

I have just assembled 2 hand water-pumps: I.E. 4 non-return valves. Sealed perfectly WITHOUT the application of the hammer on the balls. But I only tested 1 on a boiler - steam test at 42psi... The other is for hydraulic testing and exceeds 100 psi against a bit of rubber held against the end of the outlet pipe. Couldn't do that with leaking balls!
On my boilers, I used to tried the "hammer" method, as explained in a book on valves, but found a bit of precise machining of the seat worked every time with new balls...
I recognise forging as a process with its proper place, but IMHO not for seating poppet valves. - And for those who say "that is what happens in service when valves close" - I retort that with correct and cleverly designed cam profiles for durability, the valve will be lowered onto the seat by the cam-action, so "hammer" stresses are very small: just the difference between the speed the cam allows the valve to be travelling at just before the tappet clearance opens. Otherwise engines would be incredibly noisy, and would not last very long at all!
The problem I have with the hammer, is that if the valve is anything less than perfect any "out-of-perfect" shape will be impressed upon the seat, so the seat will not work well when the valve rotates in service (as it must). Whereas the "precisely made" circles of valve and seat made on your excellent and expensive machines will allow for rotation of the circular face of the valve to retain a seal against the concentric and circular seat...
I wonder where the "engineering" and "Machinist" are with your unspecified hammers being used without calibrated arms and certified drifts? - IMHO... that is?
Oh, my "humble opinion" is based on working as an engine design Engineer in a car manufacturer... and I now call myself an "amateur" as I no longer practice professionally. I therefore cannot offer anything but advice based on the best of my expertise.
K2

 

[rutzen]

I found that I couldn't get good compression on my Rumely until I re-machined the seats to a very narrow edge, no more than a few thou. I lightly lapped them in with metal polish. The compression has improved a lot since I've done more running. Oh, and I chucked the valves in the 4 jaw independant with a dial gauge to get them running dead true to re- machine them. The first try I used the 3 jaw and it just wasn't good enough.

 

[Steamchick]

I use tooth paste as a lapping compound. Do not know how it compares to manufactered lapping compounds.
I clean the tooth paste off completely whenthe lapping is done.
mike

Hi Mr Woodhands, Toothpaste is used by diamond polishers to polish those hard bits of rock, and they call it "jewellers' rouge". I understand it is a calcium mineral so it is about the same hardness and chemistry as tooth enamel, or marginally softer than the enamel so it doesn't wear it away. Yet softer materials will "lap" harder ones - like your steel valve parts - though I cannot explain "How". Though I advocate using proprietary lapping compound, as this is calibrated for grain size (relates to surface finish), and will lap valve seats much quicker than toothpaste. For a final finish, I have used metal polish - for brass or silver or aluminium or toothpaste. Yet with my limited expertise I cannot decide what is best... so the car gets just Lapping Compound, the motor bike a final "Brasso" polish after lapping. Model valves, being so small. probably appreciate the extra attention of toothpaste for that "gleam" when you smile when it runs...!

K2

 

[propclock]

Just my experience , A very narrow seat width is the key for
not perfectly precise, not manufactured ,home made model engines. I learned this the hard way. One of my earlier engines
was the Hoglett. It has a cast iron head with no valve cages.
I cut with a miniature valve seat cutter , a 45 degree face .
how wide ? not sure but looked good by eye ~25-30 thou.
The valves were made from solid in one chucking to maintain
concentricity. Leaked like a sieve. Or just simply A LOT !
I lapped with all the above suggestions. Of course the seat width
kept getting wider. And the leakage kept getting worse. WHAT? If you used blueing or magic marker to check the seating ..... perfect. I finally decided that the wide seat due to
physics beyond my comprehension, crop circles in the metal?
Was the culprit. So I was forced to make 2 more valve seat cutters. As in automotive practice. a 30 and 60 degree cutter.
with the new very narrow 45 degree seat width. "presto" perfect!
Yea presto that was 2 days of work. So now I leave valve seats
and valve cages as cut as in 90 degrees Sharp edges, a light lap or with tooth paste/ or brasso or time saver does the trick.
The wack method also works. Just my 1.414 cents worth.
It has saved me a lot of time on the many engines to follow.
PS got my HMEM stickers yesterday Yea!

 

[Steamchick]

On "Wide versus Narrow seats". The Engineering is so simple that having learned it as a young-un, I believed everyone knew this: The pressure of the seal must be greater than the pressure of the fluid, for the seal to work. As you have a fixed spring on the valve, say 1lb. and a fixed area of seat - say 0.01 sq. in. the pressure it will seal is 1lb / 0.01sq.in. = 100psi. But if the seat is wider, - say an sealing area of 0.1sq.in. - then the pressure it can seal is 1lb / 0.1sq.in. = 10psi....
In other words, twice the width of seal is half the pressure it can take, or 10 times width of seal is 1/10th the pressure it can take.
I'll let you have a think about your valves and springs and then tell me that "poppet valves have pressure helping them seal"...
OK It is a bit more complex, but your experience of "narrow seats sealing better" is the simple solution. And "proper Engineering" is that simple. Solving problems. Which is what you did to overcome your "crop-circles". (But you do need to use a bit of brain).
Enjoy!
K2

 

[scooby]

I'm going to make my valve cages over again.

The turned parts are made from the bar on right, which I got from speedy metals, it says its SAE 660 Bearing bronze on receipt.

The round stock on the left is from from McMaster-Carr. I had searched for phosphor bronze on their website and it showed me 544 bearing bronze (it also says in description that is also know as phosphor bronze)

My question is, is one better than the other for making the valve cages?

 

[Steamchick]

544 hardness = Hb 85, but 660 hardness = Hb 65. (Not so hard).
544 is also higher tensile strength etc. So that should be better... Annealing temp. >900 deg. C. So should be OK for exhaust temperatures....
K2

 

[stevehuckss396]

Bronze of any kind will be fine. Most people use brass.

 

[tornitore45]

Another reason why wider seats do not work is that we are not dealing with perfect geometric surfaces. It gets easier to "find" a sneak-by path between to imperfect conical surface as the "contact" width increase. There is more opportunity for the deviation from the ideal geometry to go in opposite directions and create a gap.
I have transitioned to no machining of the seat, too much risk of the tool to be misaligned, off center and cocked.
Leve it sharp and give it a light tap.