valve lapping

[scooby]

I was gonna work my way up from 600 to 2000

 

[TonyM]

If concentricity is ensured by the machining process with a decent surface finish then grinding is a waste of time and effort. The seat and the bore are machined from the same end without removing from the chuck and the seat and stem are machined without removing from the chuck. I have only found it necessary to lap valves on old(full size) engines where the seat or the valve has been replaced or re-cut or where there is some pitting.

 

[minh-thanh]

If drawing calls for a .015 seat, is that .015 dimensioned at A, or at B?
View attachment 131276

the word "seat" or "valve seat": it says it all !

 

[scooby]

ok I'm going to loctite new valve cages in and re drill intake/exhaust paths. Will test with pressure through spark plug hole like before.

If concentricity is ensured by the machining process with a decent surface finish then grinding is a waste of time and effort. The seat and the bore are machined from the same end without removing from the chuck and the seat and stem are machined without removing from the chuck. I have only found it necessary to lap valves on old(full size) engines where the seat or the valve has been replaced or re-cut or where there is some pitting.

Happy Thanksgiving to you all

 

[Steamchick]

The intention of lapping valves is not to make an eccentric valve align with the seat, but to improve the surface finish of the machined surfaces. If the machined surfaces are a fine finish and high degree of concentricity, then the seat will be perfect as machined. But when grind - stones are mounted on a mandril to grind the seats in industrial applications the ground surface is too coarse and needs lapping to get a finer surface finish. My opinion is that 1000 grit is OK, but that is only because that is the finest grit I have...
My Moto Guzzi workshop manual gives seats machined at 89 degrees internal angle and valves at 90 degrees internal angle, then intersection of the 2 cones is lapped to 0.5 ~ 1mm wide. But that is on valves 28 ~ 30mm diameter.
Hope this is of some use?
K2

 

[RobConway]

Hi, What did you end up doing @scooby , Valve seat cutter or valve and compound grind on a square cut seat.

Im glad i read your post as i was amazed I could hold vacuum when the valve was off its seat (using my brake bleed pump) same as you.

 

[scooby]

Hi, What did you end up doing @scooby , Valve seat cutter or valve and compound grind on a square cut seat.

Im glad i read your post as i was amazed I could hold vacuum when the valve was off its seat (using my brake bleed pump) same as you.

I used valve seat cutter and made a very small seat, haven't lapped them yet. The plan is to lap with the highest grit lapping compound I got, put in ultrasonic cleaner, then test with pressure using thing I made.

 

[RobConway]

Thanks, For now think i will re-make and install the valve cage without seat as i wont to move on to the inlet manifold and need the valve cage in place. Later ill make the cutter and do some testing before cutting the seats.

 

[scooby]

I made the valves first, then without moving lathe compound, turned the seat cutter so the angles will match

 

[Alec Ryals]

I used valve seat cutter and made a very small seat, haven't lapped them yet. The plan is to lap with the highest grit lapping compound I got, put in ultrasonic cleaner, then test with pressure using thing I made.

Lapping is not necessary if the valve seats and valves are done correctly

 

[Steamchick]

Hi Alec, fair comment, except to get those perfect mating cones takes a bit more expertise than some amateurs can achieve. Moi aussi!
Engineer's blue proves the surfaces before and after lapping.
E.g. Slip gauges stick together because of perfect surfaces, but I can't file that well to create such.
K2

 

[simonbirt]

On the Seal that I am building at the moment the valve guides are shown with a sharp edge and a 45 deg valve face, Haven’t made them yet, so unable to comment on how well they will seal ( very confusing sealing things on a Seal). I imagine than when lapped there will be a very narrow seat. The valves are only 3/16’ diameter or there abouts.

 

[scooby]

The reason I am going to lightly lap is to get a better surface finish and thus perhaps a better seal. It seals right now, but not as much as I think it should

 

[RobConway]

I assume if we can achieve good concentricity and a reasonable seat now, the running-in process beds in the seats after the first few runs ?

 

[minh-thanh]

The reason I am going to lightly lap is to get a better surface finish and thus perhaps a better seal. It seals right now, but not as much as I think it should

My way :
Use a magnifying glass to see the surface of the valve and seat
At the position of the valve and seat contact circle : No more scratches or similar caused by machining
Even a very small scratch or blemish will affect the sealing
Lapping with large grit size until No more scratches or similar caused by machining, then lapping with small grit to perfect the surface.

 

[Steamchick]

Keep the seats narrow. Check with Engineers' blue that you have full contact of the valve on the seat. Don't lap, unless the Blue shows a gap.
K2

 

[minh-thanh]

Don't lap, unless the Blue shows a gap.
K2

If the surface machining of the valve and the valve seat is perfect - it depends on the machine, the tool.....
That is not possible for me. I try to make the surface of the valve and seat the best I can but they are not completely sealed
I lapping them because I need them sealed - really sealed
I don't like : "They get more sealed when the engine is running"
Of course this is how I usually do it
If I could machine the valve and seat well enough to seal, I wouldn't need lapping

 

[gbritnell]

OK here's the straight information! First let's compare a couple of things. If you do a valve job on a full size engine with proper tooling that's rigid and accurate then you most likely will get a good seal right from the start, but not always. So let's say you have (just for comparison sake) a .001 gap on a 2 inch diameter valve. Not good but not bad in numerical terms. Now let's say that you are building a miniature engine with adequate tooling and you end up with a .001 gap on a .375 diameter valve/seat. That's a huge leak! I have built my share of I.C. engines and I'm here to tell you that the times I have had a perfect seal right off of the machine is maybe 3 out of 16 on a v-8 engine. That's not to say that they can't be sealed with a little lapping but there is no comparison between full size automotive valve machining and model engines. When machining valve seats in a head or block I never cut them on the mill, too much chance for error between concentricity and angularity. Going back to automotive practice I use the valve guide as my alignment guide for cutting the final seat. I make a cutter with a pilot shift that fits the valve guide snuggly (a couple of tenths) and turn it by hand to cut a seat which is generally no more than .025 wide depending on the size of the valve/seat. Even with this I usually have to lap a touch to get a perfect seal. To check the seal I make a tapered tube which I wring into the port so I can use my vacuum gauge on. I also put grease around the valve/guide area so that no vacuum is pulled past the valve stem.
I will say that lapping can't and won't make up for poor machining but with model engines it's almost a necessity!

 

[Bentwings]

My way :
Use a magnifying glass to see the surface of the valve and seat
At the position of the valve and seat contact circle : No more scratches or similar caused by machining
Even a very small scratch or blemish will affect the sealing
Lapping with large grit size until No more scratches or similar caused by machining, then lapping with small grit to perfect the surface.

Traditional valve lapping compound always seemed coarse to me. Ibyhinknitbbreaks down as you use it. I marked the valve seat area with blue dykem then lapped the valves to show the seat area. I used magnifying glass too. You could se scratches but they were more like something you would see using fine wet or dry paper it gets better as you use it . We had the palm stick as well as a crank thing usually the little seal surface was nearly polished . We then put the springs on and poured a little alcohol or acetone in if it didn’t leak all was good. Since it was a back and forth motion there were no directional fibers. Even after a hard nitro run the seats were highly polished or burned. Aluminum heads had very hard seats but still polished nicely

 

[minh-thanh]

You could se scratches but they were more like something you would see using fine wet or dry paper

Yes , you are correct . The surface of the valve and my seat are similar
But I'm talking about scratches....caused by machining

The lapping valve and seat can be fast or slow, just one time or many times depending on the surface of the valve and seat when machining - of course concentricity is necessary.
(concentricity between valve and seat on my engines: I don't know how precise it is but it can be called enough)
With me, most of the leaks between the valve and the seat (between the needle and the nozzles in a diesel engine) are from scratches or the like due to surface machining - Lapping is my best choice