valve lapping problem

[blockmanjohn]

Hi, I am trying to lap a 12L14 valve into a cast bronze seat. The valve is 5/16 inch in diameter, and both the valve and the seat are cut at a 45 degree angle. I am spinning the valve with an electric drill, varying the speed and using an 800 grit lapping compound. The valve has seated very uniformly, but the finish is unexpected. There are fine radial grooves that I can see and feel with my fingernail on both the valve and the seat. I thought it would be more of a uniform finish. Nothing I do seems to get the grooves out. What am I doing wrong? Thanks, John.

 

[Brian Rupnow]

I never use power to lap valves into seats. I do it by hand, using #600 grit lapping compound. it only takes about 1 minute to do it by hand.

 

[blockmanjohn]

Brian, could I repair this by lapping it by hand as you suggest?

 

[MachineTom]

The valve and seat will have a dull gray finish after lapping, This is because 500-600 is not for polishing, and there is no need for it anyway.

 

[Philipintexas]

Also, I don’t think a continuous rotary motion is recommended. Should be a back & forth. (Clock-wise then counter CW)

 

[Brian Rupnow]

I don't have an answer, because I don't really know what your parts look like now. Radial grooves sounds wrong. If you power lapped them it would leave annular grooves like the rings you see on a cut down tree. I cut my valve seats at 45 degrees. that is 90 degrees included angle. I cut my valves at 46 degrees--that's an included angle of 92 degrees. This initially gives a line contact with the seat, but a minute of spinning the valve back and forth with your fingers and moderate pressure, using #600 lapping past will widen the line contact to an area contact and they will seal quite well. I make my valve cages which include the seat and guide from brass. I use cold rolled steel for my valves. As soon as the engine fires up, the explosive charge of burning fuel will force the steel valves into the brass seats and the seats will deform to be a perfect match for the mating valve.

 

[blockmanjohn]

Brian, thanks for the info. I was incorrect in saying the grooves were radial. They are indeed annular like the rings on a tree. The other detail is that this is a valve for a mixer on a hit or miss engine. There will be no explosive force to seat the valve. They must be full contact from the start or they will leak fuel. I did clean them up by starting over with a back and forth motion with my fingers, and they show a much more consistent finish. Thanks again for all the member's help. I learned more than any book could teach me. John

 

[tornitore45]

The larger the seat, meaning the wider is the 45 degrees chamfer the lover the chance of achieving a good seal. The trick is to make the seat barely visible 0.010 or so then. The valve and seat will be shiny as machined. One starts rubbing the valve back and forth and observe the seat area and the valve. When the dull looking ring on both part is going all around you are done.
Brian suggestion of making slightly different angles on the mating parts guarantees that one starts with zero width chamfer and end up with the smallest seating area.

With a small seat, the pressure is higher assuring a better seal.

 

[Jennifer Edwards]

Hi John,

Once upon a time I worked in a marine diesel engine rebuilding shop, two things I have learned to do when lapping valves is this:

A) Cut the angle on the valves about 1/2 degree shallower than the seat. in your case 44 1/2 degrees. This is to allow the valve a little room to take the shape of the seat when breaking in the engine. I was taught that ths method created a much better seal as the engine was run in.

B) never use power to lap. I was old fashioned and used to use onr of those suction cup valve lappers rolled back and forth in my palms whilst slowly allowingthe valve to rotate a few degrees after every dozen or so back and forth motions. this should advert the circular grooves you experienced, probably from an oddly larger bit of grit that was spun around and around.

I have seen a device that looks kind of like a hand cranked drill that as you turned the handle it would both oscillate back and forth but also rotate the valve a couple of degrees after every so many oscillations. Every once in a blue moon i run across one at a car boot sale.

Another suggestion is to use bluing on the faces. After lapping a while, clean the valve and seat, apply bluing and then move the valve back and forth as if you were lapping. after a minute inspect the surfaces. The bluing should be rubbed off in an even and wide area around the seat.

Hope this helps,
Jenny

 

[XD351]

With the annular rings it sounds like the lapping compound is embedding in the valve and seat and cutting the groove . Try finer less aggressive compound , for such a small valve i would give a metal polish like autosol a go as the automotive grade stuff is pretty coarse even at 800 grit and something nearer 1200 grit would be better . Do it by hand and don’t use too much pressure .

 

[tornitore45]

As abrasive goes I use a paste of "Bon Ami" and oil. For those that are not familiar with Bon Ami https://en.wikipedia.org/wiki/Bon_Ami

It is a bit more abrasive than tooth paste

 

[Phil Haldenby]

I agree with every thing Jennifer said. I worked my whole career as an automotive machinist and that was how we did it. A very tiny valve ,if there is enough margin, you can cut a groove across the head of the valve and use a screw driver as a lapping tool.

 

[BobsModels]

John

If I understand you are using this valve on a mixer. I am assuming the valve is used to shut off the fuel supply. The port for the fuel is in the middle of the seat and you are trying to get a seal for the entire valve to seat area, not just thin line seal as would be used for an exhaust / intake valve.

Am I correct in your description? I have made a number of these, including one for a 1/8 scale Galloway. Getting that entire area to seat is an exercise in patience. Do not use power, you need to rely on the valve stem being a nice fit so you can twirl it with your fingers while applying pressure. Having messed up the first several times I tried this here is the fix I used. On one of them I just re-center the seat and took a light cut, very light because it moved the jet hole up a bit. I made a new valve, I use stainless. Then just stared over working more slowly, I also used 1000 grit paste. The other one I messed up on was caught soon enough, and I used 600 grit to get a nice dull finish and then followed up with 800, then 1000.

Like I said it is a chore to get the entire surface to match, but it should not take more than 3 or 4 minutes with a light touch. I make the valve and seat the same angle for these mixers. I do as mentioned above with the exhaust / intake valves so I get a real thin seat line.

Bob

 

[bobden72]

All very interesting as I have just finished a Cirrus and can not get any compression as the valves are leaking. I have cut the seats to be 1/32 wide and ground them in by hand, still leaks! Recut the seats and ground the valves to a 1deg shallower than the seats, blued the seats turned the valve by hand again and got a nice clear seat, still leaks. Tried making a lapping tool as described in SIC mag again no joy and from what you say the seats are now too wide, is there any remedy to get these valves and seats to seal.
I am not new to IC engines and have made a few now including a Edwards 5 and all work but this one has beaten me, and sad to say it has been put on the shelf as a failure.

 

[Brian Rupnow]

I would like to know more about these mixers. I have designed and built over a dozen carburetors, but I don't have much experience with mixers. Can anybody suggest reading material that will tell me more about them in detail?--Thank you---Brian

 

[blockmanjohn]

Bob, you are correct about the mixer valve. In fact it is for a 1/4 Galloway I am building. Do you think it makes a big difference where the intake port comes out on the face of the valve seat? Do to design considerations mine came out very near the bottom of the seat. As long as the entire valve is lapped to the seat do you see a problem with this low positioned port? Thanks, John

 

[Motorman1946]

+1 to what Jenny wrote, plus, I was at sea and we always lapped in valves using 'coarse', then when a uniform surface was observed used 'fine', carborumdum paste ( a specific number grade was not known to us, just coarse and fine!) again on until a uniform finish was obtained, and then finished with a couple of goes with just oil. We never had a valve leak. Used the procedure ever since.

Never ever use power for lapping, always use hand with the action Jenny described. I once caught a POE using an electric drill with an attachment to to the valve on a diesel genny head overhaul despite expressly telling him to do it by hand beforehand - the POE was treated to the thousand words and the attachment got quickly thrown over the wall, he'd not use that one again! As well as the axial lines with power you can also get oval seats - it has been known and observed.

Chris

 

[minh-thanh]

check the valve body,when you make it, it may be bent , if the valve body is bent even if it's very small it is difficult to make it seal to the seat, I have tried lapping valve and seat with all the above and regular ways failure, when I check the valve, the valve body is bent, I make a new valve and it seal with seat.

 

[johwen]

Hi, I am trying to lap a 12L14 valve into a cast bronze seat. The valve is 5/16 inch in diameter, and both the valve and the seat are cut at a 45 degree angle. I am spinning the valve with an electric drill, varying the speed and using an 800 grit lapping compound. The valve has seated very uniformly, but the finish is unexpected. There are fine radial grooves that I can see and feel with my fingernail on both the valve and the seat. I thought it would be more of a uniform finish. Nothing I do seems to get the grooves out. What am I doing wrong? Thanks, John.

The cause here is the grit is causing the grooves and to get a good seat you mus oscillate the valve back and forth after refacing the valve Cheers John from johwen.

 

[BobsModels]

John

I have had the port high, middle, and low. As long as the valve seats they seem to work fine.

Brian - just look up Lunkenheimer Mixers, you should get plenty of information. They used the valve to keep the fuel from dripping when the fuel tank was above the mixer. In general most all simple hit-miss engines used mixers, not carburetors. It is probably just semantics, carburetor's are in my view more complex ie a throttle. All a mixer does is control the fuel to the air stream nothing complex, one jet and vortex for some vacuum across the jet. Adjust the jet for load / speed and that's it.

Bob