Hand lapping valves

[Pablo]

I am trying to wrap my head around why hand lapping is bad. I have been reading post after post online on various forums that tell you that lapping is bad and you should get a proper valve job. I don't doubt this to be true, but I like to know why something is the way it is rather than just putting blind faith in it.

In my mind I see two surfaces that are pretty close to the correct angle with maybe some high spots and some low spots. In between these surfaces I see a fine grit compound, and as its spun the high spots hit the high spots harder than the low spots are hit thus evening things out. I imagine that as you get to finer and finer compounds you can achieve a finer and finer finish.
I've read that grinding valve seats and valves is on its way out in favor of cutting but I haven't seen a single post citing why this is more effective or if it is in fact more effective. On top of this, I have to wonder why grinding is (was) ok but lapping isn't considering it is doing the same thing, that is abrading the surface to a desired shape. The grinding stone in this case determines the shape with a grinder vs the valve determining the shape when using compound. Yes I can see that with a compound between the valve and the seat you wont get a precise angle but 44,45,46 those are just numbers, airflow doesn't care about whatever number you've decided is good.
The lack of information out there is starting to seem like "the emperors new clothes" kind of thing so if anyone can explain this to me I would be all ears.

 

[Pablo]

It always seems like right after I post a question that I've been searching everywhere for, I find an answer. Ended up stumbling onto a great post talking about how lapping generates concavity on the mating surfaces so that only the outer edges end up touching. Makes enough sense. But then I think about how I hear about a 1 degree difference in cut that people make from the valve to the seat so that they "hammer" in during operation, and I wonder if slight concavity is that much different.

 

[The Radius Kid]

It always seems like right after I post a question that I've been searching everywhere for, I find an answer. Ended up stumbling onto a great post talking about how lapping generates concavity on the mating surfaces so that only the outer edges end up touching. Makes enough sense. But then I think about how I hear about a 1 degree difference in cut that people make from the valve to the seat so that they "hammer" in during operation, and I wonder if slight concavity is that much different.

Got a link to this article?

 

[pajo]

I guess you can call me old school but I always lap the valves on any of my engines .this procedure just insures proper seatting if you will and any inperfection of the seat and the valve, but hey this is just my opinion and i know there are guys on this board more knowledgable that probably think otherwise and will chime in ..

 

[cruzn57]

the valve / seat expand when warm, so the area you spent all that time hand lapping, is now not being used.
hand lapping was ok "back when" but todays high output motors require more precise machine work,
also you cannot control the seat contact width for the valve and seat by lapping, I won't go into seat width's
but to wide will be asking for carbon to lodge between the seat and guide, and will very quickly cause a burned valve.

 

[charlief1]

the valve / seat expand when warm, so the area you spent all that time hand lapping, is now not being used.
hand lapping was ok "back when" but todays high output motors require more precise machine work,
also you cannot control the seat contact width for the valve and seat by lapping, I won't go into seat width's
but to wide will be asking for carbon to lodge between the seat and guide, and will very quickly cause a burned valve.

This is a partially correct answer but covers most of the answer. Lapping parts that are close fitting is common in high tollerence fit equipment. I've done it myself on several of my personal weapons. What it does is allow for reduced clearence and a tighter fit. This works great for items that have constant contact and slide against each other. When you have parts that reciprocate and are not in constant contact then the parts must consistantly contact in the same location. On an engine this isn't feasible for a valve in the head. As it goes up and down the valve rotates if everything is working right. This makes the contact area move so that the valve doesn't hit in the same place all the time.

 

[Pablo]

TheRadiusKid,
Here is a link to the thread with a couple of good posts on the topic. A user by the name of pumaracing posted some interesting info:
Valve lapping, for or against

 

[The Radius Kid]

TheRadiusKid,
Here is a link to the thread with a couple of good posts on the topic. A user by the name of pumaracing posted some interesting info:
Valve lapping, for or against

He's right.
If you lap the daylights out of them,they will go concave.
A quick lap to check for even contact is just fine.
I wouldn't do it any other way.

 

[ek02]

Cutting valve seats is much more accurate than using grinding stones. That being said, I am still using the old fashioned stones along with an antique valve grinder. I lap the valves with very fine compound just to be sure where the valve is contacting the seat. Lapping valves instead of doing a proper valve job worked OK on flatheads. Not so good with hardened seats and valves.

 

[The Radius Kid]

Cutting valve seats is much more accurate than using grinding stones. That being said, I am still using the old fashioned stones along with an antique valve grinder. I lap the valves with very fine compound just to be sure where the valve is contacting the seat. Lapping valves instead of doing a proper valve job worked OK on flatheads. Not so good with hardened seats and valves.

Actually,lapping is the ultimate final step.
You are correct that tooling is good,but it must be done with an accurate rig.
To be honest,in order of accuracy - best to worst:

Lapping.
Grinding.
Tooling.

The trick is that the most accurate steps also need the less accurate steps done first.
In other words:tool,grind then lap.

 

[87GNSteve]

I usually lap valves if I take the head off and apart with only a several thousand miles on a valve job to see where the seats are. Its a good reassembly technique to see where things are, not so much making things right.