Topic: How should a lapped valve look like?

[Raul CB750K1]

I'm in the middle of my CB350 twin rebuild, and I've just lapped the valves and seats. I guess this topic applies to any bike, therefore I open this thread in the "Tech Forum".

The question is that valves and seats were machined to 45 degrees, and I was told it should be good to lap them a little. I was expecting that the contact surface would be the same as the valve side, but after lapping them it seems that just a little of the valve makes contact with the seat.

I've searched through my pile of books and tech articles and have found contradictions. There is something about the contact surface been between 1,5 and 2 mm, but others say that the bigger the contact surface, the lower the pressure, thus it's better to have a smaller surface so if a microscopic particle gets in the way the valve will crush easier than if the surface is bigger. The metal is hard so lapping until the contact surface grows can take forever. There are also opinions about valves and seats expanding with the heat, and so the contact surface will be bigger at operating temp.

I include a picture of the exhaust valves, both machined but only one lapped. Should it be enough with that or should I lap them like there is no tomorrow? I applied paste three or four times, and lapped until I didn't feel the tiny abrasives do their work.

Raul

[rt]

Raul, very hard to say. I've seen valves with very 'suspect' looking surfaces working perfectly well, whereas perfect looking ones failed. I usually start lapping with extra coarse, coarse and fine graded paste and trial fit a valve when the surface 'looks good'. I then fill some petrol into the referring port and leave it for a couple of minutes. If it's leaking into the combustion room you have to take it out and repeat the procedure... I know it sounds (and is) quite time consuming, but is, as far as I know, the only method to guarantee a 100% snug fit.

[Bodi]

The lapped valve looks excellent.
The valve seat is usually ground in 3 different angles, to get a small contact ring. This looks like what you have.
What I look for is a polished ring - what I see in your pic. The valve seat should show a very similar ring, consistent width shows the valve is centred properly (ie the valve guide is centred in the valve seat) which is what you want. If the guide is not centred you'll see it on the valve seat polishing pattern; the valve will have an even ring no matter what because it's what you've been spinning as you "grind" with lapping compound.

[Raul CB750K1]

Bodi, thanks for your reply. That's what I wanted to read.... 

Here there is another picture of the intake valves. I knew the machinist shop had a good reputation and there were still traces of "engineers blue" in the seats, so that indicates a pro job. I had also seen diagrams of the three seat different angles, but didn't know exactly where the valve should make contact. Indeed, the lapped ring is in the middle of the side, with no-contact rings above and below the contact ring.

These are the original valves. They were a little pitted, and though I already had two new ones for the exhaust, I followed the machinist' advice about grinding and reusing the old ones, because the stem width were in tolerance.

What I was a little pissed off was because the valve springs were just in the verge of tolerence. I bought new ones to discover that they are just about 0-5-1 mm longer, even being new ones, so I will reuse the old ones.


The valves are already installed, so I will make the gas-leaking test just in case.

Raul

[Demon67]

I've an engine mech all my life and the lap ring looks good to me, I think that if ring is not on the edge of the valve like where you have it, its golden, and I know that people will dispute me but many years ago I worked on Piston Aero Engines and what we tryed for was a line that was about in the middle of the valve face neither thick nor thin (there was engineering orders with the specs)  and when we'd get one our engines back on its next overhaul the valves wouldn't show signs of burning and if I'm driven to to do the valves on one of my bikes thats what I'd try for.
Bill.