Topic: Is this valve still good, or is this totally done-zo?

[markreimer]

Hi there, I'm in the middle of a top-end rebuild on my cb750 K4. I was cleaning the carbon off my valves before lapping them back in.

Just for background, last year I had new seats cut into the head and the valves cleaned by the same machine shop. I've had about 5,000km since then.

To clean, I put the valves into a drill and started cleaning them with 80 grit emory cloth, as per the HondaMan book. I also have a small brass brush which I use for a final clean, being careful to avoid the sealing face of the valve.

I've noticed that some of the exhaust valves have what look like chips or pitting or something, which you can see in the photo. What I don't know though is if that is part of the sealing face or not..

What do you guys think? It looks to me like there is a well defined line where the angle changes. the lower part (to the right in the photo) is nice and smooth, no pitting. to the left of the line is where it starts to fall apart.

[markreimer]

Hmmm... I'm guessing that if valves are supposed to look like this, it suggests that these valves are pooched?

Got the photo from this thread:
http://forums.sohc4.net/index.php?topic=126735.msg1509838#quickreply

[dave500]

wire wheel valves only or don't get the sand paper on the valve face.

[markreimer]

wire wheel valves only or don't get the sand paper on the valve face.

I used the brass wheel when I was close to the face. I used emory on the back of the valve (flat face), and above the face to get the chunks of carbon off.

[754]

Let me say this.... go around the shiny face with a felt pen, wipe off the stem real good.
 Now insert in guide with very light pressure on valve turn once.
 I can't see if it's a layer of hardcoat coming off, corrosion, or carbon that did not get cleaned off. I suspect the latter.
Brass wheel shouldn't hurt the face..

[markreimer]

 

So, I think you may have been correct that it was build up, BUT... I just took a different brass wheel to the valves to try cleaning them up some more. After a second I stopped and looked in horror. The wheel scratched the crap out of the valve face. turns out the 'brass' wheel I had was actually a brass coated steel wheel. Great.

So let me revise my question - I've just bunged up my exhaust valves.  I have some nearly new API guides in the head and seats cut last year. Now that I will be ordering new exhaust valves, can I get away with just replacing the valves themselves and lapping them in, or should I at least get the seats cut again?

[dave500]

steel wire wont either,sand paper will.

[754]

They may be scratched but they would still lap..
 Why so many coat wire wheels with brass is beyond me..

[faux fiddy]

I was going to say that it looked like carbon chipping off.

You're not building a race bike are you?

Use the coarse lapping compound. Make sure to push the valve out a few thousand times between twists so can feel your  grinding  paste working.  Then use the finer stuff a few hundred times. You'll see when they'll look a lot better.

[dave500]

so they don't rust in storage?ever spun up a real old wire wheel and gotten a cloud of rust dust?

[dave500]

valve lapping,flame me now I always use the reversible drill and rubber hose trick with finesse.

[ADW]

Hi there, I'm in the middle of a top-end rebuild on my cb750 K4. I was cleaning the carbon off my valves before lapping them back in.

Just for background, last year I had new seats cut into the head and the valves cleaned by the same machine shop. I've had about 5,000km since then.

To clean, I put the valves into a drill and started cleaning them with 80 grit emory cloth, as per the HondaMan book. I also have a small brass brush which I use for a final clean, being careful to avoid the sealing face of the valve.

I've noticed that some of the exhaust valves have what look like chips or pitting or something, which you can see in the photo. What I don't know though is if that is part of the sealing face or not..

What do you guys think? It looks to me like there is a well defined line where the angle changes. the lower part (to the right in the photo) is nice and smooth, no pitting. to the left of the line is where it starts to fall apart.

Sorry, that one is done. Do not reuse.

[70CB750]

valve lapping,flame me now I always use the reversible drill and rubber hose trick with finesse.

Rubber hose but by hand and mechanic's blue to check how it seals.

For Mark:  Yes, if you have seats already cut for guides - ie you did not replace guides or cut seats with new guides in - you can just lap new valves in.