Topic: Leaking valves after lapping?

[sammermpc]

I'm well into rebuilding the top-end of my CB500, and just lapped my valves. I bright the head into a machine shop for surfacing, and the fellow there said my valves and valve guides looked fine (I don't think he mic'd them though).

Anyhow -- I've lapped them all and every one I've tested leaks! I'm perfectly willing to spend more time lapping them, but they seem fine -- I don't want to lap them to the point of damage. Is that possible? I've been pretty careful so far, and I pulled the head because of a leaking gasket, not because of compression issues (as far as I know...)

I measured the springs and they are within spec as well. I know that the answer might just be to have new valve seats cut and so forth, but I just have a feeling I'm not doing something right.





I guess my question, basically, is whether or not sometimes the answer is to just keep on lapping? Having never done this before, I don't have a good sense of how good a job I'm doing. I've probably already lapped the one pictures 3-4 times though...

[rb550four]

How did you test them?

[sammermpc]

I put the head on its side (with the springs, keepers, washers in) -- and poured gas into the port. There was some wetness after 1-2 minutes, though no real 'flow'. From what I've read they should be totally buttoned up, though.

[PeWe]

I thought it was the other way, combustion chambers horisontal facing upwards, gasoline in chamber and see how quick it leaks. I think it can be done without springs mounted, only valves in.
Are all seats as the bad EX seat on photo? 
The exhaust seat must get a fresh cut, its lapped way too much. I had almost like that when lapping down corrosion. BUT, I did not lap that much with the valve I intended to use. I used an old valve with OK angle.

Time for professionals to cut the bad seats and buy new valves that will be lapped just a little for the final seal in each port.

[trueblue]

Those valves and seats look like the contact point is too wide and need refacing.  You can keep lapping and you may get a seal, but it isn't the best thing to do .

[sammermpc]

Time for professionals to cut the bad seats and buy new valves that will be lapped just a little for the final seal in each port.

Really? That's disappointing — I feel like I've opened up a can of worms here. If I get new valves, do I also need to get new valve guides? Has anyone experience with a service like this one on Ebay? I am skeptical of finding a willing shop around here that can compete on that price, though maybe. Seems like I would need new valves anyhow? It might make sense for me just to buy a used complete head and hope it's in better shape? Will it cause issues to mix-and-match cylinder head w/ everything else?

  You can keep lapping and you may get a seal, but it isn't the best thing to do .

Do I have anything to lose by continuing to do that, or is that just an exercise in futility?

[PeWe]

Guides only if the valve will move sideways too much when you have it opened about 10-15mm. You will feel the lash. 0,1mm is very easy to feel and I think that is max. or close. The guys wroing with this know.
I got my seats cut after my lapping removing corrosion, old guides were all OK. (Honda good steel guides)

The seat must have a smooth surface. Your EX valves need to open 0,1mm or more to overcome the edge when the valve now has sunken down into the seat.
Bad flow and you'll lost low lift flow. Cut seats (3 angle job) is not expensive. The tool cost too much, though)
Valves are relatively cheap. Open the head again will cost a head gasket and other stuff, maybe drain oil ....

New valves that the workshop can machine an additional angle so the flow will be improved at low lift. Search multi angle 30 degree. Just cut just over area the valve seals against the seat. Better flow at low lift when the valve has slimmer profile. I do not know much it does. It's a quick job for a tuner.

[strynboen]

leak test..just fill fuel in the reversed head...

[sammermpc]

Pretty sure the testing is fine — I find the pouring through the ports way a little easier, as the hemispheres on the head are real shallow.

The seat must have a smooth surface. Your EX valves need to open 0,1mm or more to overcome the edge when the valve now has sunken down into the seat.
Bad flow and you'll lost low lift flow. Cut seats (3 angle job) is not expensive. The tool cost too much, though)
Valves are relatively cheap. Open the head again will cost a head gasket and other stuff, maybe drain oil ....

Can you explain that a bit more? What do you mean "0,1mm or more"? From my perspective, valves definitely don't look cheap! It seems like a whole new set will cost ~$300!

[Bodi]

Hard to diagnose.
- the contact rings do look very wide. After a 3-angle seat grind I expect much narrower contact rings. Do you have any idea of the history on this head?
- the surface on the pictured valve's contact ring looks rather rough. Fine lapping compound should leave an even specular finish (not shiny, but quite smooth).
- I don't have a head here to compare but the seats look to be deeper than normal. That comes from repeated seat regrinds. There is a limit to how deepthey should be and replacing the seats is the only solution after that.
- depending on your lapping method loose guides can have the valve rattling about causing poor sealing. Very careful lapping will get a good valve seal with worn guides but you will still have smoke etc.

[sammermpc]

- the contact rings do look very wide. After a 3-angle seat grind I expect much narrower contact rings. Do you have any idea of the history on this head?

Nope. I got it w/ 35k miles on it and have done a fair amount of maintenance. It looks like it's had a fairly handy, conscientious PO, but I don't think anything has ever been seriously overhauled.

- the surface on the pictured valve's contact ring looks rather rough. Fine lapping compound should leave an even specular finish (not shiny, but quite smooth).

Ok, that's good to know. I do have some fine lapping compound, but I haven't really given it a serious shot. I really only have been trying on one valve, to see if I can get the technique right to have it seal.

- I don't have a head here to compare but the seats look to be deeper than normal. That comes from repeated seat regrinds. There is a limit to how deep they should be and replacing the seats is the only solution after that.

OK — replacing seats, new valves, etc...it seems like I could be in the territory for a new head altogether? I'm a little befuddled, as I pulled the head not because I was noticing any issues. Is it routine to run the bike w/ leaking valves? (Not saying I want to, just curious).

It may be cheaper for me just to get a used head. There are a variety of CB500 + CB550 heads online — would I need to replace the cam & rockers too? Or just the head itself? Anything to look out for or worry about? From what I understand, CB550 heads slide right onto CB500s.

[PeWe]

See photo of my overlapped seat. after I tried to remove corrosion dimples by lapping only... No good idea.
Left it to my local tuner that did the port job over 30 years earlier. No warranty job.
- My exhaust seats were OK to recut only.
He did them all, new valves, measured spring force and found that ex springs had too weak spring force, 1.5mm shims from Kibblewhite under springs did it. Got the shining head back with valves mounted.
EDIT: New valves got all the multi angle valve job

[sammermpc]

Hmm, I see. It definitely looks like my seats are pretty banged up, then. I have multiple rings of varying depths. I will have to make a call and see what I can have done locally. For bang-for-buck, it seems hard to beat a used head, but some part of me really loves making the original one sing. New valves are not cheap, though! I take it a new seating definitely goes w/ new valvess?

[PeWe]

Check if a recut can fix the seats before giving up

[trueblue]

Any GOOD engine shop should be able to sort that head out, I emphasise GOOD engine shop, there are a lot out there who shouldn't be in business.   

[sammermpc]

OK — I brought the head by a guy I know in town. Not sure how "good" he is — he doesn't have the fancy Serdi machine I've seen being talked about. But he seems honest and competent. He took a look at the head, and said that the valves needed to be reground and the seats recut. He said he'd do a 3-angle cut on the seats — but wasn't too concerned about the actual angles.  . Anyhow, I trust him, though he may be a bit seat-of-the-pants for my taste... The price is right, though — the whole job should cost me < $100, which makes it worth my while.