Bonneville America Forums Forums Technical Forums General Tech What the heck is valve lapping!?!

So I was talking to a mechanic about replacing the valves in my cylinder head and he mentioned valve lapping, but really couldn't explain it.

Anyone? Anyone? Beuhler?

It's been a while but here goes......We're going back 30 years here but.. There is an angle under the flat face of the valve and it has to be perfect for the valve to seat properly when it is closed. Lapping is the smoothing and adjusting of that surface. Anyone please correct me if I am remembering wrong. Hey grump, where are you? You're needed here.

Jerry

Yep, you attach a tool to the valve stem and put "lapping compund" on the valve seat, draw the valve up tight and spin until both surfaces mate perfectly. Over time your head deforms slightly and the new vavles have to be matched to insure the proper seal.

We all know that the valves have angles on the faces that mate up to the valve seats, and that the valve seats have the "same" angles. Even though these angles are the same the valves may not match exactly the angles on the seat, and if the valves and seats are not matched gasses can escape under compression because of a faulty seal between valve and seat.
We need to lap the valves so that all the faces match and seat correctly. This is achieved by using a valve grinding paste, and rotating the valves back and forth so that the valves are lapped with the seats, this means that the valves and seats form a good seal and gasses will not escape until the valves open.

Cruser

Okay. Makes sense. Thanks!

Don't spin the valve with a drill or anything like that. Flip the head upside down with the valves removed from the head. Put a little lapping compound on the valve seat,slide the valve into place (careful not to get compound in the guide) There is a tool that looks like a stick with a suction cup on the end. Put the suction cup on the valve head and move back and forth between your hands. Take the valve out now and then to look and see if the whole valve seating area has been done. It should be a uniform greyish color all the way around on valve and seat.

Please don't reply to my posts. My question was answered. Move along and crap on someone else's thread.

I have 2 words for you and it isn't happy birthday.

See above.

Erwin,

Love it !! Move over !!! Can I sit here !!!!

Now that's funny.

Just as a followup, I went to my buddy's shop today and fit the new valves. I'll post pics later of the bent valves as a cautionary tale. We used a drill press to compress the valve springs, which definitely makes it a two-man job. The homemade tool he attached to the press looked like a 5/8" sparkplug socket with slots cut on either side. He also had the lapping compound and tool so now I'm all set to slap the head back on the bike and start the valve adjustment all over again.

Woohoo!

If done correctly it's the single most most boring, mind numbing job in a workshop.

Then I'm pretty sure it was done correctly.

Jeff was dying to take the grinder to the head and start porting it. A diehard Harley mechanic, he was practically salivating at the possibilities with the design of the head. I was like, "Dude, I just want to ride my bike again. Let it go."

Dang John what the heck happened to your ride? You mentioned bent valves (yuk) cam chain go south? Hit your nitrous @10 grand? please do tell! Looking forward to pic's and follow up as promised. Geez makes me hurt all over thinking about it!

I'm not too proud of the story. Let's just say I have my moments, all bad.

When I was doing my valve adjustment, all I can conclude is that I got turned around when I was taking the camshafts from the table to the bike, and I reversed them. I was in total brain-fart mode and as I slowly tightened the bearing caps down, all four valves on the left cylinder got bent. Believe me, it doesn't take much pressure - these things are pretty fragile. As soon as I saw that the engine wouldn't turn over, I knew I was f****d.



So as I type this I just finished torquing down the cylinder head nuts. I'm not rushing anything this time. I'm reading the manual very carefully. The valve adjustment was expensive enough. This little mishap cost me another $240 just in parts from Triumph (via BikeBandit.com): head gasket, 4 valves, seals, o-rings, gaskets, etc. When I get the cams back in, basically, I have to start the valve adjustment all over again.

On top of that, I've spent well over a grand on parts for an extreme make over on my bike this past winter. That project has been seriously derailed. I was hoping to have it done for the Kootenay Raid in July but I don't think I'll have the time or money to finish all of it. More details on that later.

No wonder why you've seemed a bit touchy lately.

That's not a bad analogy.

There's a phrase that carpet layers and carpenters all seem to like that applies equally to anything mechanical: Measure twice, cut once.

It's all a learning curve (which's what I told my boss when he had to replace a dang expensive layshaft at his expense coz I'd put it in the gearbox wrong ).

Dang sorry to hear that John, Other than the $$$$ seems all is on the way to resolve, That's the good thing! When she's all back together get out there and ride!

Well, I have a little time to work on it yet. It's supposed to snow this week!

Hey John, You said you will have to do a complete start over valve adjust, Just wanted to let you know I have 9 shims that you are welcome to have if you need them. The sizes are actual as measured vs. the stamped size due to them being used. Just let me know your needs and I will send them to Ya. T

1. 2.73mm

2. 2.74mm

3. 2.75mm

4. 2.78mm

5. 2.82mm

6. 2.83mm

7. 2.85mm

8. 2.86mm

9. 2.90mm

Thanks for the offer. I'll let you know.

You'll likely find that that hand lapped valves will respond in a superior manner when you get your motor fired back up.

Since this thread has increase a bit in scope, here's my old valves (testing my new camera, too):





Today I managed to get the head torqued down (again, after I torqued it down yesterday without the oil breather tube installed). I reinstalled the main sprocket and managed to not drop the thrust washers down into the engine. Drained the oil and pulled the side cover to set the cam timing (managing to destroy the side cover gasket ::sigh::).

I quit while I was "ahead."

While there are plenty of useful resources in the Tech Vault about doing a valve adjustment, as soon as things went FUBAR on me Dinqua's big bore guide was absolutely crucial in preventing countless other mistakes.

http://www.dinqua.com/Triumph/BigBore/bb1.html