Topic: Rusted Valve Seats - Can these be re-cut?

[Tree]

Removed the valves from a 750K8 head today.  Some of of the valve seats look really rusty to me, mostly the exhausts.    Think these can be re-cut?

[brokenspoke]

I was told by the experts here that they could not be, so I bought a usable one on ebay for something like $40 shipped.  I sent the ebay head to member "Mrieck" so I'm waiting for a diagnosis and then I'll have some performance work done.  Probably could have installed it as-is but I had already been anticipating spending a lot more on machining the rusty one, I figured I might as well make sure it was in pristine condition before going any further.

[MCRider]

The seats are alumimum, you have deposits on the non ferris valve seats that need to be cleaned off for a good appraisal of the base material. Seats can be lapped or recut if needed. There are special Honda cutters for the job.

Sure about that? I thought the seats were hardened steel (may be cast iron) pressed into the aluminum head?

Assuming I'm right, Tree's look too far gone to recut. But that's from a distance.

http://www.mahle.com/C12570B3006C0D49/CurrentBaseLink/W27BLLNH524STULEN

At the price of a used head, not worth trying to replace the seats.

[Ernest T]

Seats should be hardened steel otherwise the valves would pound right through the head.

[tlbranth]

My 1970 CB750 had its head rebuilt at a machine shop many years ago and the seats were recut. They were not aluminum. They're steel or cast. Take it to a decent machine shop and ask.

[MRieck]

The seats are iron. The exhaust seats are harder than the intake seats.

[MCRider]

Maybe I'm wrong, I thought the seats were cut into the head.

Nope. The picture in my link shows the steel ring which is the seat, and demonstrates how it is installed into the aluminum of the head, as a repair. i suspect the original install is similar.

On a clean head you can see the line of differentiation. The cutting tools actually cut into the steel (or iron) ring to make the angle that matches up with the face of the valve. And as mentioned the valve bangs into the ring seat. If it banged into aluminum it would only last a few moments.

[MRieck]

You have to bead blast that head to see how bad they are. Seats can be replaced no problem but it can get expensive fast. I had all the intake seats on my latest head replaced as they are soft and do not hang well with higher spring pressures necessary with heavy cams.

[chewbacca5000]

Hi Tree,

I love to hear about your avatar there did you set that engine ablaze?

[Tree]

Hi Tree,

I love to hear about your avatar there did you set that engine ablaze?

I was trying to free a seized piston by putting ATF and gas in the cylinders, burning the gas to heat and hopefully expand the cylinder so the ATF would penetrate.  In the end I had to split the case and have a machine shop press out the last piston.  I thought it made a good picture though.

I guess I'll see if I can soda blast the rust away from the valve seat and then see if I can save the head.

[chewbacca5000]

Sorry the CB750 Olympic torch didn't work out for ya.  Think is Hellva cool though! If you blast the head be sure to post some pics.  From the pics doubt there is enough material to cut, but you never know.  Sometimes that crusty stuff is not as bad as it looks, but in this case it probably is.

[crazypj]

They can be re-cut, but, it may 'pocket' the valve
Easy fix is to fit 1mm oversize valves, then they won't get pocketed and reduce compression (usually minimal anyway)
Oversize valves may be a bit expensive, but, it's probably still cheaper than getting all the seats replaced (and if the seats are rusted, you need to replace valves anyway as they will be just as rusty and pitted)
BTW, good rule of thumb is to use about 3~4% of valve head diameter for valve seat width, up to 5% for exhaust on street motors (it usually works out around 1~1.5mm  )

[Lostboy Steve]

Looks a lot like mine did. I had them cut but I haven't reassembled yet so I can't tell you either way how it will all work out.