Topic: Grinding Stock cb400f Valves

[Tom R]

I’ve heard something about a hardened face on OEM valves… true? Or can I just grind and reuse?

[bryanj]

You can lap them with paste but not grind them on a machine, also the tips can not be ground as the hardening is not deep

[HondaMan]

I’ve heard something about a hardened face on OEM valves… true? Or can I just grind and reuse?

Some of the early ones (CB750K0-K2, CB500 era) had hardened valve faces. If you have one made after 1973, they are not hard. Hand-lapping them is a common way to 'see' the sealing face and helps you tell if the valve can then seal or not. It also tends to clean away that thin hard layer of carbon in one spot (or two?) that is holding the valve open, if it leaks. The lapping will let you see how wide the sealing surface is: it should be about 0.7-1.25mm wide on the 400F.

[newday777]

I’ve heard something about a hardened face on OEM valves… true? Or can I just grind and reuse?

As in grinding the sealing face or back cut grinding to have 2 or 3 angle valve cuts for better flow? You've asked about back cuts in another thread.

I noticed reading through your posts you haven't introduced yourself and your bike (s) in the new members introduction section.

[Tom R]

I've been sorting out a Souix valve grinder I acquired and am looking at a couple of spare 400f heads in my basement. I was also given a Kwik-Way seat cutter, but all the pilots it came with were too big. However, 5.5 mm pilots are available and I have the stones in the kit already.

Whenever it gets warm enough to get back in my garage, I'd like to clean up the valves and seats on one of the 400f heads. The guides are still in spec, so I'm only looking at doing a valve job.... 3 angle with back-cut, new springs / seals and clean any casting flaws in the ports. I run the bike in a production class, so nothing over the top, just a stock rebuild.

I'd have to invest about $75 in a pilot (which could also be useful for the spare CL175 head and the GS1100 heads I have in my pile)... and probably some other costs I haven't figured out yet; but would be fun to do.
 

the tips can not be ground as the hardening is not deep

The tips seem flat... is it usual to have to grind them?

[bryanj]

You see lots with damage and what looks like erosion holes/indents.
Be very carefull cutting seats and valves its easy to holes too big

[Mark1976]

   If you don't already own or have access to the proper tools and or tooling as well as machinery to do it correctly, it becomes a pricey proposition (40yrs later and I still have to buy tooling and cutting blades on those few odd occasions). If you think paying someone to do it for you is expensive, wait and see how much of your money and your time you'll spend acquiring and learning everything necessary to do it properly takes and costs. I'm not saying you can't or shouldn't, its an investment in your time and your money.
   Many here just clean up the seats and valves re-lap them and install new seals, and you can do that. However, depending on the condition of the exhaust valves you maybe better off just to replace them. The faces on the exhausts usually show a bit of excessive runout, pitting and some recession and lapping isn't going to fix that, it'll depend on just what they've been exposed to. The exhaust valves in the 400f really get pretty beat up (heat) overtime, the intakes not so much, you'll more than likely be able to clean and reuse the intakes. Again, you'll have to take a close look and go from there.
   The seats are pretty durable, but you'll really have to look at them and then check them for excessive runout and pitting (exhaust) as well. There's not really a lot there to cut, they're pretty small compared to most of their automotive counterparts, and it sounds like most (if not all) of what you have is automotive based.
   Your sioux valve grinder chuck isn't going to adjust small enough to grind a valve of this size, (let alone within the tolerances needed) and these valves aren't meant to be re-ground anyway. These valves just don't have enough body/margin to sustain a recut, The seats will definitely benefit from a 3 angle cut, and you'll more than likely need to do a top cut to get the proper seat width and location on the valve face.
   So yeah, its an investment, but it's definitely a worthwhile one. There's members here who can do it (Mike and Mark) and once you've had an opportunity to see what they can do it'll give you a better idea of what's involved in doing it correctly.
Your bike,  your project, your choice...

[Keith]

You are correct; simply grinding the valves or lapping them in is not the correct way to do things. The reason for a three angle “valve job” is to position the contact area on the valve face and to create the correct seat width. All affects long term reliability.

[CycleRanger]

I’ve heard something about a hardened face on OEM valves… true? Or can I just grind and reuse?

Here's the Honda service bulletin about the hardened valve faces.

[MRieck]

I have a mint set of used 400F valves for sale.

[bryanj]

Note that in that bulletin it states that the valves must not be GROUND but must be LAPPED if replaced.
This causes confusion as we use grinding paste to LAP the valves whilst a machine with a rotating carborundum wheel is used to GRIND the valves.

Now i am not clairvoyant but i will guess the op wants to know if he can use grinding paste on the valve seats to which the answer is YES

[Tom R]

Now i am not clairvoyant but i will guess the op wants to know if he can use grinding paste on the valve seats to which the answer is YES

I was talking about chucking the valves up in a Souix valve grinder.

Thank you for posting that service bulletin. Great info.
I’ll keep them away from my grinder.

I have several heads (cb750, cb400f, cl175, and gs1100) I was hoping to clean up the ports, blend a little, and put a radius seat cut on. The last three heads have the same diameter valve stems, so I was thinking of purchasing at least that pilot. I have stones as small as 1-1/4”; but I think even they’re too big.
But as Mark pointed out earlier, it’s seeming much more involved than I initially thought…. Per usual, haha.

[bryanj]

Like i said i am not a clairvoyant, trouble is grinding valves to most people is using paste, they dont even teach using machinery to reface in the UK colledges any more, and to be honest a true mechanic is rare and even rarer one who does not use a computer and obd.
If you have an old school v8 muscle are there any garages you would trust with it plus try and find a Honda dealer who will work on an sohc or even anything over 10 years old

[Mark1976]

Ok Mike, I gotta ask. What the going rate for the valves. Me being a collector of valves, so to speak...

[Bodi]

As far as I know a three or five angle grind is on the seat, and valves are flat ground and then lapped in.
I heard if the Stellite coated valves but ignored that theory on '76 400F valves. These have been running for about 40 years and I haven't noticed any issues.
I do have a full set of new valves from MC-Again from maybe 30 years ago "just in case"...