Topic: F2/F3 valves: were the stem tips bevelled? Or is this wear?

[wohali]

OK here's a question for y'all. Check out these pics of my valves (warning: full rez versions of pics are 2MB each). These are off of a CB750F3 with either 15k or 45k miles; the history of the bike is a bit murky.

Were the tips of the F2/F3 valves bevelled originally? Looking at intake valve #4, it hardly looks bevelled at all - and has the least wear in the piston/cylinder too - telling me it was either not gapped right, not firing right, or something else. On the exhaust side, all 4 have bevelled wear, but again #4 has less wear than #1 (see the close-up comparisons taken on graph paper for better side-by-side.

I'm trying to determine if a) this is normal, b) if I should try and resquare the ends, and c) if so, how much is "too much" to take off to try and resquare the ends. Valve guides and seals are already slated to be replaced. I don't want to buy new parts just to buy new parts, and don't dread taking the head off again in a year or two to replace them - but I also don't want to risk catastrophic engine damage or waste my time cleaning these if they're shot.

Intake stem tops:


Intake #1 (L) and #4 (R):


Exhaust stem tops:

Exhaust #1 (L) and #4 (R):

[mazingerzeca]

Hello.
I rebuilt my F2 engine over two years ago, mainly due to the known issues with the F2 head, which leads to wear between the valves and the guides and oil consumption and so on.
Buzz at Dynoman told me that one of the things that can be improved on these heads are valve guides. Given that mine were shot I changed to bronze valve guides. He told me also that the problem could also be fixed changing the valve retainers for titanium retainers, which are harder and prevent the valve stem tops to move inside them. Yours seem to have had this problem, given the marks made by the cotters. They seem to have run too loose and probably you should change the valve cotters and valve retainers, better if you install titanium ones. I've ridden my bike in a daily base since the rebuilt without any problem.

[TwoTired]

Using Mrieck as a guide/guru...
I believe he said that the stem tip should be flat  (The outer edges are typically beveled when new, to prevent gouging during install.)
Of course, grinding the stem tip flat shortens the valve stem and changes the geometry of the adjusters pushing on the stem.  So, there are "lash caps" that are placed on the top of the stem to restore the proper length.  I'm not sure where he said these can be obtained.

As the F2/3 engine revs about 1k higher than a standard K 750, they get stiffer valve springs which increases the loads on the retainers.  I expect the titanium retainers would be a good insurance policy, as a lost retainer and dropped valve could make quite mess.

You might wish to PM Mrieck about this.  Or, do a search for lash caps in one of his prior posts.

Cheers,

[wohali]

Lash caps, that was the phrase that I was looking for. Thank you!

I would still need to know what the exact correct valve OAL is to be able to grind off the right amount to make that happen, but as MRieck says in other posts, you also really should be doing this on a proper valve grinding machine. Even with a machine lathe, I'm not sure I'd feel comfortable doing it right.

Looking more at the seats, I think I'm going to "spring" for new valves anyway and just get CycleX's F2 top end kit. These are probably salvageable for someone else out there with an F2/F3; once I've finished the restoration I'll sell them off with the other spare bits I'll undoubtedly have.

Thanks again! Now to see if the pistons can be salvaged or if I have to go 836...