Topic: How did this happen

[70CB750]

This is the # 2 intake valve and I am trying to find out how did this happen.  Since there are no signs on oil starvation or any major trauma, the camshaft is beautiful and all the pistons tops are in good shape I narrowed it down to the tappet addjuster.  The tappet adjuster screw can be either:

1. too tight, i.e. the lash is too small.  Now if this was the reason for the screw eating up the valve, it would self adjust itself to proper lash and stop. 

2. too lose, i.e. the lash is too big.  That means that instead of a gentle click and push during the cycle, the screw hits the top of the valve like a little hammer, again and again and again.  The downward movement of the arm is not constant speed, it starts slow and speeds up when the other end of the arm climbs the lobe of the cam shaft.  With the indent getting deeper, it hits it with greater speed eventually creating this 0.8mm indent.  Now when I am thinking about it, this affected valve may be even taller than others, at least on the mug like edges, the repeated hammering raised those "side walls" up instead of digging this indent in the valve.


Please,  fault my logic.

[754]

Maybe guide sticking, giving excess clearance at times. Then changing later.

[70CB750]

You mean excessive force needed to open the valve?

[Tim2005]

Going by the black paint, is this a 1978ish f2 engine? If so, this sort of problem really wasn't at all unusual on them at quite low mileages.

[70CB750]

Yes it is F2, I am jsut trying to understand why it happened.

[andrewk]

I've seen a few of these, and I have always come to the conclusion that any kind of hammering action would have to come from lash being too loose, at least at some point in the movement of the valve.

I would also think that any kind of valve bind or sticking (which would allow for lash to open way up, the have the valve slam into the tappet adjuster) would show evidence of pistons hitting valves, or heat discoloration on the guide/valve, especially if it had hammered enough to do what it's done.

Could it be chalked up to running the tappet adjuster too loose for too long?  (Lack of maintenance)

[Vinhead1957]

And a soft valve material. Is the problem with F models just guides?  I agree with the too loose theory but it may be a lower grade material used in the valve. Either way it leads to a new set of valves and complete head job

[CB750F2]

The F2s have stronger valve springs which may mean more wear. Pat

[Rigid]

Valve tip damage such as that can be stoned smooth with oil and a proper stone.  The extra clearance this work creates can be adjusted out. 

[70CB750]

I asked one of the few vendors who sell F2 parts:

The stock valves have a stellite weld on the faces and the top of the stems. The valve in the picture has had the stellite fail. There are many reasons that this can happen. The stellite is very thin on the stock valves so if they show wear they cannot be cut and need to be replaced. If a machine shop did not know this and did a butt cut on them the stellite would be too thin and would only last a short while. Worn valve adjusters is nother scenario that will cause this to happen. The adjuster screws have a round dome on them when they are new, but after years of wear they can become damaged and destroy the stellite as well. A common problem is the springs are stacked incorrectly and coil bind. This will not only destroy the top of the valves, but the keeper groves as well.

[bytio]

I had a similar thing happen to my F2 many years ago, the angle of the valves is slightly different to other CB 750s so the tappet screws tend to scratch the top of the valves and often find domes in top of the valve tips. However to get like yours, the stellite must have been removed as was the case with mine. A very well known (and expensive) CB 750 restorer in the UK rebuilt my engine, and where the valve tips had domed, ground them smooth, thus removing the coating, and after 400 miles mine looked just like yours. You need new valves or some companies can re harden them. I bought a whole set of valves and springs from cyclex and no problems after redoing the complete engine myself.

[70CB750]

Thanks, man!  There is hope afterall 

Did you replace the guides and recut valve seats too?

[bytio]

Yes I did, put bronze guides in and had seats re cut, I got a whole kit from cycle x beehive springs, new hi-flow valves, bronze guides and seals

[bytio]

Some pics of the build

[70CB750]

Beautiful!  Is it on the road yet?

[bytio]

Yes all on the road and 270 miles, runs superbly and no leaks

[70CB750]

Great job!