Took the No 6 head off the big Kelvin today, and found the damage to be at the low end of the spectrum of possibilities.
The inlet valve was very obviously bent, and it's quite clear that this was a result of a foreign body being trapped between the valve & seat when the piston reached the top of the compression stroke. There are marks on the top of the piston from impact with the valve face.
There is no air cleaner on these engines, just a vertical induction pipe about 20" long at one end of the manifold, No 6 is at the opposite end of the manifold. Its position makes it most likely of the six to suffer this sort of problem, I think.
Luckily the damage to the piston is superficial, there's no need even to draw it out. Also luckily there was a new, if slightly rusty, inlet valve in the 'ship's stores' - a bit of polishing in the lathe, 10 minutes lapping it to it's seat, & the head is ready to go back on in the morning. I'll have to dust off the big torque wrench, the main head studs have to be pulled down to 450 lbft. I've straightened the bent valve as best I can, & will regrind the face so that hopefully it will be worth keeping as a spare.
The great mystery (to me), and what prompted me to post here on the subject in the first place, is why the valve was reopening after closing. All I can come up with is that the rush of air through the gap between the bent valve & its seat was working in a way which was capable of lifting the valve fractionally. Any comments on that? The ground valve face is perhaps 3 times the width of the seating.
The engine is an older version of that shown at:
Cheers Tim
Tim Leech Dutton Dry-Dock
Traditional & Modern canal craft repairs