Shim for setting valve clearances (steel hardening question)
I've done some work on the seat of an engine head, and now i can't find
any shim that can i use for setting the valve clearances.
The
clearance is set by a 11.5mm in diameter diameter shim that is placed
just under the bucket, between the bucket and the valve stem.
The
minimum shim that i can find is 2.00mm. I'd suppose that it's difficult
to find shim under this value because this shims seem case hardened, and
under 2.00mm the core can be too brittle, and can split in two under
the load of the valve(ball valve) stem (6mm stem loading something like 60/70kg at
max rpm, spring load + inertia).
I need some 0.95 /1.00mm shims, so i must find a way to machine them by my own.
I know that the tip of the valve is hardened at least at 53 hrc.
I've think about grinding the shims and get them nitrided, so that the core will be untouched.
Someone have a suggestion for me?
You should take a look at Precision Brand, they as well as others have
1.00 mm feeler gage stock. You could also look at hardened drill rod,
key stock, or even flat stock to get your required 11.5mm thickness.
The majority of feeler gage stock is through hardened.
Measure the surface hardness of a factory shim. Then gently grind it down to your required thickness. Measure it again. If still hard, have it checked for grinding checks via liquid fluorescent mag. particle (Magnaglo). If the ground surface is softer, nitriding should be able to restore it.
A good nitrided case can be as thin as ~.005", so a .080" shim would be
no problem. Any "white layer" should be removed by lapping, not
grinding. You'd probably need at least a steel like 4140 to get a good
hard case. But there is another way.
Get a piece of pre-hardened
S7 tool steel drill rod. You'll have to grind it to size, but if you
do it gently you probably won't crack/heat check it.
S7 is a VERY tough tool steel, even at ~Rc58. Your shim stresses are probably pure compression, with little or no bending.
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