Valve stem diameter tolerance

I’m not clear what the maual means relative to valve stem diameter

This shows valve stem diameter at 5/16, or .3125 then there are two figures, -.0025 and -.0035. I assume this is the tolerance for wear, is that correct? Seems strange to have two numbers in there.
It’s a bit of an academic question perhaps since my stems are all between .309 and .310, and I’m already in there, but I would like to understand it when the machinist gives me the low-down…

John North

Nothing in the engineering world is exactly exact. Every drawing or written design specification will state the allowable tolerance for the part shown. Anything within that range meets the specifications.

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On valve stems, under the tolerance may have a tight fit which may be problematic for friction issues over the temperature range of operation. Over the tolerance may give increased tilt and wear along with excessive oil drawn down the guide.

John, you normally get more wear in the valve guide, so you need to check movement of the valve head at 90 deg to the camshaft and compared to parallel to the camshaft.