Valve Stem Wear

[euro911sc]

I wanted to determine if I needed to rebuild the heads and did so by looking at the valve stems first. I took out 2 exhaust valves and measured the stems. I set my gage blocks at the min wear spec in my spec book and set the micrometer to the blocks. This should create a go gage if the stem will not fit in the micrometer and if it does, well it is worn too much.

So, my 2 valves looked like this:



Question:

Little book has the measurement in the middle of the stem... which on my valves is in spec. The engineer in me says that it should be in spec for the entire length of the stem to be usable... They are reasonably tight in the guides... though with my inexperience here they could be loose and I'd not even really know it (why I measured on the stem) Usable or not?

Best regards,

Michael

[911pcars]

This is how valve stems wear due to the wiping action of the rocker arm as it opens and closes the valve. Open the valve about .5", then rock the valve back and forth to feel for excessive play.

If the valve(s) are under spec, replace them; the valve guides as well. This is part of a top end rebuild.

Sherwood

[304065]

Valve stems wear any old way.

All right, I finally gave in and set up on a pin gage that was exactly 8.95mm, class Z tolerance. Got it for about four bucks from McMaster.



Here is a 41 year old exhaust valve. Measurements taken along the stem about every cm. You have to be sure to hold it tight in the v-block or the weight of the head will lever it up, ruining the measurement.

[911pcars]

If you measure a worn guide, they'll exhibit the opposite wear pattern as the valve stem.

Sherwood

[davidppp]

Hello all.

There is as has been said quite a bit of variation in the wear patterns i have measured.

FWIW, the graph John postted is pretty representative..

And IMHO the inlets are almost always gone..

Kind regards
David

[cgarr]

Make sure you clean and dry the valve and guide when checking, oil will make it "feel" ok, also they never wear round, check it in all directions.

[304065]

How do you guys measure the guide? Do you use a go/not-go gage, a bore micrometer, or a bore gage?

[shbop]

John,

I've always used my Mitutoyo small hole gages and a mic. They've served me well in the past, but I'm anxious to hear if there's a new and better way.

[304065]

OK, so my new SI Valves arrived. Thank you Henry Schmidt for the reference to SI, they have a pretty good web site.

The valves are solid stainless steel vs. the sodium-filled of the original. This makes them slightly heavier: an average of 89g vs. 77.5g for the ATE originals, or about 15% heavier. That said, my engine is not a high-rpm screamer motor, and I consider the increased weight an acceptable tradeoff for the lack of taper wear found in my used valves.

I am going to measure EVERYTHING that goes into this engine to the limit of my capabilities. So I measured the new valves. They were manufactured to the low end of the factory tolerance, e.g. right on about 8.938mm.

I used a Class Z plug gage to zero the comparator, then made the measurements with the stem held firmly in the v-block. Note that plug gages are specified as either PLUS or MINUS tolerance. This means that the tolerance value is one-sided, e.g. my 8.95mm class Z gage has a PLUS tolerance of .0025mm, which means +.0025 or minus 0.0000. Yes, to the ten-thousandth of a mm. Can you see that? Hell yes, the Millimess is a very high-amplification instrument.

Anyway, the point of the tolerance is that if you zero the gage on something larger than the spec and then measure something that's exactly the spec, you end up with a reading that's smaller. This accounts, I believe, for the results shown below, which are within the tolerance range if you subtract it from the low limit.



For you grumpy decimal-inch guys: 0.012mm is around five ten-thousands of an inch.

The great thing about new valves is that there is negligible taper wear and a very thick margin.

Well, as soon as the valvesprings arrive these heads will go off to be rebuilt with new valves and springs. Then the CC measurement, then I can order the pistons.

[euro911sc]

Nice work , John! I'm thinking I'll have to with the lighter valves as I don't intend to be driving the car any where near the small RPMs

-Michael