Valve guide/stem clearance

[4sd911]

How much clearance should be between the valve guide and valve stem?
This is what I measured, these heads have 30,000 miles on them they are from a 2.7 CIS. Exhaust valves only measured at this point.

Ex. valve ------ stem ------- guide up/down ---- guide side ways
1 ------------- 0.351 ---------- 0.355 --------------- 0.354
2 ------------- 0.352 ---------- 0.355 --------------- 0.355
3 ------------- 0.351 ---------- 0.356 --------------- 0.354
4 ------------- 0.351 ---------- 0.356 --------------- 0.354
5 ------------- 0.351 ---------- 0.357 --------------- 0.354
6 ------------- 0.352 ---------- 0.356 --------------- 0.354

I'm using a dial caliper that measures to .001, for inside the guide I am using spring loaded T handle type then measure that with the dial caliper. I can see that there is some wear. Should these guides be replaced?

[tom1394racing]

Use the "wiggle" test.

Insert the valve in the guide and see if you can wiggle it and feel any movement.

If you can feel movement, then you need new guides.

[4sd911]

Yes, they do wiggle, I didn't know how much they should wiggle. So I removed one intake valve and it is quite tight, almost no wiggle. Heads are off for rebuild.

[cgarr]

You should really take them out and give the stems and guides a good cleaning with some brake clean then check them, you will be surprised. Installed to spec will even move a bit.

[matt demaria]

Although dial calipers are not that good an instrument to use here, you have made an attempt. Ex valve new is 8.95mm (.352"). My machinist likes a wear limit of .001". I like .0005' tolerance of wear limit. My machinist would toss out the valves that measured .351". Again your measuring instrument is not a precise instrument. It looks as if you should toss all the exhaust valves.
The exhaust guides are set up new with .002" to .003" clearance. This would mean that the holes should measure .354" to .355" when new. The wear limit is .360"
I am not clear on your definition of “up/down” and “sideways”. Valves will wear in a barrel type of configuration. Maximum wear will occur on the stem closest to the valve head. The second wear point is about ˝ inch below the keeper grooves. This is where the end of the guide rests against the valve when it is closed. Check with your fingernail to see if there is a ridge of wear here. If so, toss valve.
Valve guides will wear in a bell type of configuration. Maximum wear is on end of the guide below the valve seat. Looking at the valve guide through the valve seat the end of the valve guide will wear in an oval pattern. The largest diameter will be in the plane through both valves. This is because the valve is pushed back and forth in one direction from the rocker arm movement. Since this is the maximum wear point of the valve guide, the diameter should be measured here. (I use a small dial bore gauge to do the measurements.)
There are a lot of considerations here. You are on the slippery slope of engine rebuilding. Usually german cars will reach the wear limits in about 60K miles. Not to say the engine will destruct at this mileage; but when parts are measured, they will usually be beyond the published wear limits. One dilemma that you have is if you let some of the parts stay, you might be acquiring adverse effects. When the valve guides get old the valve slops around and doesn’t seat consistently on the valve seat causing unexplained missing. Also the valve seal has a hard time sealing oil out from going down the valve guide due to the increased slop. Increased slop will hinder the heat transfer that the exhaust valve is acquiring from the combustion chamber and transferring to the head causing premature seat/valve face failure.

[xtremech]

I agree that the valve guide and the exhaust guides are in need of replacement. The fact of the matter is that the exhaust stem to guide clearance that the factory calls for "new" is considered wiped-out, in comparison to what tolerances can be held with current materials and honing techniques.
One of the issues I have had in the past is that most any valve guide that is available for a 911 is already to big on the inner diameter even before installation. So... after the install, you hone it straight...and now you have a "new" wiped-out valve guide. I have all of my valve guides made with a .351 I.D so the clearance can actually be set properly. Remember, 70% of the heat in the valve is transferred back to the head through valve seat contact. Bad guide..bad seat contact...well, you know what happens next. See if you can maintain exhaust @ .0016---.0018 max on the clearance and you valve job will thank you for it.
(BTW.... Very accurate info Matt)

It has been a while since my last visit...good to be back.

[docrodg]

They are wiped out. Matt was right, you really should use a micrometer... much more accurate. Best is to measure multiple points on stem and guide and write down numbers. Take smallest stem and largest guide measure and the difference is the clearance... yours is .006 which is .1524 mm plus the inaccuracy of the dial caliper (they are limited in precision by a minimum of .01 mm) gives you .1724 total clearance. factory calls for .057 largest new intake and .077 largest new exhaust. This is kinda big already. Service replace limit is .15 Intake and .2 exhaust but that kind of wear would probably kill the valves before it got that large.

Get new valves and guides. Your engine will thank you. Also have the seats done while in there for better sealing and when returned from shop lap them in lightly.

[xtremech]

I agree. When you do lap them lightly, use nothing more than 400 grit. You may have to hunt a little to find it but it will not lie to you, 280 grit will.
MSC or GRAINGER has everything.
REMEMBER, If you have to lap more than once....someone did not do a quality job on the valve job. Typically, all Porsche valve seats are very hard.
It takes ample time and effort to get a concentric seat.
Spend the time with some brake cleaner and a lint free white rag after lapping to remove any residue on the seats and valves.
Good Luck

[304065]

This was my approach to measuring the valve stems.

Measurement, Instruments and Metrology

Ultimately I put a pin gage in a V-block, set the millimess, pulled the gage and put the valve stem in. Decades of wear made the valve stem appear straight to the naked eye but hugely tapered under the instruments. Into the garbage the old valves went, replaced by NOS ATE.

Bill, welcome back to the Forum! There is a really good thread on here about a guy who built his own flow bench. . .Homebrew Head Porting: Attempting the dark art ..... Enjoy