You want accuracy when setting the valves on your 911? This method delivers.
This simple, advanced method of 911 valve adjustment does away with the hard to get at gap between the swivel foot, (elephant foot), and the valve.
This procedure uses the gap found at the other end of the rocker arm.
You don't need that special shim holding tool, just a normal everyday feeler set.
You will use ordinary 2.5 and 3 thousandths feelers as a GO and NO-GO gauges.
Figure one.View of a 911 engine showing insertion of feelers.
Thanks to Tom Hutchinson, aka khamul02.
Referring to figure one.
This method uses the gap between the rocker arm and the cam. As shown by the red arrow and line.
The classic method as described in the previous article uses the gap between the elephant foot and valve stem.
The difficulty with this as experienced by any who have done this procedure is the close quarters around the valve adjusting screw and the need of a special shim holder tool.
Photo one.The tight area at the rocker arm adjusting screw. Notice you need to slip the special shim into the gap between the swivel foot and valve stem. No easy task.
It is simpler to slip a common feeler gauge into the gap between the rocker arm and the cam. Specifically the "backside"(base circle), of the cam.
Photo two. The GO 2.5 thousandths gauge slips into the gap. This is correct.
Notice how easily the gauge is guided into the gap.
Please note the position of the cam lobes in figure one and the photos. The rockers are against the "backside" (the low side or as it is known to cam people "the base circle"). In this position both valves are closed.
As you can see from the figures and photos it is easy to slip an ordinary .0025" feeler gauge into the gap between the rocker and the cam. This is one advantage of this method.
Why do we use a .0025" feeler?
Simple math. The ratio of the rocker as it sits against the base circle of the cam is 1.4 to1.
This means the .004" gap at the rocker swivel foot and valve stem is 1.4 times greater than the gap at the point where the other end of the rocker contacts the base circle of the cam.
So if we divide .004" by 1.4 we get .00286". Actually .00285714286",
So .00286" is a rounded off number. Plenty close for what we are doing.
To answer the question why a .0025" feeler?
Since it is smaller than .00286" it will slip easily into the rocker/cam gap.
We now have a GO gauge, .0025".
As a Go gauge it will slip into the rocker/cam gap unless the gap at the other end of the rocker is less than .0035".
Now the NO-GO gauge .003". This should not slip into a properly set gap. See photo two.
Photo three. The NO-GO 3 thousandths gauge blocked. This is correct.
The NO-GO 3 thou gauge will only slip in if the setscrew end of the rocker/valve gap is greater than .0042". Correct is .004" (0.1mm).
This means if the valve gap is set over .0042", looser than 2 TEN thousandths, the NO-GO 3 thou gauge will catch it.
You have to work very carefully to get this kind of accuracy with a dial indicator.
And of course you can only use a dial indicator with the motor out or partially dropped.
The advantage at this point is it is quicker than a dial indicator and as accurate.
You adjust the rocker arm set screw so it allows the GO 2.5 thou gauge to slip into the gap, but the NO-GO 3 thou gauge will be stopped.
See pictures two and three. Picture two shows the GO 2.5 thou gauge in between the rocker and cam. Picture three shows the NO-GO 3 thou gauge blocked.
This is what you want.
If you have read this far, save yourself some time and check the gaps with the NO-GO 3 thou and the GO 2.5 thou beforeyou adjust any rocker screws.
With this quick check you will find which valves are OK and which valves need adjustment. After all it is unlikely you need to adjust every rocker arm.
Notice you don't need to learn the "feel" to correctly adjust your valves.
The GO gauge slips in and the NO-GO is blocked when the gap is correct .
When you are done, your valves will be set within 3 or 4 Ten thousandths of optimum. No worry about whether you got it right. GO and NO-GO answered that.
No more bent or broken .004" shims. No more dropped and lost screws.
You don't need that special tool any more.
This is a simple quality control check for any other valve adjusting procedure.
Forget about the "magnetic drag or feel". This is GO or NO-GO.
Note: The dimensions in this article are rounded off for simplicity. The actual exact dimensions will vary slightly.
Pelican Parts sells the 0.0025" and 0.003" feeler stock. The part numbers are
Pelican Technical Article:
911 Valve Adjustment The Backside Method
2.5 (.0025-inch) and 3 (.003-inch) thousandths feeler gauges, metric socket set, flathead screwdriver
Porsche 911 (1965-89)
0.0025" and 0.003" feeler stock (Pelican Parts part numbers are PEL-LM-0025 and PEL-LM-003 respectively)
Properly adjusted valves
Replace your spark plugs