911 Valve Guides
The valve guides in the Porsche 911 Cylinder heads are consumable. The guides will also experience a lot of wear. There is constant friction between the guide and valve stem and the valve-train geometry puts even more side pressure on the guide.
Porsche has always used a bronze guide. The bronze has a similar expansion rate as the aluminum head. Today you can choose between manganese bronze or phosphorus bronze.
Manganese-bronze guides generally consists of 55% copper, 40% zinc and 3.5% manganese.
Phosphor Bronzes, or tin bronzes, are alloys containing copper, tin and phosphorous. The phosphor bronzes contain between 0.5 and 11% tin and 0.01 to 0.35 % phosphorous. The addition of tin increases the corrosion resistance and strength of the alloy.
The phosphorus bronze could have a slightly better wear time, but all valve guides get worn out. The exhaust side wears two to one over the intake side because of the heat and it is just a hard place to be in the engine.
Worn-out Valve Guides
Your valve guides could be worn if you see excessive smoke in the exhaust. Especially when you decelerate because of the increased manifold vacuum. Increased oil consumption can be another sign. Increased because Porsche air-cooled engines consume oil already. Normal oil consumption is between 1 to 1 ½ quarts per 1000 miles. So, if you notice your puff of smoke in the mirror getting bigger, and your engine is thirsty for oil, it may be time to look at your heads and guides.
Valve Guide Inspection and Replacement
The first way to do a quick guide check is to use a Go/No go gauge. A Go/No Go gauge is just a pin gauge. Choose a pin at the maximum wear limit of the guide. If the gauge will fit into the guide, then the guide is not usable and will need to be replaced. This is a quick test but does not always show the true state of the guide.
To get a true indication of the guide condition, a new valve should be installed. And then using a dial gauge measure the amount of “rock” in the guide. This will show the true clearance of the guide. The exhaust valve should have a new clearance of 0.05 – 0.07 mm. The guide is worn-out at 0.20 mm. The intake valve to guide new clearance should be 0.03 to 0.05 mm and is worn-out at 0.15 mm.
Removing Valve Guides
The guides are press-fit into the cylinder head. To remove them we need to machine off the top of the valve guide so that it is flush with the cylinder head. Then using a valve guide punch drive the valve guide out in the direction of the valve head. Do Not attempt to drive out a valve guide from the port side of the head. It will risk cracking the head and causing excessive damage to the guide bore if driven backward.
Once the old guides have been removed measure the old ones to get an indication of the currently installed guide size. Valve guides are available in three sizes, standard, 1st oversize, and 2nd oversize. Often different manufacturers will have different sizes. Always measure the new guide outside diameter before reaming or installing it into the cylinder head.
Using a fine ball hone go ahead and hone the valve guide bore in the cylinder head. This is to smooth out any imperfections caused by driving out the old valve guides. If the ball hone does not clean up the bore, then the bore will need to be machined. You will need to machine to the next oversize and install a matching guide.
Measure the outside diameter of the new replacement valve guides. The valve guides need to have an interference fit of -0.05 mm. If the interference between the guide and the cylinder head is greater than 0.05 mm, the bore will need to be reamed or the guide O/D reduced to meet the tolerance.
Most time I will opt for reaming the cylinder head. This is because the ream will clean up any damage that was caused by removing the original valve guides.
Installing New Guides
To install the new guides lightly lubricate the valve guide with grease. The grease will help the guide install without binding in the cylinder head. Using a 9mm punch drive the new guide into the head until it stops.
Once the new guide has been installed, it will need to be reamed. This is because as the guide is driven in, the interference fit causes the guide to close in on the inside diameter. Also, often the new valve guides are supplied semi-finished. This is to allow for the machine shop to set the correct valve to guide clearance. Ream the valve guide to a 9.00 mm size. Next hone the guide using an ultra-fine ball hone. Hone to remove any reaming marks.
Once you have installed all the guides, the valve to guide clearance needs to be checked. Using a new valve installed into the guide and using a dial gauge check the clearance. If the clearance is too tight, hone the guide until the clearance is within specifications. If the clearance is too big then the guide will need to be replaced.
Tools You May Need
To make the measurements needed in this lesson we use a few precision tools. We use a go/no go gauge for a quick measure with pins in the size we need. It is difficult and expensive to get metric pin sets in the United States. We do the math and convert the pins from standard to metric. Sometimes we buy single metric pins when needed to hit a precise number.
We use a micrometer and stand to measure the guides. You do not need the stand but we recently added it and it has been such a help when measuring smaller parts. Keeping your hands free also keeps the temperature down. Check our measurement course to see why this matters.
A dial gauge and stand, to measure the play in the old and new valves is an essential tool for this job.
Ready for Lesson Five
Now that you have finished Lesson 4, it is time to move on to lesson 5. In this next lesson on the cylinder head, we will cover cutting the valve seats.