Welcome to lesson seven in this eight-part series. In this lesson on resurfacing, we show you one of the last steps before the heads are re-assembled. We show a 2-liter head and a 2.7-liter head in this lesson, as they have different sealing surfaces.
Resurfacing the Head Gasket Sealing Surface
Re-surfacing the head gasket sealing surface should be the last machine operation. This is because you do not want to risk damaging the sealing surface if you have to repair a stud or some other operation.
The first step before machining the cylinder heads is to measure the head thickness. Porsche specifies that the cylinder heads can only be machined by 0.25 mm. Though this is not entirely true. Porsche only offered a 0.50mm oversized barrel shim to make up the difference in height and compression ratios when using machined cylinder heads. Though in reality today as parts are getting older and harder to find it is possible to machine the heads more than 0.25 mm. As long as you use a custom thickness barrel shim. You have to add shims any time you resurface a cylinder head to maintain the design.
Measuring Cylinder heads
On the 1965-69 2.0L cylinder heads you can measure the cylinder heads using a height gauge and granite block. The overall dimensions when original should be 87.10 mm +/- 0.02 mm, anything less than this indicates a machined cylinder head.
When measuring the later style cylinder heads, (1970-89 models), the head gasket sealing surface is recessed into the head. The overall height dimension will be the same at 87.10 mm. But if you measure to the sealing surface it should be 84.48mm when new. When you measure this type of head, remember to measure from the cam box sealing surface to the head gasket surface. You can not use a height gauge and block. The way to measure this dimension is by using a large micrometer and measuring in several locations.
Large Micrometers like we use
Machining Cylinder Heads
There are several ways to machine cylinder heads. They can be resurfaced using a milling machine, CNC mill, or by using a lathe. We have used all of these methods in our shop. But I find the lathe method to be the simplest to use.
To mount the cylinder into the lathe we have made a custom fixture. The fixture has holes so that the cylinder head can be bolted to it. The fixture is installed into a 4-jaw chuck. This allows us to center on the combustion chamber. Usually, when setting up the fixture I will take a light cut over the aluminum. This cut is just to make sure that it is perfectly flat with no runout on the mounting surface.
With the cylinder head installed, touch off and zero on the fixture. This way you can maintain the overall dimensions. And be the same from cylinder head to cylinder head. Make sure to cut off equal amounts from both the sealing surface and the boss so as to maintain correct clearances.
Final Steps of Resurfacing
Now all the heads are machined they can be deburred. When you resurface, some material is dragged into the head stud holes and the edge of the holes also becomes sharp. Deburring is an important step. This will make more metal shavings so do the deburring before the final clean. Make the cleaning very thorough as you are now ready for re-assembly.
Don’t forget to stamp the heads with the current undersized dimension. Remember, it is extremely important to maintain the tolerance of +/- 0.02mm. If not, the cylinder heads will not seal correctly when installed on the motor. If they have any variation, beyond +/- 0.02mm, it can result in compression leakage and or oil leakage issues. So be precise in your measurements and machine work.
Ready for the Last Lesson
The final lesson in the air-cooled cylinder head series is reassembly. All the parts and measurements come together with a few tips and tricks. Join us for this final lesson, and you have finished your heads. With your heads done you are closer to a finished engine. Be sure to check our other component restorations.