Center Air Guides. Technical Service Bulletin
This Porsche TSB, or technical service bulletin, is in regard to the center air guides. This TSB covers Porsche air-cooled engines from 1964 to 1977. Engines from 1978 and on will be fitted with center air guides that are already the modified design.
Performing the modification
The center air guides can be easily recognized by the ends. The fins go both directions from the center. This is because they are fitted up against the cylinder. We are performing this TSB after the guides have been restored. It is cleaner if we did this before the finish was applied. Just protect any cut edges.
Mark your guide at 25mm down from the top edge of the center air guide. Mark the end of the cut 19mm from the back of the guide for normally aspirated engines. On the turbo engines, the end of the cut is 10mm from the back of the guide.
Using a quality pair of tin snips, cut out the marked section. File the edge smooth and protect from rusting by applying clear nail polish or repaint the edge. Install the guides as normal, using the holding bows, making sure they are tight and snug up against the cylinders.
Effects of this TSB
The reason Porsche issued this technical service bulletin was to help reduce engine temperature. By trimming the center air guides you could lower the engine temperature by 5.5 to 8.5°C. This heat savings at this place in the engine is important. The heating of the cylinder’s ads to their expansion and this expansion puts stress on the head studs. Lowering the temp at the cylinders can help with head stud pulling.
Many factors can contribute to the head studs pulling on a Porsche Air-Cooled engine. We suggest installing time-serts if you are rebuilding an engine. This TSB can be part of a major rebuild. Or you can perform as needed. It is easy to investigate your engine to see if the center air guides have already been modified.
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