About 15 years ago I worked for a toyota dealer. Only stayed for 2 months, but the only thing worthwhile I discovered was this: I don't know the part number, but on many of their engines, they used vulcanized grommets on their valve covers. The work perfect on nailhead valve covers and valley covers. No squishing, no leaks, and very re-useable. There is too little clearance between a dual quad manifold and the valley pan to use them on that application, but there is in all all the others, including cast rocker covers.
Thanks for the tip Joe. They could be sanded down to make them thinner for the valley cover application.
