Tuesday, November 22, 2011

More investigation: The three 'P's' of restoration.

When working on the engine and gearbox assembly we have come across the following materials

Smooth magnesium (Dow Dichromate finish)  - As used in early Dino cam covers, oil filler, and some other covers
Cast Iron (painted black)- As used in the engine block
Smooth cast aluminum (left unfinished)- As used on the main cover plate on the water pump side
Shiny pressure cast aluminum (left unfinished) - As used on the heads and intake manifold
Rough cast aluminum (painted silver)- As used in the gearbox and engine sump

While all of the finishes made sense to us, it was the painted rough cast aluminum that had us wondering. Was the silver paint original? and Why would Ferrari bother painting the aluminum? Some investigation work was needed.

To answer the first question we checked a number of cars and luckily had access to some NOS parts to look at. Upon inspecting we determined that without a doubt the parts were first painted and then received their finish machining to clean off all mating surfaces.

The second question then asked: 'why paint the aluminum?' Further investigation revealed that the surfaces that were exposed to engine oil were painted in a greenish zinc chromate primer. By VERY carefully removing original paint from our NOS casting we could see that there were traces of the zinc primer on the outside where it had been painted silver. This led us to conclude that the parts were painted all in zinc chromate primer, and then the outside surfaces were painted silver, followed by finish machining.

The question still existed as to 'Why paint in the first place?' The answer came with some research into the methods used to make the rough aluminum castings. These castings were made when molten aluminum was poured into sand molds which were later broken away to reveal the finished part. Because these were gravity castings with no pressure applied during the pouring or cooling stages, the molds were weaker than those found in pressure casting. These weaker molds can cause sand and other particulates to deposit themselves loosely on the final cast part. As such, manufacturers use the zinc chromate paint (which acts as a sealing primer) to seal the outer surface of aluminum and encapsulate any sand particles that could otherwise find themselves into the oiling system and damage the engine. The outside surfaces are merely painted silver to match the cast look of the aluminum.

It appears as though with these cars the detective work never ceases. You are either looking for People, Parts, or Processes.

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