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I've heard different stories on when 9 fin valve covers were used. Someone said that 9 fins were used only on FI cars, and I've also heard that sometime in Jan - Feb 57 that GM switch over to 7 fin valve covers on FI cars. Can anyone clear this up for me including where I can obtain documentation on this topic.
The optional cast aluminum valve covers underwent a series of changes over the years and what's considered correct is described in the particular NCRS Technical Information Manual & Judging Guide (JG) for the year of Corvette of interest. The covers started life in '56 and had nine longitudinal ribs with a script 'Corvette' in the center in low relief.
These continued in '57, but were altered by grinding the inside outer lip to give clearance for FI engines air meter, and eventually changed in late '57 production to the seven rib design which had sufficient clearance for FI engines. All three versions (9-rib without cut, 9-rib with cut, and late '57 seven rib) feature the script 'Corvette' in low relief and had staggered mounting holes to agree with the make up of the cylinder heads of the day.
The '56-57 book provides a matrix of which version of the valve cover was used on what engines in '57 by approximate VIN of the car. Later, there would be other changes to the valve covers that included raising the height of the script 'Corvette', changing the mounting hole configuration from a staggered to a straight across orientation, and finally the introduction of a line of 'flash' through the 'O' in the script Corvette that resulted from damage and repair of the molds.
Early 9-fin versions of the cover are rarer as they're older and fewer cars were originally produced with this version versus the later 7-fin style with straight across mounting holes that were used during higher production periods including the mid-year cars.
57 vin 1093, a 250 hp pg, has the 9 fins and 57 vin 4450, a 283 hp t-10, has the 7 fins. i believe the juding manual gives a more specific answer but mine isn't handy. god luck, mike
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