1960 & 1959 270HP (2X4BBL) Pistons Domed or Flat Top

[Richard M.]

The specs on the GM Heritage site and those in the ST-12 are a bit confusing.

What piston type was used on the 1960 270hp(2x4bbl+HLCam) application, Flat Top or Domed?

I did some research this morning, thanks to 2 coffees to help....
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For 1960 the GMH specs say Flat top on the 270(and all lesser hp), and note (a)"Modified dome with FI". Page 31 of this document....

http://gmheritagecenter.com/docs/gm-heritage-archive/vehicle-information-kits/Corvette/1960-Chevrolet-Corvette.pdf

It does NOT say "Modified dome... with FI AND special cam", just "Modified dome.... with FI".
Did they really mean ANY FI, HL cam or not? IMO it must be FI AND HLcam due to the CR specs.

All 1960 engines used the 37754692 heads, therefore the same combustion chamber size.

It says CR for the 270hp is 9.5:1. CR for the 315hp(FI+HLCam) is 11.0:1.
This tells me the 270hp used Flat tops, the 315hp used Domed, right?

What's confusing is the ST-12 specs the CR for both 270hp & 315hp are identical at 11:1. This tells me the 270hp must use the Domed piston. Is the ST-12 wrong? It must be wrong..... Page here.
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For 1959 the GMH specs say Flat top on the 270hp(and all lesser hp), and note (a)Domed "with FI AND Special cam". Page 23 of this document....

http://gmheritagecenter.com/docs/gm-heritage-archive/vehicle-information-kits/Corvette/1959-Chevrolet-Corvette.pdf

It says CR for the 270hp is 9.5:1. CR for the 290hp(FI) is 10.5:1.
This also tells me in 1959 the 270hp used Flat tops, the 290hp used Domed, right?

Again the ST-12 specs the CR for the 1959 270HP and 290HP(FI+HLCam) are both identical at 10.5:1. It must be wrong also. Page here. and next page.
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My '59 had TRW domed pistons, 097 and solids when I bought it in 1986. Rebuilt it in '95(block never/not decked) with forged domed pistons and it's previous 461('65 fuelie heads) heads(cc'd at 68cc), Deck clearance of 0.020" get me to a calculated ~10.5:1 CR. With hydraulic lifters and a slightly hotter Crane Cam, it runs pretty nice, except I miss the lope.

Presently restoring a '60. I got the engine from the owner/friend, a 270hp short block and pieces I just needed to put together. It has flat tops, HL cam(097 Comp Cam spec'd), and stock 692 heads. Deck clearance measured was 0.040", and using FelPro standard gasket set. Calculated CR at about 8.75:1. Maybe domed pistons would have been a better choice, even though the spec appears to be flat tops.

Rich

[William C.]

Re: 1960 & 1959 270HP (2X4BBL) Pistons Domed or Flat Top

Flat tops in all except FI with solid lifter cam. 2x4 with HL cam used the cast pistons. Ignore the ST-12, it is not a difinative reference. The GM parts book (referenced here from 1960 edition) is far more accurate.

[Duke W.]

Re: 1960 & 1959 270HP (2X4BBL) Pistons Domed or Flat Top

AFAIK all 283s except FI w/Duntov cam and the '61 275 HP FI engine used the same flattop piston. These engines were rated at 9.5, but were probably closer to 9.0, and most will operate detonation-free on modern unleaded regular.

If you are rebuilding the engine I suggest you find a piston that will yield 10.3 (maximum) true as the engine will have more torque/power across the range and get 7-10 percent better fuel ecomomy on premium at only a 5 percent increase in price, so the extra torque and power is "free".

Look at this:

http://www.metroli.org/pdf/2012%20Nationals%20tuningseminar.pdf

Duke

[Richard M.]

Re: 1960 & 1959 270HP (2X4BBL) Pistons Domed or Flat Top

Thanks guys.....I will not use the ST-12 for that type of info anymore.

Duke, I was looking for that document, thanks. Great info. I'll have to read it several times to comprehend it all.

Your last paragraph on Page 8 is exactly what happened.......

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The engine was a built short block with a0.040 DC after rebuilder flat top pistons and thick FelPro head gaskets when I got it. I will find out relatively soon what it will run like.

Rich