71 Base Engine Pistons
Thread Starter
Instructor
Joined: Jun 2010
Posts: 120
Likes: 1
From: CT
71 Base Engine Pistons
I have read a lot of stuff about 71 base pistons but have yet to find the depth of the "SUMP" on the piston. It's not described as reliefs in the piston top, so I assume that refers to a dished piston top and am very curious to know the depth of the dish, short of measuring it myself when I disassemble the top end.
Want to understand how much roller cam lift choice flexibility I have before I remove the heads.
Someone must know the answer to this.................
Want to understand how much roller cam lift choice flexibility I have before I remove the heads.
Someone must know the answer to this.................
Race Director
Joined: May 2006
Posts: 18,729
Likes: 2,579
Have you looked at Competition Cams, Cam Quest tool for selecting cams?
PDF page 123 describes the pistons as sump with no dimension. Remember the valves are at a 23 degree angle so not sure how the sump depth will help you.
https://www.gmheritagecenter.com/doc...t-Corvette.pdf
PDF page 123 describes the pistons as sump with no dimension. Remember the valves are at a 23 degree angle so not sure how the sump depth will help you.
https://www.gmheritagecenter.com/doc...t-Corvette.pdf
Last edited by MelWff; Nov 6, 2020 at 01:19 PM.
Safety Car
Joined: Aug 1999
Posts: 4,862
Likes: 118
From: Texas
There are three limiting factors to valve lift. They are valve spring coil bind, valve guide/seal to retainer interference, and piston to valve clearance.
It's easier to measure these clearances when building an engine on a stand, but it is possible with the engine installed. To do it in the car, you'll need a dial indicator with a magnetic base and a pair very light springs to temporarily replace the valve springs.
With the very light springs installed on the #1 cylinder and the piston at tdc, you can push down on each valve and measure the distance until either interference occurs with the valve guide seal, or the valve makes contact with the crown of the piston.
Record that distance, then subtract about .050" and you'll have a number that is the max cam lift your heads and pistons can tolerate.
It's easier to measure these clearances when building an engine on a stand, but it is possible with the engine installed. To do it in the car, you'll need a dial indicator with a magnetic base and a pair very light springs to temporarily replace the valve springs.
With the very light springs installed on the #1 cylinder and the piston at tdc, you can push down on each valve and measure the distance until either interference occurs with the valve guide seal, or the valve makes contact with the crown of the piston.
Record that distance, then subtract about .050" and you'll have a number that is the max cam lift your heads and pistons can tolerate.
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,135
Likes: 4,244
From: US-PNW
I think I'd pull a spark plug and use a bore-scope to see what the top of the pistons look like. I could certainly be wrong in saying they probably are not dished, but I've not seen factory dished pistons for Corvettes C-1 through C-3. Can't say later than that because I just don't have the inclination to follow those generations.
