Piston ID

[240260280]

24 minutes ago, jonbill said:

I have 0.58 mm clearance in my engine. I originally had 0.56 mm and a piston just kissed the head.

0.70mm is what I used for "squish" in an engine build tool... nice to have a real world limit!  THANKS!

[ToolBoy]

3 hours ago, Zed Head said:

 Doesn't really look like a drop it in and go engine.  Measure and calculate combustion chamber volume to start.

Or, since you have it apart, just buy pistons that will give you the CR you want for a street engine.

Thanks for that. . . Sounds like a very insightful suggestion to start conversation at the machine shop. . . 

[ToolBoy]

Might not have mentioned it before but after getting the head off and turning the crank by hand, I remember noticing the tops of the pistons finished  (popping up) above the surface of the block. The head also has the relief to accept it. Might be that "73 engine. Block number is 172296. I'm unclear if these pistons were ever on the new 73 car or were they only a factory replacement? I'm gonna tear down the L26 next week and have everything from both engines available before I visit the machine shop. Should have some options. 

[ToolBoy]

. . . think I answered my own question after looking at  '1973 non-interchangability drawing posted by Hoover. Indicates It was the stock setup on that '73 car. . . Those pistons in that head. . .  

[HS30-H]

6 hours ago, ToolBoy said:

. . . think I answered my own question after looking at  '1973 non-interchangability drawing posted by Hoover. Indicates It was the stock setup on that '73 car. . . Those pistons in that head. . .  

That number '3' stamped into the piston crown in one of your photos indicates that this engine has been apart at least once since it left the factory. It is not a factory stamping.

I still believe that you are looking at H2716 replacement pistons, and that they have been incorporated in this engine during a rebuild. I also believe the H2711 stock pistons in that TSB were flat-topped, without valve pockets. 

[240260280]

Were the flycut pistons in Euro cars or were they Nismo parts?  We don't really see these in stock pistons in NA.

 

Here is Nov 1971 HS30 Sports Option Parts document excerpt showing fly cuts (B):

 

Edited September 3, 2020 by 240260280

[HS30-H]

14 minutes ago, 240260280 said:

Were the flycut pistons in Euro cars or were they Nismo parts?  We don't really see these in stock pistons in NA.

 

Neither.

Go back to post #11.

[240260280]

Hmmmm this is becoming circular.  In what markets were these pistons fitted?  I have not seen in the USA or CDN markets? 

 

Early FSM shows what we typically get on L24

[HS30-H]

53 minutes ago, 240260280 said:

Hmmmm this is becoming circular.  In what markets were these pistons fitted?  I have not seen in the USA or CDN markets? 

As I wrote before, this type of piston was supplied as a service replacement. I have not said anything about being fitted to stock L24 engines, in any market. 

You seem to be hung up on the idea that piston relief cutouts = 'high performance'. But the 'Pop-Up' pistons were designed to intrude into the combustion chamber of particular types of cylinder head, and required the relief cutouts because this brought them closer to the valves. They are not high performance pistons per se.   

[240260280]

OK thanks. Got it.