[pgtr]

I've seen 2 figures thrown around for this dimension: 1.560 and 1.585. Which was 'the' correct or nominal dimension Chevy used on it's production pistons in early 70s?

 

[machine shop tom]

1.560 for the 350 and 1.675 for the 307.

 

[pgtr]

Gotcha - thanks Tom!

BTW you wouldn't happen to know the factory spec'd CCs of the original '71 L48 (270hp 8.5:1CR) dished piston would you? From what I can tell it's anywhere from 10cc to 17ccs depending on the source...

 

[machine shop tom]

Measure the diameter of the dish and then the depth. From these figures you can compute the volume. Then add 2 ccs for the valve reliefs.

tom

 

[pgtr]

Heh heh call me lazy but I'd have to pull the heads to do that

Just now I dug through some more data at kbsilvolite site and it seems to be confirming that nominally they were 12ccs on '71 L48s.

My calcs in my spreadsheet peg it right at 8.42:1 CR (factory advertised 8.5:1) using all the nominal original specs. That's based on a guess on my part that the original head gasket was the thin metal one at ~4.1 bore and 0.16 compressed thickness. Now that I'm feeling comfortable that I've accurately baselined the original engine specs in my spreadsheet I'm going to start plugging in ACTUAL (or planned) measurements as I go along to see where things are at today (not 30+ yrs ago) and where I want everythign tomorrow.

thanks

 

[78 monte]

As far as measureing the dish then adding 2cc.The stock 350 dished pistons also have a beveled(right word??)edge to them that will also add ccs to the pistons total CC volume.
Thats prolly were some people come up with 17cc as compared to 12cc.I think when Carcraft did the multipart build up they fingered the goodwrench motor to be 7.8:1 and 8.4:1 after the heads(76cc) were milled and thin gaskets appled.

 

[jimfulco]

Not all factory dished 350 pistons have the beveled edge. My '72 does not.

 

[pgtr]

I'm fairly confident the factory was using a thin steel gasket then that was 0.016 compressed. The bore on those original thin steel gaskets back then - I dunno! Maybe 4.1 on a 350? I'm get .21 ci or 3.46 ccs.

I'm using nominal 9.025 deck height so am using .025 as deck clearance from the piston.

Regardless of bevels, reliefs or not, if I plug in 12 ccs, that gets my '71 L48 calcs real close to the factory claimed 8.5:1 (I'm calcing it at 8.42:1). (BTW I'm pretty disappointed in Desktop Dyno 2000's ability to calc compression - seems way off!)

The '70 L48 was 300hp and advertised 10.25:1 CR. but the same engine in '71 dropped to 270hp and 8.25:1 CR. WHy? 64cc to 76cc heads and flat tops to dished. I've never found any specific references to the '71 pistons being precisely 12ccs but that seems to be where the consensus leads me. I think '72 engines were pretty much identical to '71s w/ respect to pistons/heads but don't quote me.