425 BBO compression ratios

[DRIVEN]

I've got a line on a couple 425s right now. One is a Toro that's sat for a while and I'm expecting to need a full rebuild. The other is listed as 310hp (orig 2bbl) runner currently breathing through a Carter AFB. Both are listed as '67s so they should be 39* blocks. Lifter diameter is up for debate. Both have C heads.

I'm only interested in the shortblock either way. My plan is to run aluminum heads, a newer cam grind and EFI.

Here's the actual question:
Is it correct that the only difference in advertised compression ratios (9.0 vs 10.5) is from different pistons? A C head is a C head, right?

 

[Turbo Zach]

I have no clue, but I like where this is going.

 

[DRIVEN]

Haha, I thought the Toro guy might have evaporated but he's back. It's 5-6 hours away so I'm trying to get more info about condition. Photos verify Toro though.

 

[spidereyes455]

As far as I know the only difference between C heads is large and small valves. Everything I've seen lists them all at 80cc chambers, and compression was changed with either "high compression" or "low compression" pistons.

 

[DRIVEN]

That's pretty much what I thought. Probably haven't been factory pistons on the shelf for 40 years so it's not like I can call my local Oldsmobile dealer.
It's amazing how much conflicting information is out there when it comes to 425s. Lots of opinions but not much documentation.

 

[fleming442]

Why would you want a factory cast crap piston?

 

[Kennybill]

Toro has small dish pistons and 2.07" intake valves. 2 barrel engine has bigger dish pistons and 2" intake valves, the rest of the heads are the same. Toro engine has big lifters (0.921") the other std size lifters. Toro has one qt bigger oil pan with small windage tray and timing chain windage tray.
The Toronado engine has low profile intake, may come in handy for hood clearance. They are supposed to flow pretty good.

 

[olds307 and 403]

Cutlassefi have brought the Mahle 10cc dish with the ultra modern 1mm ring pack for the 425. Add those to aftermarket 7" rods.

 

[DRIVEN]

Why would you want a factory cast crap piston?

Budget concern, mainly. Was hoping I might get away with rings and bearings. Just curious about possible piston differences. Not chasing custom rods or pistons for a cruiser engine unless I really need to.

I screwed up and procrastinated away a low mile rebuild just a few miles away. Talked to the guy right after it sold. I'm not really in a hurry so I guess I can afford to be a little picky. They just don't seem to be getting any more plentiful as time goes by.

 

[olds307 and 403]

Yeah, I could find Olds V8's plentiful local and 350's were dirt cheap. Now they rarely come up and people want too much. I still want a spare Olds 350 to build with the 4" stroker crank.