Actual compression of a 68, 340?

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1968FormulaS340

1968FormulaS340

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Actual compression of a 68, 340?

I know the advertised number but I have seen posts on here that said when the math was actually done it was usually a bit off.

Just curious. Thanks.
 
Around 9.2:1 depending on the casting and machining quality, which was all over the place.
 
The only to be sure is to measure your own and calculate it. It is medium hard, but is achievable with a graduated beaker, 1/4" plexiglass, vernier calipers, and axle grease once you remove the head from the engine.

I could describe how to do it if you are willing to go through it, It just takes a little while to explain. If you really want to know, post me back and I can explain how to do it and try to keep it simple and unconfusing. Also, you will need to measure the thickness of your original head gasket.
 
68 340 block, (.020 over bore) X head, steel shim head gasket, aftermarket "stock" pistons.

9.41:1 compression ratio!
Every thing was cc, that is the exact compression.
 
I have checked a couple and one was just under 9.3 while the other was just over 9.4,this was with a deck bridge,cc'd heads,stock gaskets ect.The one thats really scary are the later 340s!.
 
lead69 said:
I have checked a couple and one was just under 9.3 while the other was just over 9.4,this was with a deck bridge,cc'd heads,stock gaskets ect.The one thats really scary are the later 340s!.
Click to expand...

With factory OE pistons?

When I take mine apart someday, I'll do a piston depth on it. It's the original 1968 std bore pistons in it, but I've had the heads cleaned up surface cut a couple of times. And I've sanding scrolled the cambers.
 
autoxcuda said:
With factory OE pistons?

When I take mine apart someday, I'll do a piston depth on it. It's the original 1968 std bore pistons in it, but I've had the heads cleaned up surface cut a couple of times. And I've sanding scrolled the cambers.
Click to expand...

With the price of 340s there were not to many being sold let alone rebuilt around here for years so when you got one they were usually factory, and its pretty quick to do,all you need is one old 340 piston to get the pin height,I really enjoy the details more than the big stuff lol.
 
9.2 to 9.4 for 340 makes me wonder what my stock 360 1992 Magnum is as it is advertised at 9.1 to 1?
Probably pretty low.......
 
i hear they were around 9.8 to 10.1 not the 10.5. i don't think they were much lower than that. the 10 to 1 BB were the same about .5 lower then advertised. by stock 383 was 9.4 with a .040 gasket with the .020 that would have made it about 9.7 from factory. but one side of the block the pistons were all at zero deck, the other side started at .005 under and got worse towards the rear of the cyl, so the machining was sloppy
 
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