Ok, I am trying to get a better understanding of compression ratios, so bear with me if I seem "confused".

Theoretical engine is a 350 - 3.48 stroke, 5.7" rods. I am looking at some Probe Forged pistons. The listed compression height is 1.562. Now my understanding of this is that if the block was zero decked, these pistons would be 2 thousanths above the block? And if so, in turn - a block with a deck height of 9.025, the same pistons would be 0.023 down in the hole?

Thanks in advance to anyone who can help me figure this out!

 

If the block was zero decked, they would be flush. You are correct about the .023 in the hole on a 9.025 deck height.

 

0 decked means to deck to the piston top. If you had a PERFECT 9.025" stock, with perfect 1.56" compression height pistons, then got it zero decked, THEN put in these pistons, yet, you'd be .002" over the top.

if/when building a motor, get block bored as needed, get pistons, get block honed to suit pistons. Then mock it up, and measure depth from block deck to top of piston, and take apart and get block decked to your liking, (0, or remove .010" or whatever you like.)

I have 1.54" compression height pistons, my block had a virgin bore (so I assumed virgin top deck), then had it decked .010", when measuring after, I had .015" deck to piston top clearance (instead of my assumed .035" or so)

never ASSume

 

Ahh, ok...got it. I just assumed that zero decked meant a deck height of 9.000.

That sucks about your "ASSumption". I was hoping to make the same assumption before ordering pistons, but I guess that's not gonna fly. So basically, if I have just a block, there is no way of knowing the deck clearance without mocking up the assembly

 

well mine worked out ok, I hogged out the heads to 64cc's, so I didn't have too high a compression ratio.
but yea, if you want it all to work out perfectly, then you have to mock it up and send the block back to the shop for decking after.

 

Pretty much. You don't know that there's no core shift in your block, your crank isn't ground slightly off, your rods are sized a little off, and the pistons don't have exactly the right compression height. Tolerance stacking can add up in a hurry.

 

Do a search on the term "quench". Use this site and Google. It will be an eye opener for you. Me I'm going with a .035" quench on my next rebuild. Reason I bring this up is that there is more to the story regarding deck height. You are on the right track in asking the questions.

 

Exactly what sonix said. I had all the machine work done on my engine, mocked it up, measured all of the pistons to the deck, and then picked a # I wanted milled off the deck to get it close (mine was .022"). The deck was flat, but not square to the centerline of the crank, i.e. the front was 9.025 but the rear was say 9.023 or something like that. I picked a # to get me close, and ended up with the pistons in the hole .0045".

 

One more thing I noticed you have not bought your pistons yet. That is a good thing because you may be leaving horsepower on the table. Also with the decking of the block your valve train geometry may have changed. So you will need to check that. By the way do a search on "valve train geometry". That along with the search on quench will put more hosepower into your engine with no extra money spent. You will be buying the right parts the first time around.