Thread: 355 build HP and Torque?

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Originally Posted by rollie715

Trying to decide between 12cc or 18cc dished pistons. Here's a link to the 12cc ones on Ebay: I'm also wondering if I should buy the .030 over pistons now before I take the block to the machinest or have him check the size, then wait for the pistons and then proceed to bore according to the actual piston sizes. I've never done this before, I'm learning a lot on the computer, but don't have much actual engine building experience.

Those pistons should work fine, plenty of squish pad and the correct compression height. I strongly recommend getting as close to 9.5:1 SCR as you can, in order to use the cam timing that I linked earlier, or something very close. Your first task will be to cc the chambers. Without that info, you have no idea where you are, so you have no idea of where to go. Haul the block to the shop and have the machinist look it over for main bearing alignment, size and roundness. The block may need to be align-honed or align-bored to get the holes perfectly round and parallel with each other.

He will measure the bore and tell you where the bore needs to be for new pistons. DO NOT EVER BORE A BLOCK UNTIL YOU HAVE THE PISTONS, RINGS AND COMPLETE INSTRUCTIONS FROM THE PISTON MANUFACTURER. Cylinder wall hone finish will depend on the ring material used. I highly recommend moly rings rather than cast iron or chrome. They break in quickly and will last longer.

Once the block is bored and honed for the new pistons, you can haul it home and check the block deck height on each corner. The machine shop can do this for you also, but their labor is more costly than yours. You will know that the crank radius for a 350 crank is 1.740", the rod length is 5.700" and the piston compression height is 1.560". Therefore, your "stack" of parts will measure 9.000". If, for instance, you measure #1 piston deck height at 0.024", use a Sharpie to write 9.024" on the block deck at the #1 cylinder. If, for instance, #7 hole measures 0.012", write 9.012" on the block deck at #7 hole. Going to the other side of the block, let's say that #2 measures 0.017" and #8 measures 0.020". The shortest hole will be #7 in this case and so you will use that as your standard from which to cut the decks. Did you read my instructions anywhere about finding piston deck height with a steel rule and a set of feeler gauges? If not, I'll outline it here for you. I know some of these guys gripe and moan about using old tools to do this stuff, but it's not rocket surgery. If you're within a couple of thousandths on the piston deck height on all 8 holes, wonderful. Pistons will grow taller as they heat up and the rods and crank will bend and stretch a little as well, so don't think that you have to have everything to the nearest ten thousandth of an inch. David Vizard found that he could set the squish as close as 0.026" before the piston began kissing the underside of the cylinder head, so we know the "stack" gets taller under engine operation.

I recommend that you purchase and read through this offering from David Vizard before going any further.....
They're showing 26 used from $10.94 plus shipping.....
Amazon.com: How to Rebuild Your Small-Block Chevy (9781557880291): David Vizard: Books

Taking a light cut on the cylinder head surface will decrease the chamber volume if you need smaller chambers to reach the desired SCR. On L31 heads, each 0.0065" (six and a half thousandths) cut will reduce chamber volume by ONE cc. Therefore, a +/- 0.020" (twenty thousandths) cut will reduce chamber size by 3 cc's in addition to trueing the head surface to flat. By the way, if you haven't magnafluxed the heads, do so before you do any work to them. They're thin-wall castings and could be cracked.

Don't buy anything else until you have engaged the auto machine shop that you will use. Those fellows will have suppliers that may be cheaper or have better products than you have access to.