Check my math please...

proposed 383 (option A)
3.75" = crank stroke "400 crank"
9.025" = stock block deck height
5.7" = rod length
1.45" = piston compression height

The compression height of the pistons would need to be:
3.75/2 = 1.875 (half the stroke length)
9.025 - 1.875 = 7.15 - 5.7" rod = 1.45" compression height

proposed 383 (option B)
3.75" = crank stroke "400 crank"
9.025" = stock block deck height
6.0" = rod length
1.15" = piston compression height

The compression height of the pistons would need to be:
3.75/2 = 1.875 (half the stroke length)
9.025 - 1.875 = 7.15 - 6.0" rod = 1.15" compression height

So my question is... which would work better? Would one compression height provide better durability over the other?

Jegs sells pistons for both rod lengths to build a "383 with a 400 crank". What are the advantages / disadvantages? Is one way better?

 

Do a search for 6 inch vs 5.7 inch rods. Too many posts to even begine to cover.

Basically though a longer rod will sideload the cyl walls less which is a good thing. Downfall is you have less piston girth, so if you are going forced induction, nitrous etc, you may be better off with more piston and shorter rod. You also have 5.85 rods as an alternative that many off the shelf pistons will fit.

 

You are making an assumption, that nothing will need to be taken off the deck to make it flat and true. Most people want to "zero deck" it at 9.000". Otherwise you can end up with odd piston sizes (like your method). Decide what rod you want to with (a trivial choice for an NA motor, IMHO) then get the block to the machinist before ordering parts.

Rich

 

Yes, this is N/A.
So the best way to do it would be to deck the block to 9.000 and do the following:

The compression height of the pistons would need to be:
3.75/2 = 1.875 (half the stroke length)
9.000 - 1.875 = 7.125 - 6.0" rod = 1.125" compression height

sound right?

 

Yes, that is correct. But still I advise ordering parts until the machinst has had a look at the block.

Rich

 

Cool! I have a 4-bolt block sitting on a stand in my garage awaiting prep. It's going to be awhile before I have the money to build it, but I need to know what direction to go.

 

I recomend 9.005 just in case you ever burn a head gasket and need to clean up small burn damage to the deck. Be sure to tell your piston supplier in all of that equation you want the piston to sit .xxxx in the hole at TDC.

 

Dont forget to figure in your desired quench. Unless you intend on picking up Cometics custom head gaskets, you will want to figure in your intended gasket into the final deck height.

For example...if recommended quench is at least .040, but you dont want it too deep, so say .045. Figure a felpro .039 head gasket, you will want to deck the block to the above mentioned height, .005, which also leaves room to make future repairs.

Edit: also involved, probably not a problem but your PTV clearance depending on how big a cam and how much you would plan to advance/unadvance it.