Why is it that i have heard that 305 and 350 small block chevys share the same block, yet why can't you punch out a 305 to a 350?? (i have heard of 335's but not 350s, 383s etc)
Is there a critical difference somewhere in the artitecture of the block perhaps? Ive been meaning to post this question for a while, and ive asked around and no-one i speak to seems to have a definite/solid answer.

Whats different?!?!

 

The outside dimensions of both blocks are identical, but the 305 has a 3.73 inch cylinder bore, and the 350/383 have a 4 inch bore. You can only safely bore any stock SBC .060. 3.73" to 4" is .250. The cylinder jugs are cast specifically for each block, and they're too thin to be bored a whopping 1/4". The 305 also shares the same 3.48" stroke as the 350, it just has the smaller bore. The 400 SBC still has the same outer dimesions as the 305/350/383, but the bore is 4.125". The only way GM was able to manage that big of a bore diameter in the same block was by siamesing the cylinders with steam ports between the cylinders instead of full water jackets. In fact, it's recommended to only bore a 400 .030 maximum. Otherwise, they're subject to overheating problems. It's a whole different story if you buy a "bowtie" performance block from GM. These are cast with very thick cylinder walls, so they can bored a lot bigger than the regular production blocks.

 

exactly there pretty much the same except for the bore size so if you had a 350 everything would transfer over to the 305 except the pistons. thats why everybody says to build a 350 instead of a 305 you can make more power with exactly the same setup because of the extra displacement.

 

Finally! Thanks Pat for the info!