what are the pros and cons of boring a B18C to 84mm vs 85mm if i plan to turbo it in the future?

 

Pros:
increased displacement, ability to run a more shallow dome on piston and maintain same static compression
Cons: weaker cylinder walls, less room for error (less opportunities to rebore)

there are some other minute effects on air flow with regards to having a larger diameter bore than combustion chamber diameter, but they're not important for the scope of your question.

 

what do you mean by more shallow dome on the pistons?

 

please post forced inducted related topics in forced induction.

 

Just to clarify, if you're planning on going to 84 or 85mm, you cannot use the stock sleeves. At this point, aftermarket sleeves are the only option.

Going larger increases displacement, but at the cost of thinner sleeves. With aftermarket sleeves, thinner (weaker) sleeves aren't much of a concern...the bigger issue is that if you were to tank a piston, there is not much room for error. 84mm sleeves can be overbored 0.5mm twice safely, whereas an 85mm block could possibly go to 85.5mm but nothing more if you plan to boost. If this is a fresh set of sleeves, go to 83mm or 84mm max...i wouldnt recommend going larger than that.

 

Another thing that should be mentioned is head gasket sealing can start to become an issue when you get 85mm+ and remedies should be taken into consideration like O-ringing for instance.