"Block relief" - L.M. Williams book

[Chris KC]

As I understand it, taking the sharp square edge off the bore on the valve side helps things somewhat, so there is no need to gouge a great big trough.  Like many block and head modifications, more harm than good can come from removing too much metal.

That's kind of how I see it - small potential gain, if everything else is on song - vs risk of buggering up a good block. I have never used a block like this so don't know what I'm missing; on the other hand my car goes alright with parallel bores.