[tubman]

Gentlemen - You have to realize the intent of the O/P, which is : "is it worth the effort to add a 4 inch crank to increase the displacement from 239 to 255?" I am just pointing out that if you're looking to increase your displacement, boring the engine out is a more than reasonable alternative to using a 4" Mercury crankshaft. Not only do you get 3 more cubic inches and less piston speed, it is MUCH less expensive. You could also update to better pistons with the metric rings everyone is so excited about. The only possible downside on increasing the displacement in this way is compromising the future rebuildibility of the engine. It is my personal opinion, that by going to 3 5/16" , that compromise is so small as to be almost nil. You can disagree with that if you want, but there are so many 3 5/16" and up bore engines out there that I am completely comfortable with my position.

With all due respect "deuce_roadster", the O/P came here asking about an increase in displacement, not whether a stock displacement engine provides adequate performance on tours. I went from a rather tired 239 in my '51, which provided "adequate performance" to a warmed over '51 Mercury; while it wasn't necessary, I am much happier with the car now. One last comment : I do not consider a 1/8" overbore in a 239" block "taking it to the max".