Increasing the Bore Size

how do you bore a motor

Boring, or increasing the bore of an engine, increases the displacement. The cylinders are machined to accept pistons with a larger than stock diameter, as is shown on the right side of the diagram.

So how big can you bore your motor? Here's the way it was done in the early 1950's:

49-52 F & M 3 5/16"  3 3/8"*
46-48 F & M 3 5/16" 3 3/8"*
39-42 M 3 5/16" 3 3/8"*
39-42 F 3 3/16" 3 3/16"
35-38 F 3 3/16" NOTE 1
32-34 F 3 3/16" 3 3/16"

F = Ford
M = Mercury
* = Will probably crack from bolt holes to edge of cylinder.
NOTE 1 = Bore as little as needed to clean up the cylinder. .060" over is usually safe, .100" over will sometimes "fall through".

 Large bores (>3 3/8") will tend to balloon-out at full power, causing increased blow-by and a loss of power. Large bores also increase the chance of cracking around the bores. If you plan to keep the motor for a long time, start with a bore size that will allow you to clean up bore the motor again in the future. I like this thought from a 1950's manual, "If you are racing in a class where every HP must be obtained, then go to the limit and don't worry about losing an occasional engine - it's to be expected in competition and there is absolutely nothing that anyone can do about it!" Somehow I don't think flathead blocks are that easy to come by today.

Here's a chart from Hot Rod magazine.

Back to the hot rod the flathead page.