I have a 261 block that has been bored to 3.875" for 278 cubic inches; it uses Venolia pistons purchased from Bob Toros in 1961. No problems. Toros used to advertise 3-15/16" pistons for the 261, but I do not know anyone who has used that size. The famous article in the May 1955 issue of HRM about the 261 dyno test by Frank McGurk indicated that McGurk felt that there was no problem with the 0.125" over bore. Has anyone out there had a problem or ever sectioned a block to see how much wall was left? Just how much wall is needed?
