Editing Ford small block engine (section)

===Design changes===
All 221, 260, and 289 engines built from July 1961 through August 1964 used a five-bolt bell housing, with all 221s and 260s being of this configuration, while 289s made after August 1964 changed to the six-bolt pattern – a change made to resolve transmission utilization issues, such as the need for larger-diameter clutches.

The block mount pads and the cylinder wall contour of the 221 and 260 engines changed in January–February 1963 with the introduction of the 289 variant – all 221 and 260 engine blocks up to this time featured "corrugated wall" construction with two [[core plug]]s on the side of each bank and engine mount hole pitch distances of 6 inch.

All three block variants from this point on featured the straight wall method of construction, three core plugs, and an engine mount hole pitch distance of seven inches. The corrugated wall method of block construction had caused cleaning difficulties in the foundry from day one and a change was phased in.