I was installing an electric cooling fan and removing the original belt driven fan from my 261 and discovered an interesting thing. The old fan had two badly cracked blades at the rivets. A very dangerous thing, I think! I could flex the broken blades by hand easily.
After I got the new fan on I went for a test drive and found that what I thought was a driveline or motor mount vibration that needed fixing was GONE! I now remember that there was a posting about fan vibration but I foolishly didn't make the connection to my vibration.
CHECK THOSE OLD FANS FOR CRACKS! A broken blade at 2000rpm would ruin your day.