Hi. I have a '65 Mustang with a 289-2V. The date code for the engine is 5C 10R. I know that this engine was assembled on March 10, 1965. What does the R mean? Also, I know the timing chain design was changed from a thicker chain to a thinner chain in March of 65, but does anybody know when? I'd rather not rip it apart before I have the parts. I just want to get that nylon gear out of there as it is 37 years and 110,000 miles old... Thanks in advance!