There were 4,874 pitchers who made at least one start within my study, of whom 4,093 were never designated an ace.  There were 781 who, in one season or another, wore the "#1 starter" tag.  16%, or about one in six.

Don Drysdale made 464 starts in his major league career; actually, 465, but one of them is missing from my data for some reason.  He pitched in 14 seasons and was designated as a #1 starter in 11 of those.  He was not a #1 starter in 1956, his rookie season, or in 1969, his last season, or in 1958, when he was 12-13 with a 4.17 ERA.  Actually, now that I look at it, even in the three seasons when Drysdale was NOT a #1 starter, he was still pretty good.

Anyway, Drysdale was a #1 starter in 411 of his 464 starts within my data, which is the highest percentage of all time.   These are the top 10:

Drysdale is not #1 on this chart because he was the greatest pitcher of all time; he is number one because he came and went quickly.  Both his training phase and his decline phase were very short, as contrasted with Tom Seaver, who had more years as a #1 than Drysdale did, but who had a fairly slow and painful departure.  Actually, Chris Sale had been a #1 in every one of his 216 career starts through 2018, but that’s only because he had not yet hit his injury/decline phase.  It gets everybody sooner or later.   Gaylord Perry made more starts as a #1 starter than anyone else within my data:

My data is missing 32 starts for Warren Spahn; if we had those, Spahn would probably be #1 on this list, I’m guessing. 

Almost all of those guys are Hall of Famers, you will notice.  One of the problems with what I do for a living is that almost every discussion is in danger of degenerating into a Hall of Fame argument, which becomes repetitious and boring. 

It is interesting, however, that among pitchers with 400 or more starts, 50% of your starts as a #1 starter is the dividing line between Hall of Famers and non-Hall of Famers.  These are the 15 pitchers (with 400 or more starts) who were #1 starters in 50-60% of their careers. 

And these are the 12 pitchers who had percentages in the 40s:

Just above 50%, 9 of 15 are now in the Hall of Fame.  Just below 50%, only 3 of 12 are in.   Below 43%, no one is in the Hall of Fame—that is, no one in my study who made 400 starts in my study and was a #1 starter for less than 43% of those starts.   Simple message:  to make the Hall of Fame, as a starting pitcher, you have to be a #1 starter for half of your career.

These are the pitchers with 300-400 starts who have the highest percentage of starts as a #1 pitcher:

And as you see, most of those are Hall of Famers.  But there are 17 pitchers in my data who made 300 to 400 starts with 50% to 66.7% of those as #1 starters, and not a single one of them has been elected to the Hall of Fame. (There are three Hall of Famers with 300-400 starts in my data and #1 starter percentages less than 50%, but two of those, Burleigh Grimes and Waite Hoyt, were in the majors several years before the study starts in 1921, and there is a lot of missing data for them post-1921, so their data is unreliable.  The third one was Dennis Eckersley, who was a #1 starter for four seasons, 37%, but made the Hall of Fame more as a reliever.) 

Anyway, you make the Hall of Fame if you have 400 starts, 50% as a #1 starter, or 300 starts, 67% as a #1 starter.  50% of 400 is 200.  66.7% of 300 is also 200.   We can conclude, then, that for a starting pitcher to make the Hall of Fame, he has to have 200 starts in seasons in which he was a #1 starter. 

There are, however, a significant number of starting pitchers who made 200 starts as a #1 starter, but are not in the Hall of Fame, including Rany Jazayerli’s favorite, Kevin Appier.  There are 35 pitchers in my data who had 200 starts as a #1 pitcher but are not in the Hall of Fame.  Eleven of those are active or recently retired pitchers:  Verlander, Kershaw, Sabathia, Felix Hernandez, Greinke, Bartolo Colon, David Price, Cole Hamels, Max Scherzer, Dan Haren, and Chris Sale.   Some of those will make the Hall of Fame; some won’t. 

That leaves 24 Hall of Fame eligible pitchers who had 200 starts as a number one pitcher, but who are not in the Hall of Fame:

200 starts as a #1 pitcher means seven years.   This leads us to a generalization:  Seven years as a #1 starter are more or less required for the Hall of Fame, although there are a few guys who made it with six, and Dizzy Dean had only four and Hal Newhouser five.  Basically, unless you are a completely dominant star like Koufax or Newhouser, seven are required, but seven does not guarantee you a seat.   But nine years as a #1 starter more or less seals the deal.  There are only four guys who had nine years as a #1 starter and have not yet made the Hall of Fame, although they are eligible:  Roger Clemens, Curt Schilling, David Cone and Kevin Brown. 

Interestingly enough, the "Years as #1 starter" designation seems to predict Hall of Fame selection as well as the career win total.  No one with less than 200 starts in seasons as a #1 starter is in the Hall of Fame except Dizzy Dean and Hal Newhouser. But the guys with 190 to 199 include several popular Hall of Fame candidates:  Fernando Valenzuela (5 years, 199 starts), Don Newcombe (5 and 198), Wes Ferrell (6 and 196, some data missing), Tommie John (6 and 193).

Several pitchers who had relatively low win totals are in the Hall of Fame, if they had seven or more seasons as a #1 pitcher:  Dazzy Vance, Jim Bunning, Roy Halladay, Bob Lemon, Lefty Gomez.  But some pitchers who had higher win totals are not in the Hall of Fame, if they had seven or less.