I was at an antique store and found a Parker 21 for sale. It was only $8, and I was about to snatch it up and then I found some cracks in the plastic shell, partly through the threads. I set it back down and went my way. Bad move? Can cracks be repaired?

$8 ? I wouldn't pay more than $5 for a good one but you may get diiferent opinions. Parker 21s are not woth much AND they are very, very prone to cracking in the grip (hood) The cracks are not repairable so you made the right decision.
The 51 is the one to look for; it was voted "Best fountain pen ever made" by some 60% of responders on the Zoss pen network recently. Great pen and parts can always be found.
Have fun!

I disagree. The "21" Mark II is a remarkably good pen. (The Mark I had some flow control problems.) Yes, many "21"s have cracks. Not all do. I have three standard "21"s and one "21" Super, none of which is cracked and all of which write well.

Mark I

Mark II

Mark II

Super


While KDW's decision may have been the correct one in the specific case, it is categorically not true that the cracks cannot be repaired. They most certainly can be repaired in some (but not all) cases. Reparability depends on whether the pen is a Mark I or a Mark II or a Super and, if it's a Mark II, how serious the cracks are and where they are located. The question is not "can it be repaired?" but rather "is this specific pen worth the cost of repair?".

Thanks for the pics, Richard. They will help me recognize future jaunts into antique and fleamarket venues. And, yes Daniel, I have two wonderful 51s as well as 2 wonderful 61s. As a matter of fact, right now, one of the 61s is my favorite pen.