Noticed my 2 piece Clare had a crack running up and down the back of its head. Figured temp changes where I carried it might be the cause. It didnt crack all the way and a piece of Duct tape sealed the leak. Anything else I should/should have done? Really like the whistle.
Stick it together with tape, glue, paint, paper, nail varnish, anything else you can find, and it might turn out to be your best whistle ever. I have an old C Gen that’s held together like that, and it’s brilliant.
As for what you could have done to prevent it, I think sometimes they just crack. Not a lot you can do about it.
You can try running some cyanocrylate glue (like Crazy Glue) into the crack to fill it. It may or may not stick well to the plastic, depending on the material. An alternative might be some two part epoxy if the crack is wide. The crack suggests that there were some stresses in the head’s plastic and the crack may have relieved them.
The Clare heads are notorious for cracking exactly as you describe. Mine has the exact same crack, and I know of 2 or 3 others. I just ran cyanoacrylate (super glue) into the crack as Paul describes. I had to repeat several times before it stabilized. You could tape around the collar of the head for extra support.
Also remember that it’s easy to swap heads on all those 1/2 inch tubes. The Clare 2-piece tube with a vintage Gen head is one of my very favorite whistles.
I’ve never had much luck with adhesives (and I have tried everything I’ve ever seen advised) on these plastic heads… they can fill a crack, and that’s fine, but they won’t really mend it. A couple of wraps of clear cellophane tape around the collar is plenty strong enough to reinforce the head and keep it from migrating further. In the past I’ve also experimented with banding the collar with thin metal, but I find that this can sometimes squeeze the collar a little too much and create additional stress which can actually force the crack to reopen. Cellophane tape is very lightweight and has enough “give” to avert this potential problem - and it’s less obtrusive as well. 
Go to WalMart and buy some of the 100% polyester thread they sell that comes on a cone (not the Coats and Clarkes thread on spools). Thiis thread can be coated with clear nail polish or lacquer without fuzzing. Other threads will fuzz (I’ve tried many). Edited to add: I was at WalMart today. The thread is Coats and Clarkes brand “Outdoor” 100% polyester thread on 200 yard cones.
Assemble the whistlehead on the tube.
Using the thread you purchased at WalMart, “whip” (a procedure that uses string to stabilize the end of a rope) the outside of the socket following the instructions here (I use method 2): http://www.ropeworks.biz/reader/whipping.pdf
Then coat the thread with nail polish, lacquer or super glue. It will be much stronger than the original plastic.
Best wishes,
Jerry
I had this problem on my Clare Two-Piece and e-mailed the manufacturer requesting recommendations for adhesive … the response was a request for my mailing address so he could send me a replacement head 
The replacement hasn’t cracked 
Any ideas on what to do with the other 199 1/2 yards of outdoor polyester thread?
you could knit a tent 
Thanks for the tips! Ill watch the crack. If it gets any wider I will do the whipping idea. May do it anyway, decorative knots and lanyards and whipping look cool.
I have an old Feadog here on my desk that split right down the ramp. It is held together with super glue and tape. It plays better now than it did before. Always try and fix them before you toss them.
My Feadogs always split that way too, when they do split. I even have an O’Briain tweaked that split both the head and the piece of plastic he glues on… crazy wierd. I had forgotten all about the fly fisherman’s fix until Jerry posted it.
Three of my red top Generations now have brown parcel tape holding a split head together. To my ear, they don’t sound any different, but somehow they seem more lovable like that, just as a car with a couple of minor dents and scratches is a closer friend than a shiny beast just out of the showroom. For the same reason, I like the worn thumbmarks on the back of the brass Generations…