I agree with Pastor,
try adjusting the original one first with some slight prying on either side. The rivet is most likely a monel rivet, which will be somewhat hard. They are typically installed placing a "bucking bar" on the smaller diameter end and a rivet gun shoots a "rivet set" against the factory head.
the bucking bar against the small end gives resistance against the rivet gun and causes that end to swell, forming a "shop head". On spear shafts the shop head is then ground or belt sanded to smooth it out.
Rive - Wikipedia, the free encyclopedia
Dan Checkoway's RV-7 Project - Tools
These two sites can give a good overview of bucking rivets, probably way more than you need to know in order to install a flopper.
A quick and easy way to do it as an alternative would be to cut the shank of the rivet so that it is only about 1/16" longer than the shaft and floppers combined width. Install the rivet, hold the factory head down onto a steel plate or the flat portion of a bench vice (have someone else hold the spear shaft). You then take a center punch and hit a punch mark with a ball peen hammer into the center of the rivet shaft. Hit it a few times to flare the end of the rivet. Smooth or blend with a file.
Or just follow Pastors advice and pry out your existing flopper. I'd probably just do that if I was doing it at home.
I'm a machinist and aircraft sheetmetal mechanic, so I'd just take the shaft and flopper to work and hit it with a rivet gun.
If I couldn't do that, I'd just tweak the one you have on there.
Sorry for the long winded response.. I just wanted to contribute something. I had to log on to deeperblue before I went spearfishing. I leave in an hour.
Take care,
Mitch
Oh, to remove the old rivet, if you decide to go that route. Drill the factory head with a drill that's slightly smaller than the rivet shank diameter. Once the head is removed, use a pin punch and ball peen hammer to drive out the old pin.
