Windsurfing Universal Joints are some of the most important parts of your kit. Don't believe me? Try to go sailing without one and let me know how it goes! hehehe ;) Routine inspection and replacement of worn parts is critical and not to be overlooked.
UJ Tendons will crack/dry rot over time. Luckily it is really easy to pull them apart and they are relatively inexpensive if they do need to be replaced. We recommend inspecting yours at least once (if not twice) per year, even if you don't use it very often. They tend to crack along the hole where the mounting bolt goes through them, so you'll need to get out your tool kit and disassemble the UJ to get a good look at the tendon. If you do notice a crack there (or anywhere else for that matter), we recommend replacing the tendon immediately.
While it is best practice to stick within the original brand's offerings when shopping for replacement parts, there is some compatibility between the brands. Specs are similar enough between some of the brands that you can mix and match parts with reasonable outcomes, but DO THIS AT YOUR OWN RISK and know that this might lead to premature wear and failure of the parts.
Chinook and Streamlined are our two most popular UJ brands. Their tendons are quite similar overall. The biggest difference is in the distance between the end of the tendon and thru-hole for the mounting bolt. Chinook's hole is approximately 5mm away from the end of their tendon, and Streamlined's hole is approximately 7mm away from the end of their tendon.
Due to this difference, the Streamlined tendon will NOT fit into a Chinook UJ, but a Chinook tendon WILL fit into a Streamlined UJ. We recommend adding a washer or two "underneath" both ends of the chinook tendon when installing it into a streamlined UJ to fill the 2mm gap between the tendon and the bottom of the cup. Leaving a gap between the end of the tendon and the bottom of the UJ cup will lead to premature wear and tear of the tendon and it will likely fail much more quickly than if the gap had been filled.
Again, mix and match UJ brands at your own risk! We recommend sticking within one brand when building out your personal stock of UJs and Extensions and replacement parts.