After any splice in dyneema, the outer braid will be more open. Upon loading, the fibers will settle back into place and the line will stretch. This is not creep, it is "constructional stretch". It is important to get this constructional stretch out before setting up the stays as it will ensure that the final length is closest to the desired length.
To get constructional stretch out, I like to tie an end to a fixed object and then pull really hard on the line, snapping it tight and letting it go, then snapping it tight again. This will take out most of the stretch. The last bit of stretch can be taken out during tensioning with the deadeyes.
I'm in the process of making new check stays for Wisdom. I connected two off-cuts that were rather long with a very long end to end splice. My next step will be to take the long lines and measure them on the mast to set the eye splices in the desired locations. The problem is the constructional stretch is an unknown factor on these lines. It is very difficult to get the length of the stays perfect with such a large unknown in their length. For this reason, I am going above and beyond with removing the stretch from the stays.
Pulling hard on the line will remove most of the constructional stretch, but if you want to get all the stretch out of the line, this additional method will be necessary. While this method will remove the stretch and minimize the amount of creep you have to deal with, it is not necessary to do with all your stays. I only did this method on the headstay and the cap shrouds as I didn't want to worry much about them stretching on me once set.
To tension the stays, you first need to attach the stay to a very secure object. I use a silver maple tree that has a very extensive root system as my anchor point. Tie the stay to the tree with a small dyneema line, being sure to do various wraps around the anchor point. The small line is not as strong as the large line you are tensioning, so multiple wraps are needed to share the load.
If you are using a tree, be sure to set a sacrificial board on the backside of the tree. This will spread the load and keep the tree alive. Thousands of pounds pulling into the bark would be very damaging to a tree.
Next attach the other end to something rather strong and heavy. I like to use my pickup truck, it weighs just over 7,000 lbs and has a very sturdy tow-ball on the back of it.
The line is laying on the ground and properly secured to the tree and the truck, now we just need to apply some tension to the line. I put the truck in gear and begin driving down the hill away from the tree at a very slow speed (less than 2mph) until the line gets tight and stops the truck.
At this point, I put the truck back in park and check the splices.
Even under this amount of tension, the outer braid can be bunched up and not relaxed yet. This is when you can work the line out and push the fibers into their appropriate positions. I let the truck hang on the line for a few hours and then put it in gear and creep forward a few more inches, then let it hang on the line for a few more hours again.
Since these stays need as much creep and stretch removed from them, I let the truck hang on the line for a few days. This will get all the stretch out of the end to end splices and give me a very accurate length to measure my stays with.
If you noticed, I'm also tensioning the deadeyes at the same time. I know the system has been exposed to enough tension when the thimbles close and their ends come together.
If you need to get the stretch out of your new synthetic stays and don't have access to very large hydraulic equipment, just know that there are always more simplistic ways to achieve your goals.