I've noticed some weirdness with Final Cut's cross-dissolves when trying to make a default dissolve of a different length. Try this:
01) Make sure the 1 second cross-dissolve is your default. 2nd click on it and choose "Set Default Transition".
02) Put two clips in your timeline and make sure they have plenty of handles between them for the dissolve. Make the handles far larger than you need to be sure. In testing this I gave each clip 3 second handles.
03) Highlight the edit point between the two clips and tap Command + T to add the default transition which will be a 1 second cross-dissolve.
04) Highlight the cross-dissolve in the timeline and tap Enter to load it into the Viewer. At the top left you should see it's duration which should be 00:00:01;00.
Here's where things start getting weird.
05) Change the length of the cross-dissolve in the timeline. Either by adjusting the numbers in the duration box or by dragging it to a different length in the timeline. Try making it 00:00:00;20 for a 20 frame dissolve.
06) Now, double-click the dissolve in the timeline (you should have seen it get shorter in both the Viewer and in the Timeline) so it's for sure loaded into the Viewer.
07) In the upper right of the Viewer grab the transition/hand icon and drag this to your Favorites bin in your Effects window or wherever you keep your custom transitions.
08) Position the scroll box in the Effects window so you can see the "Length" column. You'll see your cross-dissolve's duration has magically changed (at least for me) to something other than 20 frames. For me it consistently changes to 00:00:00;16.
09) Now when you place this custom transition into your timeline and load it into the Viewer and you'll see it's back to it's 20 frame length.
If you experiment with different methods of making custom-length dissolves you'll get inconsistent results.
01) Double-click the standard 1 second cross-dissolve in the effects window.
02) In the Viewer drag it to a different length. Say 14 frames.
03) Using the transition/hand icon drag it to your timeline.
04) Double-click the cross-dissolve in the timeline and check it's duration. For me a 14 frame duration turns into an 18 frame one when placed into the Timeline. If you count the number of frames on either side of the dissolve they add up to 18 so something seems amiss.
Am I missing something here or doing something wrong or is this an actual bug in Final Cut?