The instructions should say how much gap there is between the pipes when installed - on a 3/4" fitting, it might be more than 1/2". You could probably measure how far the pipe fits into the fitting, measure the entire distance, then figure it out, but the instructions should say. It should work if there is any slop in the pipes. If they are anchored on either side of that fitting, you won't be able to spread them enough to insert the fitting. In these cases, a repair fitting is often used. This is a fittign without a stop in the middle. You prepare the connection (clean and flux) slide the fitting over one part of a section of the pipe, place the new section (yes, you need a section) in place, then slide the coupling over the two ends then solder it in place. This can be done when both sections are anchored and cannot be spread apart to get the fitting in, since the sleeve is pushed entirely out of the way first. Hope that is clear.