No 43c is no need for worry. Intel cpus can sustain much higher temperatures.
Also a temperature reading on it's own means nothing. Whats the ambient temperature?
It's been quite hot today, and indoors feels humid, ambient temperatures inside my house at the minute are about 26 degrees.
On a cooler day your cpu temperature would be ever so slightly lower. Add to that other variances such as the thermal compound used, application method, cooler type, whether it's mounted correctly, case airflow etc.
Why are you concerned? Is it higher than it was before you rebuilt it? If so by how much? If it is only slightly hotter by a few degrees then it could be the fact that it's hotter today, or the way you applied the the thermal compound, improper application can affect temps by a few degrees for example.
Ultimately as has been mentioned, an intel cpu at 43 degrees is certainly not reason for concern. If it's higher than it normally is, then try and clean off the cpu, re apply thermal compound, not too much, you're not spreading butter on a sandwich, and re mount your cooler.