Shrink I don't believe so, warping is a good possibility though. On a lot of older things I have seen that have been repeatedly dissassembled and put back together is the area right around the bolts distort. Often times if you tighten something up to tight you will raise the metal right around the bolt hole or slightly pull the metal around the threads of whatever you bolting into so the sealing surfaces are not flat.
Laying a ruler/straightedge over the sealing surface across the bolt holes can easily confirm if it is your problem. Even smooth complete swipes across the sealing surface with a good clean mill file will often cure the problem on non critical areas. 320 grit wet sandpaper laying flat on a piece of glass can often do a satisfactory job on critical areas. Keep the paper wet and clean with a light grade oil and holding piece down flat,not letting it rock, move it in little figure 8's.
You could also use a countersink bit and a drill press to chamfer the bolt holes BUT do not try it witha drill bit and hand drill . The drill bit will bite and the hand drill will not let you have enough control to keep from digging in to deep.
