Are you installing a core of 2" x 8" x 6" ? There is a core slightly larger too which was refered to as being the H.D. unit.
I find the standard cores to be good for about ten years. The center is the same for most all of them, only the ends are different. it takes a good 2-4 hours to remove one, but then it gets worked up here at home. Most often the leaks are from water deposits getting in between the end caps and the core, then due to heat and cooling, they wedge further and further in until a leak comes in.
I usually just unsolder the two end caps with a very small tipped Acy (but can also be done with propane). This takes all of two minutes and if the leak was at the caps, boy will it show up then. Then with a foot long strip of banding steel just rod out the flues, under running water. You need to dry good, then tin up the caps and the core ends with excess clean and shiney solder. Use a 3/4" pipe clamp to squeeze each of the caps back into position, then heat and apply more solder to the edges. May need needle nosed pliers to hold certain tight spots too. One time had a leak in the middle of a core, so plugged each end of the particular tube with brazing rod snips and soldered in place.
After you are done, hook a broken ended garden hose to one of the pipes, turn on slow then hold your thumb over the end. If any leaks, they will hit you in the face. Dry and solder there. Watch it for domestic water pressure is usually greater than 60PSI, so use your thumb to regulate the pressure, not something clamped in place.
You know the unit fits, as it just came out of the heater unit two hours ago. The cleaning, inspection and soldering operation ought to consume about 60-90 minutes, which includes gathering the tools, drying and watching the television in between time. It saves you a good $30+ and don't have to drive to auto store for new one.