I don’t have experience with them personally, but I have seen a few on here claim that the new-style Mishimoto radiators with the isolation bushings have been fairly reliable.
I personally run OEM as I got one before the new old stock ran out.
I’ll bet it has to do with the relative stiffness of the designs. The factory hybrid aluminum/plastic radiator is probably capable of taking more flex than the aftermarket all-aluminum radiators as a factor of design.
Part of the issue probably has to do with the wall thickness and how the radiators are assembled. It is a common thought that thicker = stronger, and I wouldn’t be surprised if some of the aftermarket radiators simply have too much wall thickness, especially at the fillet and butt welds.
Sure, thicker = stronger, but it also means stiffer. In fact, strength is proportional to thickness squared (in flat plate subjected to bending in the weak axis) but stiffness is proportional to thickness cubed So if you have a part that is subjected to flex, sure doubling the thickness will make it 4X stronger against the same load, but against the same displacement, you still have twice the stress than you would in the thinner part.
Since the stiffness of the radiator is small compared to the stiffness of the average radiator grille (assembled with V-bar), it’s unlikely that the overall displacements are going to change much due to the stiffness of the radiator. Phrased differently, the radiator is not intended to be a structural member. So it is fairly safe to assume that the displacements of the fixed mounts aren’t going to change much as a function of radiator strength/stiffness.
As to why the aftermarket radiators are more susceptible here, I would suspect it is largely due to the methods in which they are made. OEM radiators are mass-produced, and at that scale, it is cost effective to form/stamp/crimp everything out of thin sheet metal and assemble it into what you need. Meanwhile, aftermarket radiator manufacturers are stuck with more low-volume methods, and are forced to use techniques (welding, flaring, etc.) that are more suited to heavier wall thickness stock. Some are probably also under the illusion that thicker = stronger = better, which I’m sure the average aftermarket buyer would think as well.
Rubber mounts could indeed be a fix to this issue, as could be making a super-thin walled radiator. However, rubber isolation mounts are almost certainly cheaper (at small production scales) than going thin-walled.
Rubber isolation mounts also eliminate the other trade-offs of going to thinner walls, which could include reduced burst pressure, reduced corrosion life, and increased susceptibility to puncture.
A 3rd solution that I have yet to see in aftermarket radiators is to add significant cross-stiffening independent of the core to the radiator assembly itself. Some heat exchangers are made this way, usually by cutting four triangular openings into the front mounting plate to create an X shape to carry shear forces as tension without allowing significant flex or stress in the radiator core itself. As to whether this would be better or worse than isolation mounts or a thin-wall radiator, I don’t know. Most OEMs now use a combination of thin walls and isolating bushings rather than attempt to make the radiator a structural member.
