My two cents worth: You can test to see that the heads make sufficient mechanical contact for reliable operations (I'm assuming you have already successfully tried a known good floppy drive in each of your machines to eliminate bad floppy controllers, cables, power supplies, and possibly abusive connections to the external floppy connector by previous users).
There is a two stage trigger mechanism that you can manually operate to close the mechanism without a floppy: viewed from the front, one pin needs to be pushed to the right side, then the rear pin needs to be pushed backwards (both of these pins are connected by one top mounted spring on the right top side). With the loading platform now closed, you should look to see that the two heads actually touch, with no light showing between them at all. The space between the plastic tab that lifts the upper head, and the plastic track below, should be at least 1 mm, more than the thickness of the tab. The platform should be in the fully dropped position which means the four bearing rings should be bottomed out in direct contact with the eject slide in the fully forwards position (you can wiggle this to identify the slide, it is integrally connected to the manual eject tab in front).
If you take a new US $1 bill and place it between the two heads and hang it vertically, the weight of a single penny taped to the bill should be supported, but two pennies taped to the bill should extract the bill. On my SE/30 floppy there is a small semi-hidden coiled spring pulling down against the force of the upper leaf spring that supplies this tension.
I have seen only one case of "spread head" on a liberated machine and do not know if it resulted from someone over-separating the heads in attempted preventative maintenance or forcible extraction of a stuck floppy.
There is a two stage trigger mechanism that you can manually operate to close the mechanism without a floppy: viewed from the front, one pin needs to be pushed to the right side, then the rear pin needs to be pushed backwards (both of these pins are connected by one top mounted spring on the right top side). With the loading platform now closed, you should look to see that the two heads actually touch, with no light showing between them at all. The space between the plastic tab that lifts the upper head, and the plastic track below, should be at least 1 mm, more than the thickness of the tab. The platform should be in the fully dropped position which means the four bearing rings should be bottomed out in direct contact with the eject slide in the fully forwards position (you can wiggle this to identify the slide, it is integrally connected to the manual eject tab in front).
If you take a new US $1 bill and place it between the two heads and hang it vertically, the weight of a single penny taped to the bill should be supported, but two pennies taped to the bill should extract the bill. On my SE/30 floppy there is a small semi-hidden coiled spring pulling down against the force of the upper leaf spring that supplies this tension.
I have seen only one case of "spread head" on a liberated machine and do not know if it resulted from someone over-separating the heads in attempted preventative maintenance or forcible extraction of a stuck floppy.