Following help in a previous thread, my crank was identified as a Stronglight 100, 86 BCD and I ordered a 28T ring, but it does not fit. The holes line up fine, but they are very slightly too small - I can’t get the nut/threaded insert to pass through from the back (the Allen headed bolts will go through, but that would be the wrong way around). Also, I note the ends that connect with the spider are round (they follow the outside diameter of the holes), rather than being almost square like the other two (and previous third 32T) chainring (and the spider is formed to accept the squarer end). Are there different types of Stronglight 86 BCD chain rings?
One other thing - does anyone have a link to a diagram showing how the various parts are bolted together? I want to make sure the washers go back in the right place. I am sure I took a photo with my phone when I took it apart but cannot find it :-(
Those little 86 bcd rings have become quite scarce in the last couple years! Not long ago I could always find one on eBay; now not so much. It sounds like your 28t ring was made to be the third ring of a triple, intended to be used with spacers. Is that how you're going to use it? The rounded spider arms shouldn't make any difference either way if the bolts are good and tight.
How big are the holes? Is there enough metal there to hold the bolts if you enlarge the holes? Assuming there is, then the tool you want is not a drill, but a reamer. You can get a perfectly good one at Harbor Freight for $3.29.
If you only need to remove a little material, you can make do with almost any tapered steel tool, such as a pair of scissors; twist one scissors blade in the hole, taking a thin shaving of metal off as you turn the scissors. Don't take off too much.
Washers? I don't think this assembly has any washers in it; at least, not what I'm familiar with. If you're running this as a triple, the order is: first the female part of the bolt, then the smallest chain ring, then a spacer, then the middle chain ring, then the spider, then the big chain ring, then the male bolt.