Oversills! Don’t fit them if you want your Mini to retain its originality, and you want to fetch more money when you come to sell it. However, whatever sills you fit, make sure you inject them with wax something like Dynax S 50. Apparently this stuff is used in steel cavities on oil rigs, so it should be good enough for your Mini.
In theory, an oversill should preserve the sills from corrosion better than the original vented sills. This is purely because an oversill will retain moisture longer than original vented sills. It’s a fact that when steel is repeatedly exposed to water, and then allowed to dry, this speeds up the corrosion. If steel is immersed in water, and not allowed to dry, then the corrosion is reduced by a considerable amount. I carried out a test quite some time ago using a couple off-cuts of seam trim from my Mini.
Before carrying out the test, both pieces of steel was first cleaned with brake cleaner to ensure both pieces had nothing to protect the steel. One piece was placed in a container, and immersed in approx ½” of water, and the other piece placed next to the container (not in water, but exposed to weather in August 2012).
The picture below is 24 hours later
The picture below is 48 hours later
The picture below is 48 DAYS later, I had to remove the piece of steel from the water (pictured on the right) to show the condition.
The picture below is exactly ONE YEAR later. I think you get the idea now.
So if an oversill retains moisture, it will probably last longer than a vented sill. Personally, I believe a lot of the corrosion discovered beneath an oversill was there before the sill was welded on, and not necessarily caused by the oversill it self, but there because someone wanted a quick fix and couldn’t be bothered to fix things properly.
I personally fitted oversills to my Mini, simply because I’m not particularly bothered about originality or resale value. Providing the sill is welded at the lower point approximately 5” down from the sill seam lip, then the original strength should be retained. I believe a non vented sill (oversill) adds strength to the structure because it has metal throughout the length of the sill, as opposed to the original sill with vents.
So, in my opinion, if you want to retain originality and or resale value, then fit the original vented sill. If you’re not bothered either way, then I don’t see a problem with oversills, providing you weld approx 5” down from the sill seam lip. If like me, you intend to use your Mini on stage rallies, and it’s strength you’re after, then fit the full 9” sill, and weld along the bottom as well as 5” down instead of cutting it off. Don’t forget to add a few drain holes at the bottom of the sill to allow condensation to drain, you don’t want to carry around unnecessary weight.
I guess, if vented sills were a good idea, then I would expect modern cars to have them, but they don’t.
Anyway, we are all entitled to have an opinion, so I thought I would add mine 
