Long chainring bolts VS. Short chainring bolts, or why use long and short together?
Long chainring bolts are primarily used for thicker gears and spiders, however the short chainring bolts are mostly found on complete bikes using a thinner spider and chainring.When you use the long 8mm fronts and backs of the chainring bolts, you will experience, in many cases, that they will not tighten down the gear to the spider. They will just bottom out against each other leaving the gear loose on the spider.
As for the short style chainring bolts, I believe that the backs are not long enough to go through the spider and will leave a gap between the front and the back creating a weak spot for the chainring bolt to break.
I prefer to use the long, 8mm chainring backs and the short 6mm front chainring bolts together.
I believe by doing this, we have a greater surface area that actually provides support for the chainring and helps keep it from shifting.