To grease or not to grease?

Should one grease the tapers of bottom bracket spindles before installing the cranks? Few topics spur as much controversy among bike mechanics as this question.
In the old days, Campagnolo not only recommended mounting the cranks dry, they even suggested degreasing the tapers. The concern was that grease might facilitate the crank slide up further and further on the tapers. And since many of us learned about bicycles when Campagnolo was the undisputed king of components, the word from Vicenza was treated as gospel.
More recently, I worked for Race Face as a technical writer and translator, and  their engineers disagreed. They advised: “Grease the tapers, but make sure you only tighten the bolts once, then leave them alone.” Their tests had shown that a “dry” spindle/crank interface did not result in a consistent press-fit between the parts.
As we developed the new René Herse cranks, we discussed this topic without coming to a conclusion. In the mean time, our engineer mounted the first test cranks without grease, and found that they had unacceptable levels of runout of the chainrings – the chainrings didn’t run as true as we would have liked. (“Unacceptable” means that the runout was visible even if it did not affect the function or performance.) When the runout changed each time he mounted the cranks, we realized that the cranks were not seating uniformly on the taper.
What exactly was happening? Imagine the crank and spindle surfaces as tectonic plates that slide past each other as the crank bolts are tightened. If you grease the interface, they will slide smoothly until you stop turning the bolt when it is tight. If the interface is “dry,” the crank catches on the spindle. This builds up tension, which then is released in an “earthquake.” Even with the same torque, the crank will sit differently, depending on whether the tension has been released or not, before you stop turning the bolt. With grease, the crank’s position was more uniform, as the crank slid smoothly onto the spindle.
What about the crank arms “migrating” further and further onto the spindle each time you mount the cranks? To test this, we mounted a René Herse crank, tightened it to 25 Nm, then took it off, mounted it with 30 Nm … we repeated this 5 times, increasing the torque by 5 Nm each time until we reached 50 Nm. The goal was to find out when the crank would be destroyed. To our surprise, the crank did not move any further onto the spindle. We could not destroy the cranks in this way. A 1990s Campagnolo C-Record crank that we tested for comparison slid further and further onto the crank. We stopped the experiment early to avoid breaking the classic Campagnolo crank.
The difference between the two cranks appears to be that the new René Herse cranks have a forged taper. This makes that part of the crank assembly much stronger. The C-Record crank appears to have a machined taper. Every time the crank gets mounted on the spindle, the aluminum moves – in fact, mounting the crank acts in the same way as a forging process. It is likely that this process would stop eventually, but I’d rather not experiment with a classic crank.
So for our René Herse cranks, we recommend that you lightly grease the crank spindle. Also grease the treads of the bolts, but not the underside of the bolt head. (The underside of the bolt head should interlock with the crank, so it doesn’t come loose.) Then tighten the bolts. Check after your first ride that the bolts are tight. Thereafter, leave them alone.
Crank bolts can loosen over time, but that does not mean that you should re-tighten them. Instead, remove your cranks every couple of years and inspect them for cracks. (You should do this with all cranks, no matter the brand.) Then put them back on, and enjoy them some more. Treated like this, quality cranks will last most riders for many decades.