Flight controllers in Moscow assured Sergei Volkov and Oleg Kononenko, both Russians, that the bolt would not detonate and that the unprecedented job would help ensure their safe return to Earth in the Soyuz capsule this fall.
"You should not be concerned at all," Mission Control radioed.
NASA said its own engineers were convinced the two spacemen would be in no danger, and that it would be all right for them to put the explosive bolt in a blast-proof container and take it into the space station for eventual return to Earth.
The past two Soyuz descents have been steep, off-course and bone-jarring, and the Russian Space Agency wants to avoid the problem when Volkov and Kononenko fly home in October.
Mission Control urged the astronauts, both first-time spacefliers, to use caution working on the Soyuz capsule that carried them to the international space station in April. The men took out a serrated knife to cut into the insulation surrounding the bolt - a tool normally shunned by spacewalkers because of the possibility of piercing their pressurized suits or gloves.
They had a wrench to remove the 3-inch pyrotechnic bolt, one of 10 used to separate two parts of the module during re-entry.
During Soyuz descents this past April and in October 2007, the two module sections did not separate properly, leading to so-called ballistic entries that submitted the crews to far higher gravity forces than normal.
Russian engineers suspect some of the explosive bolts did not fire. By disabling the bolts in this suspect location, there should be no mechanical hang-up during the October descent, officials said.
The lone American on board, Gregory Chamitoff, was inside the Soyuz for the entire six-hour spacewalk in case an emergency required the two Russians to join him in the capsule.
Chamitoff took books, music and a laptop computer with him to help fill his time, and could hear everything that was going on.
Each pyrotechnic bolt has the force of a large M-80 firecracker, NASA officials said.
A high-ranking flight director at Russian Mission Control outside Moscow told the crew Wednesday that the bolt could withstand shocks of up to 100 times the force of gravity and would not fire, even if they hit it with a big hammer.
NASA has a keen interest in the Russian-built Soyuz capsules because they sometimes transport Americans to and from the space station, and also serve as lifeboats.
Once the space shuttles are retired in 2010, the Soyuz will be the sole means of human space transportation until 2015, when America's new rocketship starts carrying crews.