Type and year of car would help.
a hex bolt is bolt having six face head operated by spanner. an allen bolt is bolt having a six face groove in the head and operated by a special type of L shape key called allen key.
Torx or Allen wrenches get more torque (pressure) on the bolt.
8mm
An allen key is a square key normally used for a square headed bolt or screw used in mainly self build furniture (flat packed).
to align a flat belt conveyor simply adjust the tail end pulley or the idler pulley, it is provided with adjuster bolt to adjust it in order to align the conveyor.
from the bottom 13mm loosen idler pulley bolt then turn adjuster bolt under pulley
It requires a special puller to remove the pulley. There is NO bolt are Allen head that has to be removed. The pulley is pressed on. You will also have to get a special tool to put the pulley back on too.
An allen bolt is a bolt with a head that requires an allen key to turn it.
plate in floor next to footrest (RHD) - ALLEN KEY -TOP BOLT - BOTTOM BOLT FROM UNDER THE CAR
Remove the belt, then remove the bolt holding in the idler pulley.
Pulleys are held on to shafts by either a bolt in the bottom or a set screw in the side, loosen them and use a gear puller.
You will need a three prong puller and a 7 or 8 inch bolt, no thicker than about 5/16. Remove the balancer bolt, insert your thinner bolt (the bolt needs to be long enough to stick out atleast an inch). Then hook your puller on to the balancer and center the puller bolt against the head of your guide bolt. Should come off pretty easily.
13 mm
NO it is not.
It doesn't have a bolt. Just put the puller's collar around the pulley hub, and with a bit of effort, the pulley comes off. A lot of mechanics use an air impact to drive bolt for the puller.
Take the belt off, un-bolt the idler pulley, bolt the new pulley on, and finally put the belt back on and Viola!!
Take the belt off, take the bolt off of the idler pulley, take the pulley off, put a new one on replace the bolt, and put the belt back on