You've just installed a new BB with used cranks and they have a knock or free-play in them vertically.
Crank-sets with a straight 24 mm spindle (Shimano, RF, FSA etc) rely on a good fit between the bearing caps and the crank spindle. Any small gap can be felt. GXP crank-sets need the non-drive side bearing clamped tight and the drive side cap to be a good fit on the spindle. To find the problem side, rest your fingers where the crank joins the bottom bracket and feel for movement while rocking the cranks with your other hand. Any up/down movement will need sorted. In-out sliding movement is crank adjustment.
Is it the crank or the bottom bracket?
GXP cranks clamp the non-drive side bearing and adapter. Some GXP cranks have very tight splines and simply require more grease and torque than normal to close the gap and clamp up tight. If you have play in a GXP drive side or either side of a Shimano/FSA/RaceFace 24mm crank then the fit of the bearings/caps to axle needs checked.
Bearing Cap Check
To check the bearing caps you need a bearing of 24 mm outside diameter. Common sizes are 6901 and 6802. Press the bearing into the cap as shown in the photo below. If the fit is good, the bearing and cap are fine and the axle is the problem. If the fit is loose then please contact us for replacement caps.
My Shaft is Worn
Used cranks with spindles (aka shafts, axles etc) worn enough to cause a problem are becoming common. The wear is usually caused by rust/corrosion or a previous bottom bracket locking up. It takes very little wear to create a noticeable knock or play at the crank arms. 0.2 mm is significant. There are two solutions: replace the crank (expensive) or pack out the bearing cap to fit the worn crank.
Packing & Installation
Get a roll of PTFE (teflon) plumber's thread-seal tape. Remove the cap from the bad side, then carefully wrap the tape around the snout of the bearing cap. Each turn will give up to 0.2 mm of size reduction. Peel off any dags and reinstall the cap with tape into the bearing. The tape will cause the cap to compress, shrinking the hole through the cap and taking up spindle/cap clearance. Repeat with more or less tape until a good fit is achieved.
Then ride some more. This fix is permanent, but reversible.
You may also be interested in our range of Technical Articles & Resources, written to help with bike maintenance & suspension setup.