When replacing the bridge on a mandolin it is normally necessary to shape and fit the base of the bridge to match the tops curvature.
Non- Adjustable Bridges
Mandolins with non-adjustable bridges (solid wood without thumbwheels) must be sanded in order to be lowered.
On bridges like the one shown, material is normally removed from both the base and the top to avoid altering the overall look of the bridge. This bridge is compensated, which means the crown (surface where the strings contacts it) is shaped to alter the strings length and improve intonation (the ability to play in tune up and down the neck.) The small hole (usually filled with felt) accepted a pin which was mounted to the bottom of a floating pickguard.
If more height is necessary due to a sinking top or overset neck I will often laminate more ebony to the base in order to build height and retain the original bridge. The use of black adhesive and similarly colored ebony makes this difficult to detect.
Adjustable, thumbwheel bridge
Bridges with thumbwheels or wedges such as Webber's Brekke Bridge are considerably easier to adjust.
Thumbwheels are used to raise or lower the top portion of the bridge thereby adjusting the string height above the frets. When setting the action with an adjustable bridge it is far easier to start by placing the bridge too high, lowering it as necessary After a mandolin is strung to pitch there is considerable string tension bearing down on the bridge, this makes it very difficult to adjust the wheels upward. And don't let me catch you with a pair of pliers in your hands! Impatience can provide hours of heartache...as some of my clients can attest.
Adjustable Brekke® Bridge
Ideally the feet of the bridge should contact the top across their entire foot surface. Note that some bridges arch strongly in the center and do not touch in this area. Good coupling normally equates to better sound.
Proper fitting of the feet require patient sanding to create a consistent coupling. Once the feet are in proper contact with the top, string height and intonation are set.
Correlation between neck angle and bridge height.
As demonstrated by the photo above, the mandolin's neck angle (pitch of the neck) directly relates to the height necessary at the bridge to obtain good string height. A more extreme angle, such as the one shown, requires a taller bridge to ensure the strings do not sit too low. At this height the strings still sat too close to the frets.
Likewise, instruments with a more shallow neck angle often have shorter bridges to avoid high action.
A bridge that requires frequent raising may be indicative of an instrument with a sinking top or other structural issue and should be examined.