Bridge post spacing is the center-to-center distance between the two mounting axes that support a post-mounted guitar bridge or two-point tremolo. It is one of the first dimensions to check when identifying or replacing a bridge, but it is not a complete compatibility standard.
A measurement around 74 mm can belong to several mechanically different systems. Post diameter, thread, bushing geometry, bridge holes and pivot geometry must therefore be identified separately.
In Brief
- Post spacing means center-to-center distance between the two mounting axes.
- When both posts have the same diameter, same-edge to same-edge is usually the most repeatable practical method.
- Post spacing, post diameter, thread and bushing size are separate compatibility dimensions.
- A value close to 74 mm does not identify one universal bridge standard.
- Measure the actual guitar and compare the result with the exact component or manufacturer drawing before ordering.
In This Article
Measurement Fundamentals · Tools and Preparation · Core Measurement Methods · Measuring the Actual Hardware · Reference Systems and Compatibility · KMS-Specific Compatibility · Verification and Ordering
Measurement Fundamentals
What Does Center-to-Center Mean?
Center-to-center spacing is the distance between the central axes of the two posts.
It is not:
- The empty gap between the posts
- The total outside width across both posts
- The width of the bridge body
- The string spacing
- The bushing diameter
- The distance between the outer strings
- The distance between tailpiece studs
The measurement follows the line connecting the two post centers.
On a Tune-O-Matic installation, that line may be angled relative to:
- The pickups
- The guitar body
- The scale line
- The tailpiece
The angle of the installed bridge does not change the meaning of post spacing.
Post Spacing vs. String Spacing
Post spacing describes the bridge mounting.
String spacing describes the distance across the strings, commonly measured from the center of the lowest string to the center of the highest string.
These dimensions are independent.
A bridge may have:
- Correct post spacing
- Incorrect string spacing
or:
- Correct string spacing
- Incompatible mounting holes
Both must be checked separately.
Post Spacing vs. Bushing Diameter
A bushing is the metal insert installed in the guitar body.
Its external diameter determines the size of the body hole.
Its internal thread determines which post or stud can be installed.
These are separate dimensions:
- Post spacing: distance between the two mounting axes
- Upper post diameter: section supporting the bridge
- Lower post thread: thread entering the bushing
- Bushing outside diameter: diameter of the body insert
- Bushing depth: installed length of the insert
- Body-hole diameter: hole remaining after removal of the bushing
Do not use one of these values as a substitute for another.
The Most Important Rule
Measure the feature that the replacement component must fit.
For a bridge body, this normally means:
- Post spacing
- Upper post diameter
- Bridge-hole diameter
For replacement posts, this may mean:
- Upper support diameter
- Lower thread
- Thread pitch
- Post height
For replacement bushings or Conversion Posts, this may mean:
- Body-hole diameter
- Body-hole depth
- Post spacing
- Required upper thread
For a two-point tremolo, this additionally includes:
- Pivot-post geometry
- Knife-edge position
- Stud and insert dimensions
- Baseplate clearance
Tools and Preparation
Recommended Tools
Use:
- Digital or vernier caliper
- Clean measuring surfaces
- Good lighting
- A stable guitar support
- Paper or a digital worksheet
- Manufacturer drawings where available
A rigid steel ruler can provide an approximate check, but a caliper is strongly preferable for distinguishing dimensions separated by less than one millimeter.
The number of digits displayed by a digital caliper is not automatically its guaranteed accuracy.
Record only as much precision as the tool and measuring situation support.
For example, a stable reading of approximately 74.0 mm is more useful than reporting 74.03 mm from an uncertain measurement.
Prepare the Guitar
Before measuring:
- Place the guitar on a stable, padded surface.
- Protect the finish around the bridge.
- Reduce string tension.
- Prevent loose hardware from falling onto the guitar.
- Photograph the original bridge orientation.
- Lift the bridge vertically from the posts where necessary.
Do not force a bridge sideways from its posts.
Resistance may indicate:
- Bent posts
- Misalignment
- Corrosion
- Finish buildup
- Damaged hardware
- An incompatible bridge
Inspect Before Measuring
Identify the visible system.
Check whether the guitar has:
- Thin direct-mounted posts
- Separate thumbwheels
- Small pressed-in bushings
- Nashville-style adjustment units
- Large metric posts
- Large visible bushings
- Tremolo pivot studs
- Previous conversion hardware
Also inspect for:
- Bent posts
- Leaning posts
- Loose bushings
- Different post diameters
- Replaced hardware
- Oval or enlarged bridge holes
A distorted or modified system may not produce a representative factory measurement.
Core Measurement Methods
Best General Method: Same Edge to Same Edge
When both posts have the same diameter, measure from the same side of one post to the same side of the other.
Examples include:
- Left edge to left edge
- Right edge to right edge
- Nut-side edge to nut-side edge
- Tailpiece-side edge to tailpiece-side edge
Because the posts have equal radii, the same-edge distance equals the center-to-center distance.
This method is often more repeatable than attempting to locate two invisible post centers by eye.
Conditions for the Same-Edge Method
The same-edge method is valid only when:
- Both measured sections have the same diameter.
- The same physical feature is measured on both sides.
- The caliper is aligned with the line between the post axes.
- The jaws are not positioned diagonally.
- The posts are not severely bent or leaning.
- Decorative caps or flanges do not have different diameters.
Do not measure the edge of an upper post section on one side and the edge of a thumbwheel or bushing flange on the other.
Direct Center-to-Center Measurement
Direct center-to-center measurement is possible when the centers are clearly defined.
This may be practical when measuring:
- Technical drawings
- Countersunk screw heads
- Marked drilling templates
- Empty body holes
- Parts with visible center points
On plain round posts, estimating both centers visually is usually less reliable than using the same-edge or calculated method.
Repeat the measurement before recording the result.
Calculated Method: Outside to Outside
When the posts have the same diameter:
- Measure the total outside-to-outside distance.
- Measure the diameter of one post.
- Subtract the post diameter from the outside measurement.
Formula:
Center spacing = outside-to-outside distance - post diameter
Example:
- Outside-to-outside distance: 80.0 mm
- Post diameter: 6.0 mm
- Center spacing: 74.0 mm
When the Post Diameters Are Different
The simple subtraction method does not work when the two measured sections have different diameters.
Use:
Center spacing = outside-to-outside distance - half of diameter 1 - half of diameter 2
or:
Center spacing = outside-to-outside distance - (diameter 1 + diameter 2) / 2
Example:
- Outside-to-outside distance: 79.0 mm
- First diameter: 4.0 mm
- Second diameter: 6.0 mm
Calculation:
79.0 mm - (4.0 mm + 6.0 mm) / 2 = 74.0 mm
Different post diameters on one guitar may indicate:
- Replaced hardware
- A repair
- A conversion
- Measurement at different component levels
- A damaged or non-original system
Investigate before ordering parts.
Inside-to-Inside Measurement
Center spacing can also be calculated from the gap between the inside edges.
For posts of equal diameter:
Center spacing = inside-to-inside distance + post diameter
For posts of different diameters:
Center spacing = inside-to-inside distance + (diameter 1 + diameter 2) / 2
The inside gap can be difficult to measure accurately when:
- Thumbwheels obstruct access
- Posts are short
- Bushings have large flanges
- The caliper jaws do not sit squarely
- The bridge is installed at an angle
Use this method only when the reference points are clearly accessible.
Measuring the Actual Hardware
Measuring Installed Bushings
When the posts have been removed, the spacing may be measured between identical installed bushings.
Use:
- Same flange edge to same flange edge
- Outside-to-outside minus one flange diameter
- Inside-to-inside plus one flange diameter
This determines the spacing of the bushing axes only when:
- Both bushings have identical flange diameters.
- The flanges are concentric with the body inserts.
- Both reference surfaces are accessible.
- Neither bushing is tilted or loose.
The visible flange diameter is not necessarily the same as:
- Bushing-body diameter
- Body-hole diameter
- Internal thread diameter
- Required Conversion Post diameter
Do not order a press-fit component from the visible flange measurement alone.
Measuring Empty Body Holes
When the original bushings have already been removed, measure:
- Center-to-center hole spacing
- Diameter of each body hole
- Depth of each body hole
- Condition of the surrounding wood
A body hole may not be perfectly round because of:
- Knurled original inserts
- Compressed wood
- Finish buildup
- Previous adhesive
- Removal damage
- Earlier modification
Measure each hole in more than one direction.
Do not remove secure bushings solely for a casual compatibility measurement unless removal is required for the intended conversion.
Measuring a Loose Bridge
A loose bridge can provide useful secondary measurements.
Check:
- Mounting-hole center spacing
- Diameter of each mounting hole
- Slot length where holes are elongated
- Bridge-body width
- Minimum installation height
- Maximum installation height
- String spacing
However, bridge-hole spacing is not always identical to one exact post spacing.
Bridge holes may include:
- Functional clearance
- Adjustable inserts
- Locking sleeves
- Oval slots
- Manufacturing tolerance
- Wear
- Plating thickness
The guitar’s mounting system remains the primary reference.
When available, compare the bridge with the manufacturer’s technical drawing.
Reference Systems and Compatibility
Common Tune-O-Matic Reference Examples
The following figures are examples from specific current replacement products.
They are not universal standards for every bridge carrying the same general description.
Example system | Published spacing | Mounting detail |
TonePros AVR2 ABR-1 replacement | 73.75 mm | 6-32 posts |
TonePros NVR2 Nashville replacement | 74.0 mm | 10-32 lower thread, 4 mm post top |
TonePros T3BT metric large-post bridge | 74.0 mm | M8 × 1.25 lower thread, 6 mm post top |
Schaller GTM | 74.0-74.5 mm compatible range | Available with M4, M5 or M8 mounting hardware |
Floyd Rose Original six-string tremolo | 74.3 mm | Two pivot studs, M7 × 0.5 stud thread |
This comparison demonstrates the central compatibility problem:
Several unrelated systems occupy the same approximate 74 mm range.
Post spacing alone cannot identify which one is installed.
Traditional ABR-1 Measurement
A traditional ABR-1-style installation commonly uses:
- Thin direct-mounted posts
- Separate thumbwheels
- No pressed-in bridge bushings
- Approximately 73.75 mm post spacing on the referenced TonePros AVR2
- 6-32 thread on the referenced imperial system
Metric M4 direct-mounted versions can appear similar.
Measure or identify the thread separately.
Nashville Measurement
A Nashville-style bridge normally requires separate measurement of:
- Center-to-center spacing
- Upper post-section diameter
- Lower post thread
- Bushing dimensions
- Bridge-hole diameter
The referenced TonePros NVR2 uses:
- 74.0 mm post spacing
- 10-32 lower thread
- 4 mm upper post-section diameter
A visually similar metric system may use different hardware.
Metric Large-Post Measurement
Large metric Tune-O-Matic systems commonly have:
- Large visible bushings
- Broad post tops
- Integrated height adjustment
- Large bridge holes
The referenced TonePros T3BT uses:
- 74.0 mm post spacing
- M8 × 1.25 lower thread
- 6 mm post-top width
Its published spacing is therefore the same as the referenced Nashville replacement, while its mounting hardware is fundamentally different.
Adjustable Bridge Fit Does Not Create a Universal Standard
Some replacement bridges are designed to accommodate a small spacing range.
For example, Schaller specifies its GTM bridge as compatible with post spacings from 74.0 to 74.5 mm and offers separate M4, M5 and M8 mounting options.
This does not mean that M4, M5 and M8 systems are interchangeable.
It means the bridge body has sufficient mounting tolerance when used with the correct selected hardware.
Always distinguish between:
- Bridge-body spacing tolerance
- Post diameter
- Post thread
- Bushing system
Measuring a Two-Point Tremolo
For a two-point tremolo, post spacing is measured between the central axes of the two pivot studs.
Use the same principles:
- Same edge to same edge when both measured sections are identical
- Outside-to-outside minus one diameter
- Direct center measurement from a technical drawing
- Bushing-center measurement when the studs are removed
However, matching pivot spacing does not confirm tremolo compatibility.
Also check:
- Knife-edge geometry
- Pivot-stud head shape
- Stud thread
- Insert dimensions
- Baseplate width
- String spacing
- Saddle layout
- Tremolo-block position
- Block depth
- Body-cavity dimensions
- Recess routing
- Fine-tuner clearance
- Arm-system clearance
- Available intonation range
Floyd Rose Original Reference
The current Floyd Rose Original six-string technical drawing specifies:
- Pivot-post spacing: 74.3 mm
- Stud thread: M7 × 0.5
- Stud head diameter: 8.5 mm
- Insert body diameter: approximately 9.8 mm
- Maximum knurled insert diameter: approximately 10.1 mm
These dimensions apply to the documented Floyd Rose Original system.
They should not be assumed for:
- Every licensed double-locking tremolo
- Every low-profile tremolo
- Seven-string systems
- Eight-string systems
- Manufacturer-specific variants
- Modified guitars
Use the drawing for the exact tremolo model being installed.
Knife-Edge Position Matters
On a pivoting tremolo, the functional contact occurs between:
- Knife edge
- Pivot-stud contact surface
The post centers may match while the knife-edge geometry does not.
Possible differences include:
- Knife-edge distance
- Stud-head diameter
- Groove shape
- Baseplate thickness
- Pivot position relative to the saddles
- Baseplate overhang
A tremolo must not be forced between studs simply because the nominal spacing appears close.
Six-Screw Tremolos Are Different
A vintage-style six-screw tremolo does not use two bridge posts.
Its mounting may instead be specified by:
- Distance between the two outer screw centers
- Individual screw spacing
- String spacing
- Baseplate screw pattern
Do not describe the string spacing or outer screw spacing as two-point post spacing.
Use the manufacturer’s drawing for the exact six-screw system.
KMS-Specific Compatibility
KMS FlowTrem2 Compatibility
For a KMS FlowTrem2 installation, post spacing is only one part of compatibility.
Also verify the dedicated FlowTrem2 requirements for:
- Pivot hardware
- Baseplate clearance
- Tremolo cavity
- Fine-tuner area
- MONOLITH block
- Block height
- Available body depth
- String count
- String spacing
- Required radius
Do not confirm FlowTrem2 compatibility from a single post-spacing measurement.
KMS Conversion Posts: 7 mm vs. 12 mm
The KMS Conversion Post descriptions 7 mm and 12 mm do not refer to bridge-post spacing.
They describe two different lower mounting sizes for the body holes left after suitable original bridge bushings have been removed.
KMS production dimensions are:
Small-Bushing Version
- Lower press-fit diameter: 7.2 mm
- Lower section length: 14 mm
- Upper thread: 6-32
- Upper threaded height: 14 mm
Large-Bushing Version
- Lower press-fit diameter: 12.2 mm
- Lower section length: 18.5 mm
- Upper thread: 6-32
- Upper threaded height: 14 mm
The correct version depends on the actual mounting hole.
It is not selected from:
- The approximately 74 mm bridge-post spacing
- The guitar brand alone
- The country of manufacture alone
- The visible bushing flange alone
Do Not Assume USA vs. Import
Common tendencies exist, but they are not universal.
A USA-made guitar may use:
- Direct 6-32 posts
- Small bushing-mounted Nashville hardware
- An ABR-style bridge on hybrid posts
- Previously converted hardware
An import guitar may use:
- M4 direct posts
- Small metric bushings
- M5 hardware
- Large M8 posts
- Manufacturer-specific components
Measure the actual instrument.
Verification and Ordering
Common Measurement Mistakes
Avoid:
- Measuring only the empty gap between posts
- Measuring outside to outside without calculation
- Using the same-edge method on posts of different diameters
- Measuring different component levels on the two sides
- Holding the caliper diagonally
- Confusing string spacing with post spacing
- Confusing tailpiece-stud spacing with bridge-post spacing
- Confusing bushing flange diameter with body-hole diameter
- Measuring only the loose replacement bridge
- Assuming that approximately 74 mm identifies an ABR-1
- Assuming that approximately 74 mm identifies a Nashville bridge
- Assuming that approximately 74 mm confirms Floyd Rose compatibility
- Rounding before the measurement has been evaluated
- Reporting more precision than the measurement supports
- Ordering by guitar model alone
When Posts Are Leaning
Leaning posts can produce an inconsistent measurement depending on where the caliper contacts them.
Check spacing:
- Near the base
- Near the top
- At the bushings where possible
A changing result can indicate:
- Bent posts
- Loose bushings
- Compressed wood
- Misaligned installation
- Unequal adjustment components
Do not compensate for structural misalignment by forcing a replacement bridge onto the posts.
Photograph the Measurement
For compatibility support, provide photographs showing:
- The complete bridge area
- The bridge removed
- Both posts or bushings
- The caliper position
- The displayed measurement
- One individual post
- The visible thread
- A side view of the mounting system
- The guitar model or serial information where relevant
The caliper jaws and both reference points should be visible in the same image.
A photograph showing only the digital display does not prove what was measured.
Compatibility Measurement Checklist
Record:
- Guitar manufacturer
- Exact model
- Approximate production year
- Country of manufacture
- Existing bridge or tremolo type
- Post spacing
- Upper post diameter
- Lower thread diameter
- Thread pitch
- Bushing flange diameter
- Bushing-body or body-hole diameter where known
- Bushing or body-hole depth where required
- Bridge mounting-hole diameter
- Minimum bridge height
- String spacing
- Clear photographs
Not every replacement requires every measurement, but post spacing alone is rarely sufficient.
Quick Measurement Sequence
- Identify the general mounting system.
- Protect the guitar.
- Reduce string tension.
- Remove the bridge where necessary.
- Inspect posts and bushings.
- Select one identical feature on both posts.
- Measure same edge to same edge.
- Confirm with an outside-to-outside calculation.
- Measure the upper post diameter separately.
- Identify the thread and bushing system.
- Compare the results with the exact manufacturer drawing.
- Photograph and record the measurements.
- Confirm full compatibility before ordering.