Summary
ISCG-05 is a standardized frame interface for mounting chain guides, bash guards, and related drivetrain-protection devices near a bicycle’s bottom bracket. Introduced in 2005, it uses three threaded tabs positioned on a 73 mm bolt circle.
The frequently quoted 55 mm measurement is not the bolt-circle diameter. It refers to the approximately 55.92 mm center-to-center spacing between the two lower mounting holes.
Key Facts
Full Name: International Standard Chain Guide 2005
Introduced: 2005
Category: Frame Interface Standard / Concept
Also Known As: ISCG ’05, ISCG05
Mounting Interface: Three threaded frame tabs
Typical Fasteners: M6 × 1.0
Bolt-Circle Diameter: 73.00 mm
Lower-Hole Center Spacing: 55.92 mm
Primary Uses: Upper chain guides, full guides, lower rollers, and bash guards
Commonly Found On: Trail, enduro, downhill, and selected e-MTB frames
Predecessor: Original ISCG, also called ISCG Old or ISCG03
Directly Compatible With Original ISCG: No
Overview
ISCG-05 was developed as a revised version of the original ISCG chain-guide mounting standard. Its purpose was to give frame and component manufacturers a consistent interface for mounting drivetrain-retention and chainring-protection devices independently of the bottom-bracket cup.
Early chain guides were frequently secured by a backplate trapped between the frame and bottom-bracket cup. These systems could work well, but their position depended on bottom-bracket installation, shell width, cup design, spacers, and available crank clearance. Bottom-bracket service could also disturb the guide’s alignment.
The original ISCG standard addressed this problem with dedicated frame tabs. ISCG-05 enlarged and more precisely defined the pattern to improve clearance around contemporary bottom-bracket shells, external bearing cups, chainrings, and guide backplates.
ISCG-05 subsequently became the dominant frame-mounted guide interface. It remains common even though narrow-wide chainrings and clutch derailleurs have reduced the need for full upper-and-lower guide systems.
Interface Geometry
An ISCG-05 frame normally has three threaded tabs surrounding the drive-side of the bottom-bracket shell. Depending on the frame material and construction, the tabs may be welded, machined, forged, bonded, or molded into the frame.
The mounting holes lie on a 73 mm bolt circle. They are not evenly spaced at 120-degree intervals, so their angular position is also part of the standard.
The lower two holes are approximately 55.92 mm apart when measured center to center. This is the easiest dimension to check in a workshop, but it should not be confused with the 73 mm bolt-circle diameter.
Some modern frames use only the two lower ISCG-05 positions for lightweight bash plates. These can accept products specifically designed for two-tab installation, but they do not provide the complete three-bolt interface required by many upper guides and full guide systems.
How It Works
Guide Backplate
A chain guide or bash guard uses a backplate with holes or curved slots matching the ISCG-05 tabs. The backplate bolts directly to the frame and becomes the foundation for:
- An upper chain-retention cage
- A lower guide or tensioning roller
- A taco-style bash plate
- A full chainring guard
- A combined guide-and-bash system
Curved mounting slots allow the backplate to rotate around the crank axis. This accommodates different chainring diameters and component layouts.
Lateral Adjustment
The frame tabs establish the mounting plane, but the guide must still be aligned laterally with the chainring. Most systems use guide-specific spacers or shims between the backplate, upper guide, or roller.
Mounting-standard compatibility therefore does not guarantee correct chainline. The guide must have enough adjustment for the installed crank, spindle, and chainring offset.
Load Transfer
During normal operation, an upper guide should contact the chain only when it begins moving away from the chainring. A lower roller may continuously control the return span of the chain, depending on the guide design.
Bash guards experience much greater loads when they strike rocks or trail features. ISCG-05 transfers these loads through the backplate and into the frame tabs. The larger pattern gives the backplate a broader mounting footprint, but it does not make the tabs indestructible.
A hard impact can bend the backplate, damage the threads, or crack the tabs and surrounding frame.
ISCG-05 vs. Original ISCG
| Specification | Original ISCG | ISCG-05 |
|---|---|---|
| Common Name | ISCG Old / ISCG03 | ISCG-05 |
| Introduction | 2003 | 2005 |
| Bolt-Circle Diameter | 59.24 mm | 73.00 mm |
| Lower-Hole Center Spacing | 47.77 mm | 55.92 mm |
| Typical Fastener | M6 × 1.0 | M6 × 1.0 |
| Current Use | Legacy frames | Current frame standard |
| Directly Interchangeable | No | No |
The standards may appear similar because both use three M6 fasteners around the bottom bracket. Their different hole positions prevent direct interchangeability.
Some older guide backplates were produced with slots for both standards. Conversion plates have also existed, but they can change guide position, chainline adjustment, clearance, and impact strength. Compatibility should be confirmed for the specific frame and guide rather than assumed.
What Attaches to ISCG-05?
Upper Chain Guides
An upper guide surrounds the top of the chainring closely enough to prevent the chain from lifting or moving sideways. Modern versions are commonly paired with narrow-wide chainrings and clutch derailleurs.
Full Chain Guides
A full guide combines an upper cage with a lower roller or pulley. These systems provide additional control but can add drag, noise, weight, and maintenance.
They remain most relevant for downhill racing, severe chain-growth conditions, or bikes where maximum retention matters more than drivetrain simplicity.
Bash Guards
A bash guard protects the chainring from direct impact. Modern taco-style guards cover the lower portion of the ring without enclosing it completely.
The guard must match the chainring-size range. A guard designed around a 32-tooth ring may provide inadequate coverage for a 36-tooth ring or excessive ground clearance loss with a smaller ring.
Combined Systems
Many current products combine an upper guide and replaceable bash plate on one ISCG-05 backplate. This provides retention and impact protection while avoiding the drag of a lower roller.
Compatibility Considerations
Chainring Size
Every guide has a supported chainring range. The mounting pattern can fit correctly while the guide remains incompatible with the installed ring.
Oval chainrings must be checked at their largest effective radius.
Chainline
Standard, Boost, Super Boost, wide-spindle, and e-bike cranksets can place chainrings at different lateral positions. Confirm that the guide offers sufficient adjustment for the actual chainline.
Frame Clearance
ISCG-05 does not guarantee universal product fit. The guide can interfere with:
- Chainstays
- Suspension pivots
- Motor housings
- Bottom-bracket cups
- Crankarms
- Idler pulleys
- Oversized chainrings
Current manufacturers sometimes list specific frame exclusions despite those frames having ISCG-05 tabs. For example, OneUp’s ISCG-05 guides identify several frame and suspension-layout exceptions.
Bottom-Bracket Compatibility
ISCG-05 mounting is mechanically separate from the bottom-bracket cup, but the accessory still occupies space around the bottom bracket. Large press-fit shells, external cups, bearing preload collars, power-meter spiders, and unusual crankarms can create interference.
E-MTB Compatibility
Some e-MTBs use ISCG-05, but many mid-drive frames use motor-specific guide mounts instead. The motor housing can also prevent a conventional backplate or bash guard from fitting.
A guide’s ability to bolt to the tabs does not establish e-bike compatibility. Check the guide manufacturer’s motor and frame restrictions.
High-Pivot Frames
High-pivot bikes commonly use an idler pulley and proprietary chain-management components. A conventional ISCG-05 guide or bash plate may interfere with the idler, chain path, or suspension hardware.
Installation and Setup
A general installation sequence is:
- Confirm the frame uses ISCG-05.
- Verify the guide’s chainring capacity and frame compatibility.
- Inspect and clean the tab faces and threads.
- Remove the crank if required by the guide design.
- Position the backplate against the tabs with any approved spacers.
- Install all mounting bolts loosely.
- Rotate the backplate into its specified position.
- Align the upper guide and roller with the chainring.
- Set guide height for the installed chainring.
- Torque all fasteners to the product manufacturer’s specification.
- Check crank, frame, chain, and suspension clearance.
- Test every cassette sprocket under light pedaling load.
Some products install without crank removal, but this is a feature of the guide—not an inherent benefit of ISCG-05.
Mechanic’s Perspective
The most common ISCG-05 service problems are incorrect standard identification, poor guide alignment, inadequate bolt engagement, and damage following a bash impact.
Identifying the Mount
Measure between the centers of the two lower holes:
- Approximately 55.9 mm indicates ISCG-05.
- Approximately 47.8 mm indicates original ISCG.
Do not order parts based only on a frame photograph or a listing that says “ISCG.” The term is often used generically.
Inspecting the Tabs
Before installation, inspect for:
- Cracks around the tab roots
- Bent or twisted tabs
- Damaged or contaminated threads
- Paint buildup on the mounting faces
- Uneven or non-coplanar mounting surfaces
- Previous filing or frame repair
A backplate should sit flat without rocking. Do not pull a distorted backplate against uneven tabs by tightening the bolts; this can bend the guide or place additional stress on the frame.
Facing, filing, or straightening frame tabs requires caution. Carbon tabs and damaged aluminum weldments should be evaluated according to the frame manufacturer’s repair policy.
Selecting Mounting Bolts
Bolts must be long enough to provide adequate thread engagement after spacers are installed, but short enough to avoid bottoming in blind holes.
A bolt that bottoms can feel tight while leaving the backplate loose. A bolt with insufficient engagement may strip the frame during an impact.
Use the specified washers, thread treatment, and torque values. OneUp, for example, specifies 5 N·m for its current backplate bolts, but that value should not be assumed for every guide or frame. Its complete procedure is available in the manufacturer’s installation instructions.
Setting the Upper Guide
The upper guide should sit close enough to contain the chain without rubbing during normal operation. Check:
- Inner and outer cage clearance
- Chain movement in every cassette sprocket
- Clearance at the largest point of an oval ring
- Guide position after final bolt torque
- Access for chain removal and cleaning
A guide set too low or too far inward can rub continuously. One set too high may fail to contain a derailing chain.
Checking Suspension Movement
On a full-suspension bike, cycle the rear suspension through its travel while watching the guide, chain, chainstay, and linkage. Chain growth and changing chain angles can reveal interference that is not visible at static ride height.
Post-Impact Inspection
After a substantial bash strike, inspect more than the replaceable guard:
- Check whether the backplate has rotated.
- Look for bending around the mounting slots.
- Inspect all frame tabs and their surrounding material.
- Confirm that the chainring is not bent.
- Recheck bolt torque and guide alignment.
A visually intact bash plate does not guarantee that the backplate or frame escaped damage.
Modern Use
Narrow-wide chainrings and clutch derailleurs have made full roller guides unnecessary for many riders. ISCG-05 remains useful because it allows riders to add only the protection they need:
- Lightweight upper guide
- Two-tab underbash
- Taco-style bash plate
- Combined guide and bash
- Full downhill retention system
It is common on enduro and downhill frames and remains available on many trail bikes. Cross-country frames often omit it to save weight and simplify bottom-bracket construction.
On e-MTBs and high-pivot bikes, proprietary interfaces are increasingly common, so ISCG-05 should not be assumed even on aggressive platforms.
Advantages
- Standardized frame-mounted interface
- Independent of bottom-bracket clamping
- Broad accessory availability
- Adjustable for multiple chainring sizes
- Supports retention and impact protection
- Long-term industry adoption
Limitations
- Not compatible with original ISCG
- Does not guarantee frame or crank clearance
- Tabs can be damaged by bash impacts
- Adds weight and manufacturing complexity
- Guide alignment still requires spacers and adjustment
- Some e-MTB and high-pivot layouts require proprietary systems
Common Misconceptions
“ISCG-05 Uses a 55 mm Bolt Circle”
Incorrect. The bolt-circle diameter is 73 mm. Approximately 55.92 mm is the spacing between the two lower hole centers.
“Every Three-Tab Mount Is ISCG-05”
Original ISCG also uses three tabs. The pattern must be measured or confirmed through frame documentation.
“ISCG-05 Guides Fit Every ISCG-05 Frame”
The bolt pattern may match while the backplate conflicts with the frame, crank, motor, idler, or suspension pivot.
“A Guide Cannot Affect Pedaling”
The mounting tabs do not affect pedaling, but a misaligned upper guide or excessively tight lower roller can add friction, noise, and wear.
“Bash Guards Protect the Frame”
A bash guard protects the chainring, but impact energy is transferred into the guide, mounting bolts, tabs, and frame. These areas still require inspection after a major strike.
Notable Implementations
OneUp ISCG-05 Chain Guide and Bash Guide: Modern adjustable systems using replaceable guide and bash components.
e*thirteen LG1 and TRS Systems: Gravity and enduro guide families offered in ISCG-05 configurations.
MRP G-Series Guides: Full and upper-only retention systems for trail through downhill use.
Two-Tab Underbash Systems: Minimal chainring protection using only the lower ISCG-05 mounts.
Related Terms
- ISCG
- Chain Guide
- Bash Guard
- Narrow-Wide Chainring
- Chainline
- Chain Growth
- Clutch Derailleur
- Bottom Bracket
References
- e*thirteen: ISCG Standard Identification
- OneUp Components: ISCG-05 Guide Installation
- OneUp Components: ISCG-05 Chain Guide Specifications
- e*thirteen ISCG and ISCG-05 Technical Drawings
- MRP Chain Guide Installation Manuals
- Frame-manufacturer compatibility and service documentation