The big end of a connecting rod surrounds the crankpin. In the illustrated arrangement, its removable cap completes the housing and supports a lower bearing shell, while two screws secure the joint. This page separates those parts for identification and compares a measured bolt stretch with limits supplied by the user.
Connecting Rod Head Interactive Calculator
Compare a measured rod-bolt installation stretch with a range supplied for that exact fastener and connecting rod. The exploded animation identifies the cap, bearing shells and screws. The example values are not a bolt specification.
Equation Used
- Measurement is the change in one bolt’s length, not its total length or cap movement.
- Entered target range and comparison limit apply to the same measurement and component; the calculator does not establish their suitability.
- A valid target interval requires lower target no greater than upper target.
- Animation separates rigid parts for identification; it does not model bolt elongation, tightening sequence, torque or engine operation.
This is a dimensional comparison, not a preload, yield or fatigue calculation. Enter a comparison limit from the applicable component specification. The calculator compares the dimensions and flags reversed target limits.
The cap, bearing shells and screws
The upper rod body and lower cap form the big-end housing. Two semicircular bearing shells line its bore. The screws shown beside the bore join the cap to the rod body; they are distinct from the bearing surface on which the crankpin runs.
The animation withdraws the screws and then separates the cap with its lower shell. It reverses to show how the parts fit together. This is an exploded component demonstration, not a maintenance procedure or the movement of an operating engine. The parts remain rigid and the cap does not repeatedly open while the crankshaft runs.
The construction is based on an OEM exploded overview. Shapes and separation distances are illustrative. The calculator’s stretch inputs control the comparison chart, not a fictional deformation of the rod housing.
Comparing an installation measurement
A stretch measurement compares a bolt’s loaded length with its own baseline length. The range on this page must be supplied for the actual fastener and connecting rod. ARP’s installation guidance describes measuring bolt length before and during installation and following the relevant manufacturer’s instructions.
This tool performs the comparison arithmetic. It does not supply a torque value or decide whether an engine assembly is serviceable. The same numerical stretch can mean different things for bolts with different geometry and material.
The four comparison results
Let s be the measured installation stretch, a the supplied lower target, b the supplied upper target, and c a supplied comparison limit. All are in inches.
The measurement result is s. Shortfall below the lower target is max(0, a − s). Percentage of the target midpoint is 100s / ((a + b)/2). Signed distance to the comparison limit is c − s.
A zero shortfall means only that the measurement is not below the lower target. It can still exceed the upper target. Likewise, 100 percent means the measurement equals the interval midpoint, not that a bolt has reached a calculated load capacity.
If a exceeds b, the range is reversed. The page flags that condition instead of painting a valid target band. The numerical outputs still display the entered arithmetic so the error is visible.
Example comparison, not a fastening specification
For an entered measurement of 0.0040 in, a lower target of 0.0055 in and an upper target of 0.0065 in, the shortfall is 0.0015 in. The target midpoint is 0.0060 in, so the measurement is 66.7 percent of that midpoint.
With a comparison limit of 0.0080 in, the signed distance is 0.0040 in. That subtraction does not establish a yield margin or preload. These example settings only demonstrate how the comparison works.
If the measurement is changed to 0.0070 in with the same targets, the shortfall becomes zero but the chart correctly labels it above the entered range. That is why the full interval must be considered, not just the shortfall card.
Installation stretch and permanent elongation
Installation stretch is measured while the fastener is loaded. Permanent elongation is a change that remains after unloading. They are different observations. A user-entered threshold for the first cannot, by itself, diagnose the second.
ARP’s design discussion includes component loads, geometry and material considerations beyond a single length measurement. None of those variables is present in this calculator, so it cannot calculate clamp force, yield stress, rod fatigue life or an acceptable engine speed.
Dimensional comparisons alone cannot establish bolt yield, fatigue strength or suitability for reuse. Make reuse and replacement decisions using the applicable component guidance and inspection.
Rod-head and stretch questions
Does the cap move away from the rod while the engine runs?
No. The animation is an exploded assembly demonstration. The cap is part of the assembled rod in operation.
What does the supplied comparison limit mean?
The limit is a length threshold supplied for comparison. Subtracting the measured stretch gives the distance to that threshold; it does not calculate material yield.
Does zero shortfall mean the measurement is within range?
No. It can be above the upper target. The chart and status compare both interval boundaries.
What if the target limits are reversed?
The page marks the range invalid and removes the normal target-band display. Correct the lower and upper values before interpreting a range result.
Do the default values apply to every rod bolt?
No. They are demonstration values. Use information for the exact rod and fastener, including its measurement procedure.
Component and measurement references
Volkswagen Jetta and GLI Quick Reference Specification Book, February 2014, printed page 33/PDF page 47: exploded piston and connecting-rod components, including cap, bearing shells and fasteners.
ARP: Fastener Installation Overview: bolt-stretch measurement and the distinction between loaded extension and permanent change after disassembly. ARP: Design Procedures for Automotive Bolts: component load and geometry considerations that go beyond a dimensional comparison.
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