Installation Tolerances for Piston Pins / Connecting Rod Bushings (Porsche Engines)
Dear all,
Following a customer inquiry, I have prepared a brief overview of the installation tolerances we use
for piston pins in the brass or bronze bushings of connecting rod small ends on Porsche engines
built by Albert Motorsport.
This overview is intended as a practical reference to ensure consistent
dimensional standards for future work involving connecting rods and pistons.
Background:
The values below refer exclusively to the fit between the piston pin (wrist pin) and
the small-end bushing of the connecting rod – not to the bore in the piston itself.
The data are based on proven standards from engine manufacturers and component suppliers (Mahle, JE Pistons, KS Kolbenschmidt,
general mechanical engineering practice) and apply to steel or bronze bushings typically used in Porsche and other high-performance
engines.
Recommended Installation Tolerances (Piston Pin ↔ Connecting Rod Bushing)
| Piston Pin Ø | Bore in Connecting Rod Small End (after reaming/honing) | Theoretical Clearance | Fit Type | Notes |
|---|---|---|---|---|
| 20 mm | 20.008 – 20.015 mm | 0.008 – 0.015 mm | Close sliding fit | For production and sports engines; typical for 991 GT3 R engines |
| 21 mm | 21.008 – 21.015 mm | 0.008 – 0.015 mm | Close sliding fit | Requires good lubrication; standard for smaller wrist pins |
| 22 mm | 22.010 – 22.018 mm | 0.010 – 0.018 mm | Precision sliding fit | Common for performance or racing engines; typical for 997 GT3 Cup engines |
| 23 mm | 23.010 – 23.020 mm | 0.010 – 0.020 mm | Precision sliding fit | Typical for Porsche 964/993; ensure controlled oil supply |
Processing Notes:
Measurement conditions:
-
All dimensions refer to a measuring temperature of 20 °C.
-
Measure both pin and bushing diameters at several positions.
Bushing installation in connecting rod:
-
The bushing is press-fitted with an interference of approx. 0.03 – 0.05 mm.
-
Always ream or hone to final size after pressing in, as the bore diameter decreases slightly.
Target fit:
-
The pin should slide with slight resistance, without binding.
-
No perceptible side play should be present.
Material recommendation:
-
High-strength brass or bronze bushings (e.g. CuSn8, aluminum bronze) have proven reliable.
Rule of Thumb from Porsche Engine Building:
For piston pin bushings, the general guideline is:
Clearance ≈ 0.0004 – 0.0008 inch per inch of pin diameter,
which corresponds to approximately 0.01 – 0.02 mm for 20–23 mm pins.
Summary:
The specified tolerances ensure:
-
a stable oil film,
-
low friction and wear,
-
thermally safe behavior even at high engine speeds.
For motorsport or special applications, the clearance may
be increased by approx. +0.005 mm if required.