Piston And Cylinder Bore, Checking

WARNING: This page is about a different car, the 2021 Audi RS 7 Sportback. However, it is still accessible from the selected car via links, so may be relevant.

Pistons, Checking

Fig 1: Checking Piston Bore

Courtesy of AUDI OF AMERICA, LLC

-- Measure approximately 10 mm from the lower edge at a 90° angle to the piston pin axis using a 75 to 100 mm external micrometer.

Maximum variance from the specified size: 0.017 mm.

Piston Diameter in mm
	Audi A8	Audi RS6 / RS7 / S8
Specified size	84.45 ¹⁾	85.98 ¹⁾
¹⁾ Measurement with coating (thickness = 0.04 mm). The coating wears off.

If the wear on the coating -arrow- on the piston shaft is so well advanced, as shown in the image, the pistons must be replaced.

Courtesy of AUDI OF AMERICA, LLC

Cylinder Bore, Measuring

Fig 2: Checking Cylinder Bore (Dimensions)

Courtesy of AUDI OF AMERICA, LLC

-- Using the :VAS6078 , measure in a diagonal sequence at three positions sideways -A- and lengthwise -B-.

Maximum deviation from the specified dimension: 0.005 mm.

Cylinder Bore Diameter in mm
	Audi A8	Audi RS6 / RS7 / S8
Specified size	84.51 ¹⁾	86.01 ¹⁾
¹⁾ measure 50 mm inside the cylinder bore.

Piston Ring Gap, Measuring

Fig 3: Checking Piston Ring Gap

Courtesy of AUDI OF AMERICA, LLC

-- At a right angle to the cylinder wall, push the piston ring from above down into the lower cylinder opening until it is approximately 15 mm from the edge of the cylinder.

-- Use a piston without a piston ring for pushing it in.

Piston Ring	New mm	Wear Limit mm
1. Compression ring	0.15 to 0.25	0.35
2. Compression ring	0.30 to 0.45	0.55
3 - Part oil scraping ring	cannot be measured

Piston Ring Height Clearance, Measuring

Fig 4: Checking Piston Ring Groove Clearance

Courtesy of AUDI OF AMERICA, LLC

-- Clean the piston ring groove before checking.

Piston Ring	New mm	Wear Limit mm
1. Compression ring	0.03 to 0.08	0.15
2. Compression ring	0.02 to 0.07	0.15
3 - Part oil scraping ring	cannot be measured