Porsche Air-Cooled Connecting Rods & Crankshaft. How to Inspect and Measure.

Inspecting the Con-rods and Crankshaft during disassembly

How to inspect and measure these crucial parts of your Air-Cooled Porsche Engine

As you take your Porsche Air-Cooled engine apart, you are always looking for obvious signs of damage. Start inspecting your connecting rods and crankshaft as you disassemble the engine.

Some damage is very easy to see, you can’t’ miss big broken engine chunks. But if the parts are just worn out you may not be able to see it, then the wear must be measured. After an initial clean it is time to take a closer look at your engine parts.

Some damage is easy to see.

In this lesson, we are taking a closer look at the Connecting Rods and Crankshaft.

Connecting Rod Inspection and Measurement

The connecting rods should be washed and completely cleaned before the inspection can begin. There are two bores on the connecting rod that will need to be measured.

The first is the little end bushing or the rod end bushings. This is the connection point for the connecting rod to the piston. The bushing can be measured using a pin gauge or using a split gauge and micrometer. The specifications for the rod ends are 22.03 mm when new and discard when worn to 22.08 mm

Inspecting the Con-rods and Crankshaft during disassembly
Inspect the con-rods & crankshaft as you disassemble the engine

Set up the connecting rod to measure

To measure the big end of the rod, first, the two halves need to be assembled and torqued to factory specifications. Next using a bore gauge measure the big end of the connecting rod in three places to ensure that it is round.

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Fowler Full Warranty X-tender-E Electronic Dial Bore Gage Gauge Set, 54-646-401, 1.4-6" Measuring Range
iGaging Bore Gauge Electronic Digital Absolute Precision Gage Super High Resolution 2"-6"/0.00005"
Ginode 5/16" to 6" 6 Pcs Telescoping Gage Set Range Micrometer Precision T-Bore Hole Gauge
Anytime Tools 2" - 6" Electronic Digital Precision Engine Cylinder Hole Bore Gauge Resolution 0.00005"
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Fowler Full Warranty X-tender-E Electronic Dial Bore Gage Gauge Set, 54-646-401, 1.4-6" Measuring Range
Fowler Full Warranty X-tender-E Electronic Dial Bore Gage Gauge Set, 54-646-401, 1.4-6" Measuring Range
$222.29
iGaging Bore Gauge Electronic Digital Absolute Precision Gage Super High Resolution 2"-6"/0.00005"
iGaging Bore Gauge Electronic Digital Absolute Precision Gage Super High Resolution 2"-6"/0.00005"
$139.95
Ginode 5/16" to 6" 6 Pcs Telescoping Gage Set Range Micrometer Precision T-Bore Hole Gauge
Ginode 5/16" to 6" 6 Pcs Telescoping Gage Set Range Micrometer Precision T-Bore Hole Gauge
$34.99
Anytime Tools 2" - 6" Electronic Digital Precision Engine Cylinder Hole Bore Gauge Resolution 0.00005"
Anytime Tools 2" - 6" Electronic Digital Precision Engine Cylinder Hole Bore Gauge Resolution 0.00005"
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Note the 2.0L, 2.2L, and 2.4L engines use a different diameter connecting rod than the 2.7L engines, so always check the specifications for the engine you are working on first.

Connecting rod, set up for measurment

Connecting rods that are found to be in specifications can be put straight back into service. Rods that have worn bushings or are out of round will need to be serviced before they can be put back into service. We balance the rods to within 5 grams of each other at this point, using a balance scale to measure each end.

Crankshaft Inspection and measurement

The first operation is to wash the crankshaft. This just needs to be a quick wash to remove oil and debris from the crank journals that can affect the measurements. Before measuring perform a visual inspection. Signs of heavy scoring or other heavy damage, and you will need to replace the crank.

Heavy scoring on the crankshaft journals
Heavy scoring on a crankshaft

Using a Micrometer to measure the Crankshaft

If the crank passes the visual inspection, then it is time to measure the journal sizes. Always make sure to pull the correct specifications for the crank you are measuring before starting to measure.

A quick way to check a crank is to set your micrometer at the minimum usable dimension and see if it can be passed over any of the journals. If not, then that means that the crank is going to be most likely serviceable depending on the level of scoring or marking on the crank.

Measuring a crankshaft with a micrometer
Measuring a crankshaft

The next step would be to measure the journals to get the true dimension. With a true dimension, you can decide whether to just have the crank polished or to send it out to be machined. There are three undersized amounts from Porsche. 1st under, is .25 mm, 2nd under, is .50 mm and 3rd under is .75mm.

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iGaging 0-6" Digital Electronic Outside Micrometer Set 0-1", 1-2", 2-3", 3-4", 4-5", 5-6" /0.00005" Large LCD Inch/Metric
Mitutoyo 293-347-30 Digimatic Micrometer, 3"-4", IP65
Fowler 54-870-004-0 Full Warranty Xtra-Value II Electronic Μm Set with Grey Enamel Finish, 3-4"/75-100 mm Measuring Range, 0.00005"/0.001 mm Resolution, 0.0002"/0.005 mm Accuracy, Black
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iGaging 0-6" Digital Electronic Outside Micrometer Set 0-1", 1-2", 2-3", 3-4", 4-5", 5-6" /0.00005" Large LCD Inch/Metric
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$395.00
Mitutoyo 293-347-30 Digimatic Micrometer, 3"-4", IP65
Mitutoyo 293-347-30 Digimatic Micrometer, 3"-4", IP65
$260.37
Fowler 54-870-004-0 Full Warranty Xtra-Value II Electronic Μm Set with Grey Enamel Finish, 3-4"/75-100 mm Measuring Range, 0.00005"/0.001 mm Resolution, 0.0002"/0.005 mm Accuracy, Black
Fowler 54-870-004-0 Full Warranty Xtra-Value II Electronic Μm Set with Grey Enamel Finish, 3-4"/75-100 mm Measuring Range, 0.00005"/0.001 mm Resolution, 0.0002"/0.005 mm Accuracy, Black
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Grinding a Crankshaft

Though a machine shop is happy to grind and repolish the crank to one of these three under-sizes, getting bearings is complicated by that decision. Third-under is very hard to get for the case and the connecting rod bearings. We believe that third-under is no longer available. 1st and second under will increase the price of the bearings you need but can be sourced.

Oversized main bearings
Oversized Main Bearings cost more than standard main bearings

Best to go shopping before you send your crankshaft out! Your case should be returned to standard with a close and hone. We covered casework here. Connecting rods will also have room to close and hone so the backs of the rod bearings would also be standard. But if you undersize the crank you must get the inside of the rod bearings and case bearings to match.

Crack testing a Crankshaft

Once your crankshaft is clean and has passed visual inspection and measures ok, you must crack test.

Is a ring test good enough? You can tap a crank to make it ring. A really cracked shaft will not ring, it will just thud. But a crankshaft can still be cracked and ring. It is best to test the crank and or rods before you move on and spend any money on the crankshaft.

Crack Testing a crankshaft or any other enginepart you want to re-use is a good idea.
Crack Testing a crankshaft or any other enginepart you want to re-use is a good idea.

There are many different methods for crack-testing; Magnetic field test, phosphorus test, or X-ray, are a few. You should contact your machine shop to inquire about a crack test.

Re-Building your Porsche Air-Cooled Engine

I hope you are getting the idea that measurement is key to re-building a Porsche engine. Get the right tools to measure correctly. Learning how to use the tools is also essential to rebuilding your air-cooled Porsche motor.

Next up, we are going to inspect and measure your pistons and cylinders. If they pass the same stringent inspection and measurement as the connecting rods and crankshaft, then they can also go back into service!

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