BEARING CLEARANCES - Engine building basics Air-cooled engines

For air-cooled engines to be correctly measure engine bearing clearances in an engine you will need the following tools.

• Micrometer: For measuring journal diameters. Dial Bore Gauge: To measure the inside diameter of the bearing. Plastigage: A simple tool to double-check clearances. Torque Wrench: For correctly torqueing down the bearing caps. Feeler Gauge: To verify the alignment of bearing caps.

Bearing clearances are quite small, typically between 0.002" and 0.005" for main bearings, with a wear limit of 0.006". A general guideline is to aim for 0.001" of clearance per 1" of journal diameter. For instance, a 2.45-inch small-block main journal would have a bearing clearance of approximately 0.0024 inch. 

Here's a more detailed breakdown:

• Tools:

  Micrometre: For measuring journal diameters. (1st image)

• Dial Bore Gauge: To measure the inside diameter of the bearing. (2nd image)

• Plastigage: A simple tool to double-check clearances. (Please search other articles for this process)

• Torque Wrench: For correctly torquing down the bearing caps.

• Feeler Gauge: To verify the alignment of bearing caps.

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• Measurements:

  • Crankshaft Journal Diameters: Use a micrometre to measure the diameter of each main and rod journal. Doing 1 at a time, moving onto the next steps to confirm clearance before repeating on the next journal.

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  • Measure the micrometre opening. now you have the micrometre at the size of the crank journal, insert the bore gauge into the gap and carefully zero it.

    
    
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  • Fit the bearings into the case: Once fitted (along with the main bearing dowels) insert the bore gauge into the correct bearing for the journal you set the micrometre to.

    you need to measure, and see how far off zero the bore gauge reads. in several points for each bearing. ensuring the bearing is round. (top to bottom, side to side, and also each side)

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  • Main Bearing Clearance: Make a note of all measurements taken, ensuring clearance is in spec.

  • Connecting Rod Bearing Clearance: Repeat the above steps for the rod journals.

    
    

  • Any corrections needed can sometimes be rectified using another set of bearings, or a mixture of several sets. alternatively you can get the crank polished to give you an extra bit of clearance.

    
    
  
  

• Assembly Lube:

  Apply plenty of assembly lube on the bearings during installation to ensure proper lubrication and prevent metal-to-metal contact during break-in. 

• Break-in Oil:

  Use a straight 30W oil without friction modifiers for the initial break-in run to help the rings seat properly. 

Additional Notes:

• Surface Finish:

  The surface finish of the crankshaft journals can also impact bearing clearance. A surface finish of > 5 RA (arithmetic average) may necessitate larger clearances. 

• Oil Viscosity:

  The oil viscosity used in the engine should align with the bearing clearances and lubrication needs. 

• Debris:

  Be mindful that debris in the oil can affect bearing clearances and potentially cause damage, consider this. A human hair is 0.001" and your clearance may only be 0.0025". so any dirt/swarf from machine work can easily block oil galleries or jammed between the bearing and crank. often resulting in damage.

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• Clearance Range:

  The ideal clearance range for main bearings is generally between 0.0020" and 0.0025", with a wear limit of 0.006". 

• Bearing Thickness:

  When ordering replacement bearings, ensure you select the correct size for the crank and case you have. often your machinist can advise on the correct bearings needed. but ultimately the responsibility is on the engine builder, which is why getting your clearances checked and set correctly is important to do.

(please do your own research into the correct use of measuring tools and to familiarise yourself with checking bearing clearances, as Aircooled hut is not responsible if things go wrong)

Aircooled Hut UK LTD