BMW M42 Stroker

Our Customer wanted to change the stroke on his BMW M42 engine by fitting a longer stroke crankshaft from a later BMW engine.  The finished pistons needed to suit a bore size of 86.50mm and give a compression ratio of 10.5:1. The stroke of the engine was changing from 81mm to 88mm.

A sample piston was supplied from the 81mm M42 engine.  This gave us the basic data for the piston including the compression height* (CH) , valve relief  depth  and angles, pin diameter and pin boss clearance for the conrod.

The engine stroke change from 81mm to 88mm, means an increase of 7mm so, the compression height of the piston was reduced by 3.5mm (half the stroke)

In order to manufacture the pistons to a specific Compression Ratio of 10.5:1 we needed the following information, that our customer supplied for his engine:

Bore size: 86.50

Stroke:  88.00

Deck height /clearance*:  0.015” 

Gasket thickness: 1.6mm

Cylinder head chamber volume:  33cc

Valve lift increased from 9.7 to 10.6mm so valve pocket depth increased by 0.9mm 

Compression ratio required: 10.5:1

The finished piston shows the reduction in compression height, and the adjusted bowl volume to give 10.5:1 with the increased stroke.

*Deck height /Deck clearance:  Is the height of the piston below or above the top of the block at Top Dead Centre (TDC).  A piston level with the block will have zero deck clearance.  A piston below the block has a positive deck clearance, and a piston above the block will have a negative clearance.

*Compression Height:  Is the distance between the centre line of the pin bore and the top of the piston.

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