Perkins — Head thickness .. ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... 102,79/103,59 mm (4.047/4.078 in)

A Perkins service procedure — check it applies to your exact model before use.

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Procedure

1. Inlet.. ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ..13,00/13,027 mm (0.5118/0.5128 in)
2. Exhaust ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ..14,00/14,027 mm (0.5512/0.5522 in)

Source: manual:1000 New Serie Workshop Manual .pdf. Figures and steps are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.