ORIGINAL: gkamysz
Pe, is 1mm of end gap is acceptable in a <25mm bore engine? An OS FS-91 begins life at about .003", 0.08mm gap. A Magnum FS-91 I measured after 400-500 hours had a gap of .012", 0.3mm, the OEM replacement ring gap was the same! The bore was fine. I happen to have a NIB Magnum 91 at the time and it also measured 0.3mm.
Greg, The value is based on 0.15mm wear on the ring cylinder contact face. Some engines indeed need a long period of running before this value is obtained.
A more sophisticated number would be 15% of local ring cross section (Tring). This accounts for about 5% new condition play with ring ends closed.
Influence on ring gap would then be 2*0.15*Tring*pi for those who like to calculate.
The ships diesels I worked with had 35mm ring sections, 15% of which is 5.25mm. This would be a replace ring gap of 25.7mm. Close enough to the 20mm according to the maintenance books