Nigel R Posted April 14, 2021 Share Posted April 14, 2021 In the OS 2 strokes I have recently been fettling & refurbing a little, the liners seem to have a step on the top surface. It is lower, on the inside, and a fraction higher on the outside. This is the surface in contact with the gasket. Why is the step there? I'm at a loss to understand the reasoning behind it. Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted April 14, 2021 Share Posted April 14, 2021 A pic and model would be good if you can load one. Can't recall a lip on the inside of a cylinder. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted April 14, 2021 Author Share Posted April 14, 2021 It is not a lip... I'll try and draw something. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted April 14, 2021 Author Share Posted April 14, 2021 Quote Link to comment Share on other sites More sharing options...
Nigel R Posted April 14, 2021 Author Share Posted April 14, 2021 The step is not large at all, several thou I would guess. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted April 14, 2021 Share Posted April 14, 2021 (edited) I've had a look at an old 32 heli engine in my spares/junk department and you're right, there is a small flange (as illustrated in the picture you posted while I was typing). My suspicion would be that it's simply part of the production process allowing an accurate ground surface to be machined for the gasket without touching the top of the bore. Edited April 14, 2021 by Martin Harris - Moderator Update Quote Link to comment Share on other sites More sharing options...
Nigel R Posted April 14, 2021 Author Share Posted April 14, 2021 (edited) OS SF motors, by the way. Hmm. You think the bore might be plated (ABC/ABN liner) before the outer surface is ground? It does mean the gasket compresses only on the outside of the liner top surface. Edited April 14, 2021 by Nigel R Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted April 14, 2021 Share Posted April 14, 2021 Looking at the old head, there's a similar sized machined groove to take the gasket and the top of the flange so it looks like any (long disappeared) gasket would have only been wide enough to fit on the higher land. Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted April 14, 2021 Share Posted April 14, 2021 I see what you mean now . I thought you meant in the actual cylinder bore . My guess is that it allows the liner to press into the soft alloy head gasket /Shim and make a seal without over tightening and possibly distorting the head, also avoiding the corner where the shim can bend up slightly preventing the cyl from seating properly Quote Link to comment Share on other sites More sharing options...
Nigel R Posted April 14, 2021 Author Share Posted April 14, 2021 Thanks chaps I suppose as the head protrudes into the liner, there is already a very narrow gap between head and liner, so there would be no benefit to sealing the entire width of the liner top surface. Also, I guess no combustion is going to happen right up in that tiny gap. Quote Link to comment Share on other sites More sharing options...
Robin Colbourne Posted April 16, 2021 Share Posted April 16, 2021 By reducing the contact area the contact pressure is increased. It also avoids the need for having a really sharp corner (which would be stress raiser) between the projecting centre of the head and the gasket surface. At a guess, carbon will also build up between the head and the lower step, potentially improving the seal? Quote Link to comment Share on other sites More sharing options...
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