the outlaw Posted March 29, 2020 Report Share Posted March 29, 2020 (edited) I'm looking at needing a new piston / ring for my Fantic 240. It's a 1984 model, and I'm the second owner of the bike which I've had since 1987, so I don't know its history in its first three years of its life. Looking at the piston fitted I see it had markings on it '3P' and 'D'. And the inner of the piston has Fantic Motor moulded in it, so that might suggest it's not an after-market piston. (Research suggests Mahle produced the pistons for the Fantic factory, so whether they then moulded them with Fantic Motor within I'm uncertain). Has anyone any knowledge of how an original piston for the 240 was marked? I know that after-market pistons are marked in overbore sizes of A to G, but somehow doubt that it would have had three sizes up from standard to a 'D' in its first three years of life, especially as I've not had to take the barrel off or service the top end in the subsequent 32 years. (In fairness it's lived a gentle life). So any ideas appreciated as to whether I might still have the original piston fitted or whether the '3P' and 'D' are indeed marking that show I have a replacement Edited March 29, 2020 by the outlaw Grammar Quote Link to comment Share on other sites More sharing options...
huski Posted March 29, 2020 Report Share Posted March 29, 2020 The letters A-G refer to the piston diameter and are microscopically different.The piston would be matched to the cylinder to obtain the correct clearance at the factory.Its not unknown for people to fit the next size up in a worn bore,but it's not the right thing to do.They don't all start out with an A piston Quote Link to comment Share on other sites More sharing options...
trapezeartist Posted March 29, 2020 Report Share Posted March 29, 2020 Overbore sizes are generally marked as +0.020" or similar. Letters (typically A to D) are grades. Mass production isn't precise enough to make bores and pistons as accurate as they need to be. So they are measured after machining and marked accordingly. D pistons go in D bores, and so on. Quote Link to comment Share on other sites More sharing options...
the outlaw Posted March 30, 2020 Author Report Share Posted March 30, 2020 (edited) Many thanks huski & trapezeartist for your replies... that is enlightening but makes sense. Its reassuring too that after 36 years, its only just started to sound a bit worn on the top end. I'll start with a new ring and small end, and treat it to a new piston only if these measures fail to do the job and quieten it. Thanks again, Nigel McGoldrick Edited March 30, 2020 by the outlaw Quote Link to comment Share on other sites More sharing options...
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