daniel cooper Posted February 23, 2018 Report Share Posted February 23, 2018 I have a ty250 1974 and the piston is stamped next to the arrow on the piston 6-33 can anyone advise me what this means as want to put a new set of piston rings in and not sure what size is required from the id markings on the piston . Thanks. Quote Link to comment Share on other sites More sharing options...
feetupfun Posted February 24, 2018 Report Share Posted February 24, 2018 The rings you need to buy are determined by the diameter of the cylinder bore, the cross-sectional shape of the rings and the shape of the ends of the rings. A model TY250 came out in 1974 and has two rings of the same type with the top ring set down from the top edge. Later model TY250 twinshocks came with an L shaped top ring and a lower ring shaped like the TY250 A rings. Those numbers you posted are not the size of the piston or the rings. They look like something the manufacturer put there to keep a track of what type of blank the piston is and are not helpful for you. Sometimes but not always the oversize of a piston is stamped on the crown. Oversize numbers are nothing like 6-33. They are usually a number that represents the number of hundredths of a mm the piston is bigger in diameter than standard. They can also represent thousandths of an inch. Typical Japanese piston oversize stampings would be 25, 50, 75, 100. Imperial oversize stampings would be something like 010, 020, 030, 040 First work out what the piston is. A model, BCDE model or aftermarket supply, then measure the bore diameter. If it is a Yamaha or Wiseco piston you should be able to get rings. It is rare nowadays for people to replace just the rings on an old trials bike. A piston kit and re-bore is more commonly chosen due to the similar labour component for both jobs. A decent photo may speed up identification of the piston. 1 Quote Link to comment Share on other sites More sharing options...
daniel cooper Posted February 24, 2018 Author Report Share Posted February 24, 2018 Hi thank you for your post due to the age of the bike and not knowing it's past history I thought I would play it safe and replace the little end bearing and replacing the rings etc, the bore is in very good condition, your post has been very helpful, may be a rebore and new piston is the way to go or am I being to fussy the bike runs well but has a bit of piston slap (is this usual for this type of engine). Thanks for your help. Quote Link to comment Share on other sites More sharing options...
feetupfun Posted February 24, 2018 Report Share Posted February 24, 2018 They are a notoriously rattly motor and can function perfectly for many years despite obvious piston rattle. Because you have the motor apart, I would recommend that you decide if it needs new rings or a rebore by measuring the piston ring end gap using the standard method. Also be aware that some brands of pistons are more rattly than others. Quote Link to comment Share on other sites More sharing options...
b40rt Posted February 24, 2018 Report Share Posted February 24, 2018 9 minutes ago, feetupfun said: Also be aware that some brands of pistons are more rattly than others. Guy I use for re-bores uses a closer tolerance for customers who run in engines "sympathetically" Makes for a much quieter engine. 1 Quote Link to comment Share on other sites More sharing options...
scifi Posted February 24, 2018 Report Share Posted February 24, 2018 (edited) One of the specifications on older engines was the piston ring gap, usually 10 to 15 thousands of an inch. If when you put the ring in the bore, it was less, then you would need to grind or file it to that measurement. This is a very tricky job, as rings are very brittle, and two stroke rings have the locating pin, which means that the ring ends need to have that step included. There are also figures for 'sliding clearances' that the re-borer will need to know, but I suspect that the use of synthetic oil could reduce what is needed, also water cooled engines can have tighter tolerances than air cooled. The Haynes Manual gives some good information and figures for the TY50 80 125 and 175.. Edited February 24, 2018 by scifi Quote Link to comment Share on other sites More sharing options...
feetupfun Posted February 24, 2018 Report Share Posted February 24, 2018 Piston manufacturers recommend a certain piston/cylinder clearance be achieved at workshop temperature during the rebore for their piston, so that when someone uses the bike to plough through deep sand in an Australian summer, there will still be enough piston/cylinder clearance to avoid seizure. For those of us who have mechanical sympathy for their motorbike engine and not get it extremely hot, we can get away with less piston/cylinder clearance (at room temperature), which means less piston rattle when we ride trials. What I was referring to about different pistons is that some pistons are made from an alloy that has a higher coefficient of expansion than others. This means that the difference in piston/cylinder clearance from cold to hot is greater in some pistons than others. In the case of those high coefficient pistons, there needs to be more clearance at workshop temperature and at trials temperature to be able to have enough clearance if the bike is ridden in a way that gets the motor very hot. Because of this requirement, the standard clearance recommended for high coefficient pistons is greater than for low coefficient pistons. If the person doing the rebore uses the standard clearance as recommended for the piston, a high coefficient piston will rattle more than a low coefficient piston at trials temperature. I was pleased to read the instructions that came with a piston I bought recently because it had three different recommended clearances for the piston, depending on the intended use of the bike it was going into. 5 Quote Link to comment Share on other sites More sharing options...
daniel cooper Posted February 24, 2018 Author Report Share Posted February 24, 2018 guys thanks for info on your posts Quote Link to comment Share on other sites More sharing options...
tony27 Posted February 24, 2018 Report Share Posted February 24, 2018 If you get a new piston make sure it is the correct 1, lots of pistons are listed as fitting but won't due to I suspect rod length Biggest giveaway that it won't fit is the bottom edge of the skirt, TY pistons are tapered out to the transfer cutout, DT pistons aren't & foul the crank before reaching the bottom of the stroke. TY pistons are noticeably lighter, I'll see if I can find a link to a thread on OZVMX I did on the differences after I bought the wrong 1 Quote Link to comment Share on other sites More sharing options...
jon v8 Posted February 24, 2018 Report Share Posted February 24, 2018 I think it would help if you could post up some pictures of the bore and piston,along with measurements of ring gaps and piston to bore clearance etc. My old banger was built up with components from several boxes and just new rings and crank seals.The piston certainly didn't live with the barrel its in now from its past life. Rattles quite a bit, but goes VERY well,me and my kids thrash it around the woods with little care.But all the measurements were in the parish to start with. So all I'm saying is don't assume its worn out because it rattles a bit,from memory even Mick Andrews dismissed the noise from my TY at a training day my son used it for. 1 Quote Link to comment Share on other sites More sharing options...
guy53 Posted February 25, 2018 Report Share Posted February 25, 2018 I posted a picture of a DT and a TY piston side by side a while back if my memory serve me well I will check if I still have it. Guy Quote Link to comment Share on other sites More sharing options...
guy53 Posted February 25, 2018 Report Share Posted February 25, 2018 Here it is, A DT piston is a little heavier, and you have to file the bottom of the skirt, but they work fine. Guy 1 Quote Link to comment Share on other sites More sharing options...
tony27 Posted February 25, 2018 Report Share Posted February 25, 2018 Just for reference here is the OZVMX thread. Looks like you came to the same conclusion but I ended up finding the right piston & passed mine on to someone that needed it for a DT2MX which it was for http://forum.ozvmx.com/index.php?topic=29029.msg283770#msg283770 Quote Link to comment Share on other sites More sharing options...
scifi Posted February 25, 2018 Report Share Posted February 25, 2018 Although I only have the ty175 Haynes Manual, the figures should give you some idea when a re-bore is necessary. The tightest piston to cylinder specification is 1.6 -1.8 thou, and when this goes to 3.9 thou the cylinder is classed as needing a re-bore. These sort of measurements are not easily done at home, and need the machine-shop bore gauges... hope this helps.. . Quote Link to comment Share on other sites More sharing options...
OW10 Posted July 14, 2021 Report Share Posted July 14, 2021 (edited) I know this thread is old but what is 1.6 thou? I’ve never seen a decimal point used in a fraction before. Edited July 14, 2021 by OW10 Quote Link to comment Share on other sites More sharing options...
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