wallace Posted December 29, 2004 Report Share Posted December 29, 2004 I have noticed a lack of compression and a fair bit of knocking from the top end of the engine and this morning removed the head and cylinder barrel for inspection. The bore measures 67.98mm and the piston 67.73. I measured this with a digital caliper. I could only get an accurate measure of the bottom part of the bore as there seems to be a part of the top of the bore not touched by the piston. This impedes the prongs of the caliper from finding a accurate measurement. The piston and bore look to be in good order, apart from the gap of aprox 0.20 between piston and bore. The piston does seem rather slack on the small end bearing. I will definitely replace this part, but I am in doubt as to the need for a rebore and new piston. I am not hard on the bike and do not compete in trials. Its more for my fishing transport than anything else. Any advice/input would be most welcome. Thanks in advance.......Gordon Mills Quote Link to comment Share on other sites More sharing options...
marky g Posted December 29, 2004 Report Share Posted December 29, 2004 You cant really accurately measure bore wear without a bore gauge, but its quite easy to tell if it needs a rebore, as there will generally be a distinct wear ridge just below the top of the bore. If its not going to be used for competition, and if the bore itself is not badly worn, then lightly hone the cylinder, and rebuild with a new piston, rings, and small end bearing, not forgetting to use new circlips on the new piston. If you do find it needs a rebore, try to get hold of a genuine Yamaha piston, and get the job done by someone who knows what they are doing with motorcycle work. To stop it rattling again pretty soon after being rebored, get the bore clearance set to 1.5 thou, and run in very carefully. All of my TY's have rattled (I've had 4), Two were brand new and started to rattle after about 20hrs use, nothing to worry about, they just keep on going . I think they must make em loose to stop them nipping up in the early days I must admit it can get on you tits though. Quote Link to comment Share on other sites More sharing options...
wallace Posted December 30, 2004 Author Report Share Posted December 30, 2004 Thanks for the quick replies lads, I put it all back together this evening with a new small end bearing and gaskets. The compression has improved. Turned out the lack of compression was due to the cylinder head gasket leaking. Cheers, Gordon. Quote Link to comment Share on other sites More sharing options...
charliechitlins Posted December 30, 2004 Report Share Posted December 30, 2004 The sneaky way to check for cylinder wear without a bore gauge is to use that "wear edge" mentioned by HondaRS. Put the ring in the bore and use the piston to push it up from the bottom until it is very near the top of the bore, in the un-worn part. Measure the ring gap. Push the ring down into the bore using the piston (you're using the piston so the ring will stay square to the bore) and measure the ring gap again. You can do this again near the bottom. The tricky part is remembering your High School math and translating the difference in circumfrence to figure the difference in diameter. Remember, pi-r-square...no...pie are round, cornbread are square. Although...they all knock. Quote Link to comment Share on other sites More sharing options...
wallace Posted December 31, 2004 Author Report Share Posted December 31, 2004 Compression a wee bit better with the new gaskets, still knocking though. I,ll have to learn to live with that. Happy that I,ve had a look inside and saw nothing to worry about. On a more positive note, I successfully painted the fuel tank. Its made of soft plastic as you all know. I emptied it and ran it through the dishwasher using 2 tablets and a half cup of soda. It came out clean as a whistle so I dried it, gave it a scuff with 400 wet or dry and walloped on the primer. Stuff from Halfords called Plasticote. A few coats of the primer followed by a couple of coats of gloss and it looks good, has not wrinkled, and in the morning i,ll give it a final coat of clear laquer. Watch this space Quote Link to comment Share on other sites More sharing options...
wallace Posted December 31, 2004 Author Report Share Posted December 31, 2004 Shame you tried painting the fuel tank HondaRSIf that happens I,ll spend the Quote Link to comment Share on other sites More sharing options...
marky g Posted December 31, 2004 Report Share Posted December 31, 2004 If that happens I,ll spend the Quote Link to comment Share on other sites More sharing options...
charliechitlins Posted December 31, 2004 Report Share Posted December 31, 2004 I've always thought that the paint and stickers peel because of fuel and fumes seeping through the plastic. I wonder if a good sealer inside the tank would stop that. Quote Link to comment Share on other sites More sharing options...
marky g Posted December 31, 2004 Report Share Posted December 31, 2004 When I bought a set of fresh tank stickers for my old Pinky they had like a sliver backing that you put on first then apply the outer sticker to it (I presume this was to combat the effects of the petrol vapour) Doe's this apply to TY Mono Yams only or 80's bikes in genral? I've just sprayed my SY250 Tank and it seemed to be made out of a different type of plastic than the old yams did. Quote Link to comment Share on other sites More sharing options...
charliechitlins Posted December 31, 2004 Report Share Posted December 31, 2004 Thats exactly why the paint comes off Charlie, and people have tried using sealer as you suggest. I would imagine if you could find a good quality sealer that was flexible (most use resins that set solidly), and used it on a brnd new tank, that it would work perfectly. Trouble would be when you tried to clean the inside of a used tank, well enough to make sure sealer stuck properly.......... I use a sealer that I get at an aircraft supply...of course their insurance company tells them to write "Not for use in aircraft" on the label. It stays slightly flexible and is alcohol resistant. You do, though, have to prep the tank with acetone or MEK. I don't know how that would affect the plastic. Quote Link to comment Share on other sites More sharing options...
wallace Posted January 1, 2005 Author Report Share Posted January 1, 2005 The paint that I used is designed for use on plastic. I,m sure that the tank was well degreased in the dishwasher particularly as I used caustic soda christals and 2 tabs. Tank looks great at the moment and is curing in a heated cupboard. Keeping my fingers crossed Quote Link to comment Share on other sites More sharing options...
wallace Posted January 1, 2005 Author Report Share Posted January 1, 2005 It never even occurred to me that petrol could percolate through plastic. Oh well its done now Quote Link to comment Share on other sites More sharing options...
wallace Posted January 1, 2005 Author Report Share Posted January 1, 2005 You're so right Mr HondaRS, I gave it a dunt with my knuckle this morning and a great big chunk of "coating" came off. One consolation, a good hammering with a rolling pin or similar will probably be enough to strip the tank again. Looks like I,m going to make Sammy Miller even richer PS Mr HondaRS, please consider updating your profile? All the best for the coming year......Gordon Quote Link to comment Share on other sites More sharing options...
wallace Posted January 1, 2005 Author Report Share Posted January 1, 2005 If I had known that tank covers were available I would never have touched the paintwork. I,ll look into it this coming week. I do'nt want the bike looking like its just out of the showroom you understand, just looking decent and running reliably would be fine. You're a very helpful chappie, Mr HondaRS for a "Scarlet Pimpernel" Quote Link to comment Share on other sites More sharing options...
woody Posted January 1, 2005 Report Share Posted January 1, 2005 For a tank cover try Steve Goode motorcycles in Welshpool. He used to stock them so may have some left. 01938 850544. That number is off the top of my head but he advertises in TMX if you want to check it. Quote Link to comment Share on other sites More sharing options...
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