mikeyc Posted March 8, 2009 Report Share Posted March 8, 2009 I have a 1974 TY250 that I recently bought and before I started the bike I pulled the cylinder off.I found the piston which is standard bore was cracked.I bought a new piston 1st oversize and was going to replace the bearings on the crank.Is it a good idea to use the stock rod and replace the rod bearings or should I buy a new rod? Is there a way to tell if the low end is good or bad besides the obvious? Thanks for any help Mike Quote Link to comment Share on other sites More sharing options...
alan_nc Posted March 8, 2009 Report Share Posted March 8, 2009 Mike, Couple of questions: You said you went to the 1st oversized on the piston (was the cylinder scored?). I would check all the tolerances as you take things apart. Check the end play of the crank. Once you have the cases split, go ahead and replace seals. I just pulled my 74 TY250 apart (ring broke and broke ring land on the piston), cylinder was perfect and still well within spec. I replaced the crank bearings, including the crank pin and bearing, and the seals. Note: Get the part numbers for the parts you are going to buy off the web listings. Check the big on-line Yamaha boat suppliers. They can get the same parts and I have found them to be 30 to 60 percent cheaper than the motorcycle suppliers. Good luck..... take your time. Alan Quote Link to comment Share on other sites More sharing options...
mikeyc Posted March 8, 2009 Author Report Share Posted March 8, 2009 No,the cylinder is in good shape but I figured being that old I would bring it back to spec.It's only 30.00 for boring and I have a nice round cylinder.I am not sure how to check the crank,There dosnt seem to be any up/down play.Is it ok to replace the rod bearings and not the rod? I really want tthis motor to be right so I dont have to tare it down for another 10 years.I have never had this bike running so I dont know the history of the motor,I would guess it was sitting for a good 6 years. Quote Link to comment Share on other sites More sharing options...
dond Posted March 8, 2009 Report Share Posted March 8, 2009 If there is no dicernable up and down play on the connecting rod then it does not need replacing. It probably will last the next 10 years. But there are no guarentees on things like that in life. If you do take the crank apart definatly replace the pin, bearing, washers and con rod. the rod is the outer race for the bearing and I would not think of replacing only part of the parts once you are in there. DonD Quote Link to comment Share on other sites More sharing options...
mikeyc Posted March 8, 2009 Author Report Share Posted March 8, 2009 What about side to side slop with the rod? Is that not important? Thanks Mike Quote Link to comment Share on other sites More sharing options...
stork955 Posted March 8, 2009 Report Share Posted March 8, 2009 Gday, and Yes, side clearance is vital. More than about 25 thou is too much. Specs say 0.6 mm which is 24 thou! ( Not a bad guess...)Keep sticking larger feeler gauges down the side of the rod till you can go no further. If you are replacing one bit you may as well do the lot as they all wear together and it must be dismantled completely anyway. Dont forget to check the small end also. This overhaul work is best done by someone who does this all the time, you need a bit of gear to press it apart and back together, and then true it all up. Yamaha will have all the bits you need. HTH, Cheers, Stork. Quote Link to comment Share on other sites More sharing options...
mikeyc Posted March 8, 2009 Author Report Share Posted March 8, 2009 Thank you very much for the info,The bottom end checks out OK.I have rebuilt a few motors when I was younger but I seem to be getting really paranoid about things as I get older.I am gonna just bore it,new rings,bearing,pin and run it. Quote Link to comment Share on other sites More sharing options...
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