Do i have to change the Piston and/or Rings? On Yamaha RD125

Discussion in 'Bike Chat' started by kcnw9, Jul 10, 2007.

  1. kcnw9

    kcnw9 Member

    Messages:
    628
    Hi, my bike (Yamaha RD125) has been a wanker recently and has been hard to start and sometimes not starting at all even when warm, So i checked my compression by putting my thumb over the plug hole and kicking it over and it doesn't blow my thumb off the hole so i guess the compression is shite.

    Am i right in thinking this means the Piston or/and rings are fucked?
    Are they harder to change than a gilera piston?
    If the barrel is scored, can it be rebored?(i think thats the wording)

    thanks
    kcnw9
     
  2. capri dave

    capri dave Active Member

    Messages:
    5,076
    In theory changing the top end on a RD is exactly like a 2 stroke L.C
    125 scooter engine!!
     
  3. kcnw9

    kcnw9 Member

    Messages:
    628
    Well ive never done a 125 one but i changed the piston on my gilera 50 today, to get some practice and see what im doing.

    What will i need to buy, i was thinking Piston kit, small end bearing and head gasket, does that sound right?

    thanks
    kcnw9
     
  4. kcnw9

    kcnw9 Member

    Messages:
    628
    Hi, i took my cylinder head off just now to have a look and so far i can see my piston has lots of carbon build up on the top and i can see my barrel is ok(i think). My barrel has a few light scratches on one side, they seem very light, so you cant feel them when you run you finger across them. Is it ok to have this like that?

    Also notice my piston is 1.25 oversize and doesn't move around in the barrel so if i need a new piston i will get 1.25 oversize again right?

    thanks
    kcnw9
     
  5. wobbly_trials

    wobbly_trials Active Member

    Messages:
    6,676
    Rides:
    RD350, SR125, ICE125
    same principles, yes you need the same oversize unless your barrel is marked then you will need a re-bore and nicasil.

    you hve to remember thae RD125 motor has much finer tolerances than most scooter motors so putting it together you will need to be patient and check everything.
     
  6. kcnw9

    kcnw9 Member

    Messages:
    628
    OK well to get it re-bored will cost a lot so i hope i don't have to do that, I think it is fine as i haven't seized it or anything major for it to get mashed up. Here is a pic of the barrel, you can just make out the scratches:
    [​IMG]
    Are these bad or are they minor?

    Here are a few pics of the piston:
    [​IMG]
    [​IMG]
    [​IMG]
    Do i need a new piston? I think it looks OK, I will probably order one anyway and fit it soon but I'm going on a 150 mile ride next week so i don't have time to run in a new piston :msn1:

    At the moment I'm taking the carbon shit off the top of the piston and polishing it up.

    thanks
    kcnw9
     
  7. wobbly_trials

    wobbly_trials Active Member

    Messages:
    6,676
    Rides:
    RD350, SR125, ICE125
    polishing the piston won't help at all, tbh it looks ok, looks like someone has ridden the bike a little too hard from cold but nothing major, i would get a fresh pair of rings and try it.
     
  8. kcnw9

    kcnw9 Member

    Messages:
    628
    Yeah i was thinking the same, I just read in my hanynes manual its good idea to polish the piston :wtf: i done it now and it looks quite good. I would of brought a new piston but i cant find them anywhere online that does 1.25 oversize :( I cant even find somewhere that does 1.25 piston rings :tdwn: Dunno what to do now then

    thanks
    kcnw9
     
  9. JakeSP

    JakeSP New Member

    Messages:
    391
    Try Grampian Motors, Always had what we've needed...
     
  10. kcnw9

    kcnw9 Member

    Messages:
    628
    thanks i will give them a ring tomorrow :up: Just a quick question, what way do the piston rings go on the piston?
    [​IMG]
    The picture is not clear so i did it in paint :pimp:
    [​IMG]

    thanks
    kcnw9
     
  11. tweak

    tweak Member

    Messages:
    652
    The ring groove has a pin which you will find on the side of the piston that faces the inlet side of the engine. This pin is offset in the groove, in this instance it is toward the crown (top) of the piston. Make sure the tapers go like this:

    CROWN

    \\\\ PIN ////

    SKIRT

    ;)
     
  12. wobbly_trials

    wobbly_trials Active Member

    Messages:
    6,676
    Rides:
    RD350, SR125, ICE125
    seconded
     
  13. kcnw9

    kcnw9 Member

    Messages:
    628
    I just gave that place a ring and they have it :D :D Thanks JakeSP :up: top man :punk:

    kcnw9
     

Share This Page