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Aprilia RS125 Top End rebuild advice

Discussion in 'Maintenance and Servicing' started by chansthename, Jun 11, 2015.

  1. Hey all,

    I have an RS125 (2010) that needs a top end rebuild soon. I plan on doing it myself and have done research to see exactly what I need to do and I feel comfortable that I can deal with it myself.

    The thing is I'm looking at buying a piston and there are 5 sizes. I wasn't sure what size to purchase and don't have the tools to measure it. I hear that I am supposed to find a stamp on the cylinder that will tell me what size to buy but I am unsure where it is (haven't found it).

    I'm planning on buying from here: https://www.pjme.co.uk/acatalog/Aprilia_RS125_Engine_Top_End_.html (the mikita kit), as you can see there are 5 different sizes varying by 0.01mm to pick from.

    Do you have any advice?

  2. You will find it once you get the head off. Unless you can mic it, you won't be able to guess. If it need a boring, the machinist can bore it to a certain oversize. 10tho, 20 thou etc.
  3. You can't bore the cylinder out unless you spend big dollars re plating .

    There should be only one size , just buy it here in Australia you won't be saving much if any at all.

    With my 1997 rgv 250 there is only one size bore!
  4. I have a brand new cylinder and piston to match if you want them. Been sitting in a box for 3 years now .
    • Like Like x 1
  5. This is an area which requires specialised advice, chansthename.

    There is some variation in how engines are constructed. This determines what should be done to do an upper cylinder rebuild.

    Once upon a time, cylinders were the same metal right through from the bore to the cooling fins. Think Iron. They could be bored a number of times until you eventually ran out of thickness, and there was a lot, and many rebores were possible. Then you had to replace the cylinder.

    Later, "sleeves" were put in cylinders. These are like tubes, providing a harder, less porous metal surface in the cylinder but were thin - limiting one to maybe 3 oversizes before the sleeve needed to be replaced, But it allowed the cylinder body to be made of a lighter alloy. Many, if not most, later motorcycle engines are sleeved. If you resleeve the engine, you go back to stock displacement, then have a number of rebores possible.

    Nickasil was a plating approach - it is very hard, but not reborable (replace the cylinder or sleeve)

    That 5 sizes of piston are available suggests that rebore is possible, and that your engine is sleeved (the cylinder is alloy on the outside, is it not?)

    To rebore an engine takes the measurement tools that only an automotive engineer, or VERY good mechanic will have; To resleeve takes the ability to heat and cool parts and press fit. Both a rebore and a resleeve will require final machining/honing to ensure accuracy with the piston. - my mechanic sends his work out - the engineer tells him whether there is enough "meat" left to rebore, and whether a new sleeve is needed, what piston to get, and does the work involved in getting the old sleeve out, the new one in and the bore of the cylinder the right size for the piston. These are things that cannot be done by a backyard mechanic unless they are an accomplished fitter and turner and have the time, expertise and the specialised mill, jigs required for setting the job up. Can be done. Talented fitter& turners can make whole engines.

    A home mechanic CAN do an upper cylinder overhaul without all of this by finding someone who can do the machine work for you. It is possible to take the mechanic out of the equation, but they have the line of supply of parts clearly in sight, already existing relationships with specialist machinist services who can "batch" jobs to keep the cost down. A sympathetic mechanic may be willing to assist you by providing advice, sourcing parts (they can often buy them more cheaply than you can), and sharing portions of the job which might be simply beyond the capabilities of the home mechanic. Sadly, this is not often the case, as usually they do not want to share any part of the responsibility for what might go wrong with the job. This can look like a "grab for cash" but is usually more related to them protecting their reputation.

    I'm no mechanic, but there are things I've done for myself. I draw the line, personally at a light cylinder "hone" - machine work? Out the question.

    Actually, Blueyy's solution would be a good one - an already matching set is a straightforward replacement. This I would come at, chansthename. If you really want to tackle it, in the simplest way possible, there is your answer. Up to you to confirm that it is the right cylinder and piston.
    • Like Like x 1
  6. What size is it?

    (No, I am not joking.....there are some kits for the 125 which take it out to 150 or 160 cc, and, if it were one of those kits, I'd be interested in knowing how much you want for it.)
  7. If the cylinder isn't worn and will remain as is, it's safest to replace the piston with the same size that came out.
  8. from memory its a standard size "Mitaka cylinder kit for the Aprilia RS 125 Rotax 122 Type from 1997- 2012"

    From PJME.co.uk

    I have a parts bike Cagiva mito if you want that instead :p
  9. Wouldn't just a new piston & piston rings be needed if it's a first rebuild?
    Back in my MX days I reckon I did at least 3x pistons before the cylinder looked anywhere needing a resleeve. But that was with a brand new bike where I could account for every hour of its use. 2nd hand bikes can be a mystery.
  10. Tempting....very tempting....some spares for my Raptor 125 would be nice to have.

    Will PM you.
  11. With it being plated with Nikasil, if the bore is in good nic ie no excessive scatches, no gouges and no flaking. Replace the piston with what it had in it.

    You don't rebore Nikasil cylinders, you just get them replated.

    Also how do you know it needs a top end?

    Also if you have a service history or the piston is in good nic, it may only need rings.
  12. My intention was to just get a new piston and rings of the same size as the current cylinder, I just wasn't sure how to find the size.

    That's the plan :)

    I don't have a service history and have done a minor service myself but didn't open up the engine. The manual states that a top end rebuild should be done at 16,000km, most people around here who used to own the RS125 stated it was worth doing at 8,000km. I'm currently at 12,000km.

    I have no reason to believe that it needs a top end and if I was going to do rings I figure it's probably worth doing the piston too since I'd be opening all that up.

    That's the one I was looking at but they had select-able sizes in it :(

    I might give the closest aprilia dealer a call and see what they say. I have no idea where to get australian parts from since I purchasedthe bike with the intention of importing all parts since the Aprilia network is quite small (and pricey and time consuming)

    Thanks for all your help everyone, will report back whenever I get some stuff done.
  13. #13 mike8863, Jun 13, 2015
    Last edited: Jun 13, 2015
    The manual for the engine is available online, search this title "APRILIA 125 ,Rotax 122 Manual Reparatie.pdf" it is 12mb in size.

    During a top end rebuild you replace the following, piston, rings, gudgeon pin, bearing, circlips and gaskets. Also a good idea to check the condition of the power valve and clean said...
    You do not just replace the rings...

    The measuring of the cylinder diameter is done in the top dead centre range, not at the bottom of the cylinder

    The pages you are currently most interested in are...
    Image1. Image2. Image4.
    • Winner Winner x 1
  14. Boom, that 1 (AB in this case) in the figure is exactly what I am looking for. Thanks a lot.
  15. 16,000km sounds fairly like a fairly substantial service interval but there's no need to replacing parts that aren't worn. It's common practice to replace just the rings on a two stroke.

    Checking the piston through the exhaust port and monitoring the cylinder compression would be my first port of call in assessing the top end condition.

    When you decide to tear it down, measuring ring end gap, piston wear, cylinder wear and visually checking the condition of the piston and Nikasil will tell you how often you need to replace said parts.

    The only gasket to be that would be disturbed I'd imagine is the base gasket, no reason to replace it unless it is damaged.

    On the flip side, if you're not the first owner and have no service history, a complete freshen up of the top end may keep your mind at ease and provide an accurate base for assessing top end wear.

    Also if this bike is still under warrant, do as the manual states obviously.
  16. After many years of running small bore GP and 250/350 improved touring 2 stroke race bikes I have to disagree with some of the 'advice'...RS125 is a higher revving, hi output engine, not a low revving chookie..
    Check, measure, and above all do things properly. Cutting corners to save a few cents will bite u in the long run.
    besides a topend rebuild kit for your bike is about A$100....to my mind, replace the lot, its cheap insurance.

    Base gaskets come in different thicknesses in order to be able to set the squish clearance.....correctly setting squish clearance is necessary when dealing with a hi output 2 stroke for a number of reasons.
  17. 16,000km intervals doesn't scream high output, race motor to me. Most "chook chaser's" are replacing their piston's at around 120 hours which does not equate to anywhere near 16,000kms.

    Riding on the street is night and day compared to riding on the track and requires the appropriate maintenance schedule.

    Some people are getting as high as 300+ hours from their pistons whilst others are replacing them at 40 hours.

    There's nothing wrong with replacing the whole lot, just it's hardly cutting corners if the parts are still serviceable. Personally I think inspecting parts, keeping service records and replacing parts as needed is ultimately more reliable and economical than the manufacturer's maintenance schedule. The downside obviously being more labor involved.

    I wouldn't be setting the squish with base gaskets, much more common to do it by getting the head cut and domed. Doing it with base gaskets changes the compression and port timing. Unless this bike was detonating, was hard to jet or he was chasing more performance, I wouldn't stress about setting squish, though if you got the time and money, go for it.
  18. I'll just add, I'm not trying to pedal my advice as gospel, if anything just trying to promote doing your research and making you own decisions.

    And apologies for pretty badly derailing this thread LOL!
  19. Hi Blueyy, I just pulled the barrel off my RS125, I am interested in the kit you have, would you be able to PM me? I have no idea how to do it??? Cheers.
  20. With a Nikasil cylinder, some manufacturers will supply a number of pistons for the standard bore size. Due to mass production, the finished product may be slightly off. It looks like your nominal bore size is 54mm. But some might finish up at say 54.01mm. Same also with the pistons. A bad matching of piston & bore can lead to clearance being too loose or tight, to satisfy a manufacturers stringent standards. Manufacturers measure each cylinder & piston and mark them with a letter to make it simpler on the assembly line. Match an "A" cylinder with an "A" piston. Match a "B" cylinder with "B" piston etc. This however only applies to a new cylinder. If you are putting a new piston in a used cylinder, then this matching of parts is not always necessary, or even advisable. When putting a new piston in a used cylinder, I will usually choose the biggest piston, to allow for any wear in the cylinder & keep piston to bore clearances as close to the minimum recommended as possible. There is usually only a very slight difference between the biggest & smallest piston, which only extremely accurate measuring equipment will pick up. For the backyard rebuilder, you would be hard pressed to feel any difference in the piston to bore clearance if using just a feeler gauge. So in the grand scheme of things, I think it matters more to fit a new piston, than what it does to select a matching piston.