Honda RS125 advice (piston change interval)

Discussion in '2-Stroke Machines' started by RTD, Jul 12, 2021.

  1. RTD

    RTD Well-Known Member

    For those of you running 125’s how closely are you following piston change intervals? I bought one for my 14yr old with a fresh motor at the start of the season. Owner’s manual recommends pistons at 500kms, based on total laps run we’re sitting at about 420kms & the next weekend would add another 250kms, I’m leaning towards replace to be safe but if it’s excessive I’ll run it another weekend. This is a track day only so a small performance loss is not an issue.
  2. Boman Forklift

    Boman Forklift Well-Known Member

    If it is OEM piston you can generally get some more time. One thing to check is if the ring is starting to stick in the piston ring land.

    In the shop manual it talks about taking a triangular stone or file and running it around the piston ring land to eliminate that sticking.

    When we first started I would pull the head and check this, by seeing if the piston would slightly wiggle or rock back and forth in the cylinder. Later I got to where I could check via the exhaust port after removing the pipe.

    Im no huge expert but we ran a RS125 for 3 years. If you want to call and ask questions send me a PM and I will give you my phone number.
    RTD likes this.
  3. nigel smith

    nigel smith Well-Known Member

    When my son was on 125s I followed the maintenance schedule religiously. We seldom did dedicated track days, so that was race mileage. I never once took out a piston that didn't look like it could have gone a lot further, but I figured pistons are cheaper than ambulance rides. The only time I ever had piston trouble was when I tried a wossner. I don't know if they are still on the market, but just say no.
    RTD likes this.
  4. nigel smith

    nigel smith Well-Known Member

    I spent a lot of time laying on my back with a flashlight, poking at the piston ring with a pencil through the exhaust port.
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  5. Boman Forklift

    Boman Forklift Well-Known Member

    I too had some issues with wossners. I had trouble getting OEM for awhile so I ran wossners. If you warm up the bike properly and heat cycle them more attentively you can make them work. But they are definitely more work than OEM in my experience.

    Wossners are forged and OEM is cast. Years ago I would have always figured a forged piston is better, but not with RS125’s.
    expat and RTD like this.
  6. DonTZ125

    DonTZ125 Purveyor of Neat Toys

    IIRC, forged and cast pistons need different running clearances. Drop a forged piston into a cyl sized for a cast piston (or vice versa - I forget which was finickier), and you run a risk of cold seizing, especially on something like an RS with that god-awful bridged port.
  7. knedragon29

    knedragon29 Well-Known Member

    Always check ring end gap , that's the most important thing especially with the last batch of HRC pistons . The rings were a little big and causing all kinds of fun if they weren't dressed . Older stuff were pretty good out of the box with very little dressing . Stock stuff if you are running big heads and little compression you can get great mileage out of them . I always ran 24 to 1 for fuel /oil mix . Never had any luck with wossener or wiseco .
    Also check what the ring looks like that's used on bridge area , thats where you will see the most wear and loose compression . If its wearing hard the bridge may need more relieving . I always just pulled the cylinder , that never lies . Pulled the piston off for inspection and then reassemble . Its a 15-20 minute job . Stock stuff is little more forgiving and less to keep up with . Kits stuff is a tear down pretty much every session . We always dressed the ring land ( exhaust side with the file , all the way with some 600-800 paper ) . Really polish it and oil for assembly , you won't see hardly any sticking .
  8. knedragon29

    knedragon29 Well-Known Member

    I ran 2 rings to a pistons life , big compression and I was 175-180 lbs and rung its neck .
  9. cyclox

    cyclox moving chicane specialist

    Yep, forged requires more clearance. VHM specs 0.07 mm for their forged pistons. Had to have a cylinder rehoned to get proper fit, and have been running the VHM ones without a problem.
  10. Boman Forklift

    Boman Forklift Well-Known Member

    FWIW, I was told by Wossner that they made the pistons to the appropriate size to work with the standard bore? I do know I had my son make sure he was at least 40C before even riding down pit road onto the track....I can't actually remember when he would crack it but I think slightly warmer was okay 45ish? I had a friend and his son usually ran OEM and he would barely warm it up at all and just rip out on the track. Then they tried Wossners one time and he stuck it right away.

    I'm going off 10-12 year old memory but I think Wossners were just a hair over $100-125 back then and the OEM was around $285-300. that's for piston, ring, wrist pin and circlips.
  11. The Beer Hunter

    The Beer Hunter Well-Known Member

    Bit of tangent, but has anyone run the VHMs in TZ250s? My parts source says there are close to no more OEM 5ke pistons left in north America. Word has it the VHMs run nice and they have a good price point compared to Yamaha.

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