Decided its about time to replace the piston and rings as me rs has done nearly 17000km now!! Just wanted to kno what bits im gonna need, obviously im gonna need the piston and rings, base and head gasket but wasnt sure if i need the wristpin (the bit that holds piston to conrod) or if theres any other seals i need. Also gonna put new small end bearing in. And how much running in is it gonna need when its all put back together?? Any help appreciated, hoping to do it nextweek.
Hi there,
Buy a top end gasket set that should include 3 sizes of base gasket, head gasket water cover gasket and spark gasket.
I always use the middle size base gasket.
Buy a piston kit, that shouls have the
-Piston
-Gudgeon pin
-rings
-cir-clips
You will probably have too buy small end bearing separatly.
When the time come's to fitting it post another message here, because there are alot of thing's you need too know about how too fit it.
Jamie
Cheers for all that! So what kind of things do i need to kno when i come to do it?? Ive read the guide on this site about replacing the piston on the rs50 so i got a rough idea what im letting meself in for, its getting the rings slid into the cylinder that looks the difficult bit..
Just take your time and when u put the circlips on make sure they are properly seated in the groove where they go, to eliminate any chance of them falling out when u get ur bike running again!!! that can cause serious damage, i did that to my rs50 last year
Hi again,
Ok, well don't know what the RS50 guide says but i'll go through the things you should know.
When you have the barrell off; coat the inside with a thin layer of 2stroke oil; also a thin layer on the piston. Also the gudgeon pin and a little on the small end bearing.
The oil will help you slide the gudgeon pin into position also.
Make sure there is a very light coating of oil inside the ring grooves also.
When fitting the ring's be careful as they are brittle and can break easily.
Also there should be either a 'T' or some other writting on one side on the piston rings, this side must be at the TOP, if there is no writting them fit them either way.
The rings must be located properly in order to seal them up, there is a small notch in the ring groove where the ring ends must meet.
NOTE; BE CAREFUL WHEN SLIDNG THE PISTON INTO THE BARREL THAT THE RINGS DO NOT GO OFF LOCATION.
what kinda price are we looking at 4 the piston kit and headgasket?
Found out about a piston kit and they said £114 not sure if thats with or without vat tho! Gotta check the price of the other bits..
Hi
I just use a base gasket and a new piston kit. The head gasket O rings can be reused several times, and you dont need to disturb any other gaskets except the power valve gasket. Piston is around £80 from Peter Knight, base gasket around £1.50, power valve gasket around £3.50. Clips dont cost much (they dont come with the piston from Rotax). Use the same size base gasket as you are already using (unless you want to do a load of work measuring combustion chamber volumes with the piston at TDC to ensure you use the correct one)
At that mileage I would think about replacing the gudgeon pin and the little end bearings. They should be find, bot for the small amount of extra money and ZERO extra work you may as well for piece of mind.
By the way, the plastic gears for the counter balancer should probably also be replaced at about that mileage
All the best
Keith
Whats the standard size base gasket fitted on the 2001 model?? Theres 3 sizes isnt there..
do you not have to hone the bore a little with some fine paper like you do when changing crosser pistons to help the new rings bed in?
HI there Keith,
Do you know if the standard rotax122 pistons that Peter Knight sell's are the same pistons in the RS125.
I would think they are, but not sure.
Do you know if they are cast or forged ?
The piston kits from Prox.co.uk are forged and teflon coated, they looked a good job.
Unfortunently I only had mine in an hour before something awful happened.
Let me know becasue I always bought CR125 pistons for my old motor as Aprilia was £140 I think for the kit.
Hi
The pistons he sells are the standard Rotax pistons. These are the same quality as the ones used in the RS125 as standard (Rotax make the engines)
Not sure about the teflon coating. Not sure how long it would last
I thought CR125 pistons required you to machine a small amount from the bottom of the barrels to fit into a Rotax engine. A guy who used to do some engine work for me raced a Rotax engined kart and used Honda CR125 pistons, but to fit them he had to machine 1.2mm from the bottom of the barrel
All the best
Keith
Hi Keith,
I didn't have to modify the barrel to fit cr125 pistons.
Same bore and stroke.
Hi
This was over 10 years ago, so not sure what model of CR125 pistons he was using. The difference was the height of the piston from the gudgeon pin to the top of the piston.
Mind you if you are happy with a single ring piston then just buy one from the Rotax Max kart engines. Seen those advertised in karting magazines for £35 + vat.
All the best
Keith
Not happy with them any more, went throught too many.
I'll stick too standard pistons from now on !!
Cost less in the long run.
Cheers Keith,
Jamie
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