How to tell if top end needs a refresh?

Michael Taksa

Hi fellow shifter racers.
I am going to refresh my top end soon. (97 cr125)
I already have the piston ring and both gaskets. Here is my question:
How can I tell if the piston needs to be replaced? Same for wrist pin, and bearing. Circlips will be new if I change the piston ofcourse, but how do I tell if the pistion is bad? Also how can I tell an RS piston from stock piston? I am not sure what is in there now. I bought the kart used and was told "I think some stuff was done on the motor". Not enough info to go on is it?

If it is an RS piston and I replace it with stock am I risking piston damage in case the compression was increased on the motor?
Just to clarify before everyone tells me to call a motor builder... I have no idea who built - up the motor if at all

Thanks.
Thanks.

Geoff Dodge

Ok Michael, i hope this helps.
It is true that the RS piston is a domed top, but so is a (95 i think, been awhile since we ran them) stock piston. Another easy way to tell the difference is the stock piston will have a window in the sleeve. You will know what im talking about if you are looking at a stock piston. The Rs has no window, and instead has 2 very small drilled holes. There is an arrow on the top that points to the direction of the incoming charge. The RS piston is significantly longer in legnth than a stock. The RS also has more piston above the wrist pin, thus raising compression. One way to tell if the piston is an RS is the number of base gaskets used. most that use an rs use 2 base gaskets, which keeps the piston from hitting the head. (do not simply add a base gasket to run the rs however, there is other machine work that needs to be done) If you install the stock piston check the squish on the motor. Do this by bending a piece of solder to 90 degrees and sticking through the spark plug hole with the motor bolted together. then turn the motor over a few times. Take the solder out and use calipers to measure the size of the now squished solder. I want to say you need at lease 20 thousanths. I cant remember off the top of my head however, so you will want to check that. If the motor has been configured to run rs, you will find the motor has very little horsepower when you put the stock piston in. We usually change the whole piston and ring after 2 races. hope it helps...geoff

John Hoppa · Joined: 30 Jul 2001 Posts: 202

There are guys out here that know a lot more than me, but this will get you started. RS piston is a domed piston, compared to a flat stock piston. Domed piston is one way to increase compression. Replacing a stock piston instead of RS fine, no damage should occur, could be a bit slower.

Pistons get very hot in operation and then cool down after running. Doing this 30-40 times in a life of a piston has a tendancy to create weak points in the pistons, which could cause it to break apart. Change the piston every 6-8 hours to avoid this. I change everything, bearing, ring, clips, etc.

Stock ring should last 6-8 hours, some folks use RS rings that last 2-3 hours. I have a RS ring which has about 6 hours on it and it still holds compression ok (Changing before next race, though). Some racers change rings after every race weekend, some more. Depends on how much time/money you would like to spend. This should get you started.

Michael Taksa

I just thought of another way to find out...
Does anyone know the high/low compression values? I mean if the thing has an RS in it, I would probably put an RS back in it, but if I put stock in it, I would loose a whole bunch of compression, so does anybody have compression numbers? if you say something like 12:1, would it means 12 atmospheric pressures?
Thanks.

John August · Joined: 02 Aug 2001 Posts: 140 Location: El, Cajon, CA

I change the stock ring every. Or, change the top end when compression drops 10-15lb from peak. The 91cr is the domed piston. I believe the rs piston is .040 taller than stock 91cr. On a Swedetech, you can run the 91cr by removing the metal spacer under the cyl.(but still use 2 stock base gaskets), you'll some power-I don't think it's a whole bunch.

Michael Taksa

John, what is your peak pressure???
What does it drop 10-12 psi from?
Thanks.

John August · Joined: 02 Aug 2001 Posts: 140 Location: El, Cajon, CA

quote:
Originally posted by Michael Taksa:
John, what is your peak pressure???
What does it drop 10-12 psi from?
Thanks.

Mike: By pressure I mean compression as measured with a compression guage. Each engine builder has there own preference for compression(how the cut the head). In general peak compression would be around after you put in a new top end and run a 2-3 heat cycles on the stand, though break-in may be longer w/ an rs piston and ring. If running an rs piston w/ standard bore I break-in 4 heat cycles on stand and make real sure the motor is up to temp. when I go out on the track(don't want to cold stick

).

Michael Taksa

All of this is new to me. Thanks for replies.
I will start another thread on breaking engine in.....