CP Piston fitted..noisy??

Hi guys, I have a noise in my engine and am wondering if you can help.

My '05 YZF450 motor blew last year, blasting a big hole in my cases and cracked my barrel..wrecked it (but I have managed to weld the cases back up, but still in a box waiting to get rebuilt).

In the meantime, an '05 WR (with 5 speed, woohoo) motor was bought, with a 480 big bore, but no piston (the owner had lightened it too much), it had a new crank with 30 mins on it, and a new Hot rods rod with same time. I managed to machine the end of the WR crank so I could still use my YZF ignition system.

I got a 98mm CP piston for it, all clearances were correct.


After the first meeting, a loud ticking noise was heard, turned out the main bearings were knacked, so in went new ones and a new cam chain.

I still have a ticking noise (no where near as bad as before), more of a slight clatter really. Its not there at idle, or at partial throttle, just a anything above that.

There is no vibration from the engine at all, its dead smooth, and I have taken the head and barrell off and nothing is touching and valve clearances are correct to the manual.

The bike is fast, I race it in supermoto in the UK and it holds its own against the Open bikes in the A final, where I am mid pack.

My question is..is it the typical forged piston slap due to the CP piston whats in it?

The piston has massive clearence at the top near the crown, so I am guessing (and hoping) its just piston slap, but it doesnt really go away when its warmed up and I am careful about warming engiens up. Its also not getting any worse and I have done two races as it is, both at fast tracks.

Any other pointers of where a worrying noise would originate from?

I have taken the bash plate off, it didnt nake any difference.

The balancer shaft is still in (the YZF didnt have one in it).

It seemed to be worse when I jetted down??? But not sure on that.

Any pointers?

Or nothing to worry about. :thumbsup:


Pistons are supposed to have "massive" clearance at the top. If you look at any of them, you will see that the diameter is cut back from just below the oil ring all the way to the top. This is the ring land area of the piston, and the cut back is to allow clearance for carbon to build up without wedging the piston into the bore. The piston clearance is measured on the front and rear face of the skirt at the a point even with the wrist pin bore.

Piston slap occurs only under a load. As such, it's a half speed noise, occurring once every second revolution, synchronizing with the power stroke. Odds are that you are hearing primary gear backlash, which actually diminishes under load and with speed. There is a possibility that your primary drive gear nut is loose on the crank, though, so if you think it's noisier than it should be, you might check that.

The shape of the piston is important and also the pin off set, check to see if the cp piston has an off set pin .the oem piston has some pin off set we found through trial and error that the piston works very well with no pin off set quieter and more power .also check to see if there is clearance between the bottom of the piston and crank wheels .

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