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Bigbugberg 11-15-2012 05:34 PM

2009 ktm 400xcw
My little one is drinking oil...

Looking to do the top end but the microfiche has two part numbers 78330007000I Piston size 1 and 78330007000II Piston size 2.

WTF? They're both 95mm so what is the difference?

JTucker 11-15-2012 05:41 PM

One ring or two.

Bigbugberg 11-15-2012 05:44 PM

Is the second an oil sweep or another compression ring?

JTucker 11-15-2012 05:56 PM

Compression ring, most of the XC bikes had two ring and the SX bikes were one. You can use either. BTW, Vertex makes most of their pistons.

Bigbugberg 11-15-2012 06:05 PM

Both piston numbers have two rings, and the second is an oil sweep (pn 78030032200).

So what is the difference in the pistons?

Bigbugberg 11-15-2012 06:08 PM, Vertex only makes gasket sets for this bike...

LukasM 11-15-2012 08:15 PM


Originally Posted by JTucker (Post 20053696)
One ring or two.


Size one and two refers to size and not rings.


Pretty sure that one is meant for a fresh cylinder, the other for one that's been used and then diamond honed. Tiny size difference, about 0.01 mm if I remember correctly.

Foot dragger 11-17-2012 10:18 AM

KTM has done updates with newer style piston/rings I believe,talk to someone in the know and make sure your getting the best kit,measure the bore carefully. Check the valve seals,make sure they're not leaking and sucking oil.

gatorgrizz27 11-17-2012 01:09 PM

There are very slight, as in less than 0.001" variances in machining the cylinders and pistons when they come from the factory. Basically KTM sorts the cylinders and pistons by slightly under or over specified size, and matches them. You will have to mic your bore to be sure which size it takes, the service manual should tell you the limit. Last one I did I asked a very reputable KTM mechanic that is a buddy of mine if I could just buy the larger size piston since the bore would have worn enough that either one would fit. He said no, the Nikasil really doesn't wear at all, and it is important to mic it before you order the piston. It sucks to have to tear it down and leave it apart for parts.

WetSideRider 11-17-2012 08:39 PM

On my Husaberg 390 (same piston) the size was stamped on the piston. As stated above, you don't bore the cylinder, you replace like rings or ring and piston kit.

neumie 11-18-2012 07:44 PM

Lots of info on ktm
Definitely look there for info and experts.

dyno_dave 11-30-2012 05:14 AM

Piston size
I have the same bike. There is a stamp on your cylinder, a I or II, indicating its size. On my bike the stamp is near the area which houses the timing chain tensioner.

The original pistons are manufactured by Mahle not Vertex.

Let me know how much you paid and where you bought your kit. Mine has over 10k miles on it now and is due for a replacement. When I originally put rings in it at 3k miles the piston was near the high limit for clearance.


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