B piston in a C cylinder

[Dussucker3000]

My buddy showed up yesterday to do the top end on his 250x. I get to the point of putting the piston on and realize it's a B and he's got a C cylinder. We went up with it anyway. We heat cycled it once and he left. Today it's making a knocking sound like a worn big end crank bearing. Could the undersized piston make that noise?

 

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[mlatour]

Did you measure the side clearance, ring end gap etc ?

Excessive play, piston slap is perhaps a possibility.

 

This is on a YZ250X ?  I'd be surprised a brand new bike came with anything but an A or B sized cylinder.

[Motox367]

I wouldn't think that would do it it'ssuch a small diferance.  I always ran D pistons but had C cylinders 

[YZDOC]

Piston slap is mor of a clatter sound, knocking is bottom end or top end bearing play. You replace piston pin bearing, and how many hours on the bike? 

[Dussucker3000]

We didn't replace the bearing.  It did not seem to have any play though.  85 hours on the bike.  He's the second owner so not sure if the original had done a top end.

 

[YZDOC]

Wait, you did a new top end, and didn't replace a $15 wrist pin bearing? 

Did you check ring gap? 

Is it a clatter, or a knock?

[Dussucker3000]

It's a knock.  Sounds like he needs to come back here and we can take it apart again to recheck everything.

[Motox367]

2 minutes ago, Dussucker3000 said:

It's a knock.  Sounds like he needs to come back here and we can take it apart again to recheck everything.

Ya 80hrs is to long for wrist pin re use on that type engine.  Probably what it is. Allways put new oem bearing with piston at same time ? when you take it apart grab hold of piston with left hand as your pulling up with that hand smack top of piston with right palm you'll feel it if loose. 

[Theo]

From B to C sized pistons, at least for my 01 YZ250,  there is a difference of 7 microns: I can't believe that this is causing problems. For sure a B piston would be better: this one will be out of tolerance sooner, but not after a heat cycle.