how essential is cross hatch for new piston?

Putting a new piston in my YZ426, the bore looks and measures great, but it's perfectly smooth with no cross hatch pattern. Will I be ok to put in a new piston or do I need to take it somewhere to have it honed quick for a cross hatch?

won't seat the rings and it will have a lot of blow by along with low power and oil consumption. It needs to be roughed up a bit.

Putting a new piston in my YZ426, the bore looks and measures great, but it's perfectly smooth with no cross hatch pattern. Will I be ok to put in a new piston or do I need to take it somewhere to have it honed quick for a cross hatch?

 

The cross hatching is there to support the oil in suspension on the cylinder walls, not to 'wear down' the rings, so they will seat.

 

 

You need the crosshatching pattern to reduce heat/friction

 

You sould measure your bore for spec before attempting it your self.

The cross hatching is there to support the oil in suspension on the cylinder walls, not to 'wear down' the rings, so they will seat.

You need the crosshatching pattern to reduce heat/friction

You sould measure your bore for spec before attempting it your self.

What is the spec for the bore? With a bore gage I am getting between 3.741"-3.742 depending on where I measure. 95mm = 3.740 so I am within .001-.002 of stock, is that in spec?

Also do the yammies have a coating on the bore like the nikasil that Honda uses?

Yes, the bore is Nikasil plated.

 

The spec is that the cylinder must read round/straight within .002".  Measure the bottom, below the travel limit of the rings, then the top, above the rings, and several points up and down within the ring sweep.  If the cylinder passes this inspection, the best choice is to hone with a "bottle brush", or spring-ball hone such as those from Brush Research.  If using their products order by bore size (95mm).  320 o4 240 grit Silicon Carbide or Aluminum Oxide, nothing more aggressive.  The oversize for spring tension is built in, so no need to guess how much bigger to get.  Follow the guidelines in this post: http://www.thumpertalk.com/topic/399309-new-piston-06yz450f-advice-needed/#entry3565759

 

If, after a few quick passes, there remain any vertical scratches that are significant enough to catch a fingernail in, you should replace or replate the bore.

Thanks for the info. If I measure below the travel of the rings I get 3.7405 and the largest point in the ring sweep is 3.742, so looks like I'm in spec by .0005 overall and about .001 within the ring sweep. I just dropped it off at the local shop to be cross hatched so I don't have to wait for a ball hone to get here. But I think I will order a couple, one for the YZ426 and one for my 2 CRF450R's

They might both take the same hone. 

IIRC the CRF is a 96mm bore

I just got the wiseco piston, it measures at 3.737", and the box recommends .003" piston to bore clearance, which would be dead on with a stock 95mm bore at 3.740". So with my bore running at 3.742" at its largest point, this gives me .005" clearance. Am I going to have issues? The service manual also shows a limit of .004" clearance...

Edited by alping45

I'm considering sending the jug off to Powerseal to strip, replate and hone/fit to piston for $184. Would this be worth while and or neccessary at this point? Are there any cheaper alternatives?

.003" is pretty loose, if you ask me.  That would explain the noise those make, perhaps.  Yamaha's manual calls for from .0016 to .0026" with the OEM forged piston, and their pistons run from 3.738-3.739". 

 

Your cylinder is pretty close to tolerance limits as it is.  If it were me, I would have the bore replated to specs, and use a different piston.

After having the shop hone the cylinder, I can now see many spots of scouring and light gouges in the bore. I am definitley sending it out to Poweseal be replated and fit to piston. I already have the Wiseco piston, so I'm sticking with that. Would it be safe to have the clearance fit tighter than wiseco's recommended .003"? Powerseal said they will fit it tighter but will not warranty against seizure.

No, it won't be safe to use anything other than the manufacturer's recommended clearance.  The alloy they're using is probably less resistant to thermal expansion than others, so the need the extra clearance.

I tried a little experiment to compare thermal expsansion, not sure how conclusive it is but here's what I did: I had 3 pistons, the new wiseco, the old vertex piston I removed from the 426, and also an old factory piston from a CRF450r.I mic'd them all at room temp and recorded the results. Then I threw each piston in a pot of boiling water for about 5 mins and mic'd each one immediately after taking them out. Every piston expanded almost exactly .005", within a few tenths. I wish I could have included a factory YZ426 piston in the test, but this atleast tells me that the wiseco doesn't expand any more than the other 2 pistons. However the rate at which they expand could be different. From what I've read, as a forged piston is more dense than cast, the expansion rate is faster and could possibly expand before the cylinder has enough time to heat up and expand. I suppose I could try boiling them for shorter, pricisely controlled amouts of time and check the results to see if the differ.

Since all pistons for a WR, including OEM, are forged, the mention of cast pistons has no bearing on the matter. 

 

212 ℉ is not very warm for a piston, either, since nominal crown temperatures run closer to 450-500 ℉ in operation, cooled mostly by the vaporization of incoming fuel.

Hmm good info. Maybe if I get bored I'll throw em in my heat treat oven at 450f and cook em for a few mins each and perform the same test.

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