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Piston Sizing?

By i_wheelie_longer,
in YZ400F/426F/450F/450FX

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i_wheelie_longer

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I am going to be replacing the piston and rings in my 2006 yz450f this winter. It will be the 4th piston/rings in the bike (including the stock piston) and I usually just replace with OEM parts. I have recently become aware of different piston sizing, such as A, B, C, and D. What do I need?? I am guessing they are slightly different bore sizes to accommodate for possible wear that may have been caused to the cylinder. I used to just go to my local bike shop and order a new piston, and they have never mentioned anything about these different sizes. Please help, I am confused.

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grayracer513

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Yamaha appears to list only one size, as they have all along, as far as I know. Aftermarket companies sometimes offer sizes in increments of fractions of thousandths of an inch so that the precise fit can be achieved.

Normally, the bore cannot show any measurable wear (and often doesn't, due to the extreme hardness of Nikasil) to be reusable, since any more than a thousandth or so means that the plating has worn through. At this point, the cylinder must be refinished or replaced. So, if the bore is usable, the OEM piston will fit correctly.

With an aftermarket piston offered in different sizes, one has to precisely measure the bore, do the math, and determine which piston to buy.

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i_wheelie_longer

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thanks for the help! looks like i'll be getting another OEM piston like planned. I just wanted to double check with the sizing thing because in the back of my mind it was worrying me.

I just replaced my cam chain approx. 20 hours ago when I replaced my last piston... do you think i should replace the cam chain again when i put a new piston and rings in this time?

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grayracer513

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Not to thread jack, but is the cylinder supposed to have a cross hatch pattern on it? My cylinder isn't worn through the Nikasil, but doesn't have a cross pattern, it's more like a piston travel pattern, does my cylinder need to be honed? Can you hone Nikasil cylinders?

https://www.thumpertalk.com/forum/showthread.php?p=4380759#post4380759

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i_wheelie_longer

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Just curious, but why would you be replacing a piston every 20 hours?

Good question... I replaced my piston 20 hours ago, but since the winter is coming up and I will be bored and have plenty of free time, I figured I would replace my piston again, check valves, etc so that I am ready to go next season and hopefully ride all season on the piston.

This way I won't have to tear my bike down during race season and rebuild it midway through the season.

Any input with replacing the cam chain at approx 20 hours?

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TDW

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Heres an interesting video on the subject of honing a cylinder, and maybe would be a good reason to consider using a slightly larger or selective sized piston other than the standard OEM. This method is more likely to cause some measureable change to the cylinder diameter.

Unfortunately most of us dont have the proper measuring tools to be able to verify accurately the diameter of a cylinder, and there is not alot of wiggle room when it comes to piston fit in a cylinder. Wiseco usually recommends .0015 and that is kind of hard to measure.

Interested in others people thoughts and opinoins on this method.

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R_Little

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A lot of times the cross hatching is still there even though you cannot see it.

The bore gets fouled with aluminum from the piston.

Take some WD40 and a scotchbrite pad and rub in a circle.

as long as you don't have any sctatches u can feel with your finger nail the bore will be fine.

these Yamaha engines are tough!

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grayracer513

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Heres an interesting video on the subject of honing a cylinder, and maybe would be a good reason to consider using a slightly larger or selective sized piston other than the standard OEM. This method is more likely to cause some measureable change to the cylinder diameter.

It's guaranteed, and in fact intended to cause a change in cylinder diameter. The operation shown is done to accomplish one thing: the precise fit of a non-OEM piston to the bore. Why would you ever need to do this? The likely reasons are:

  1. The cylinder was replated, and has to be fit to a piston
  2. An aftermarket piston requiring a greater than standard clearance needs to be fit
  3. The cylinder is showing wear in excess of tolerance

Note that in the third case, only an aftermarket oversized piston may be used, as honing a cylinder straight/round again would increase clearances excessively.

In both the second and third case, one needs to be concerned that the amount of plating remaining in the bore is adequate to tolerate the removal of as much material as will be honed away. This is usually not a worry in the case of a replated bore, because a known amount of plating has been applied. Remember that the Nikasil is only .003-.004" thick after it's applied, and once finished to size, it can be as little as .0025" from the factory (metric equivalents: 0.076-0.10mm and 0.064mm)

The advice I gave for the use of a bottle brush, or ball hone, was given for the purpose of re-using a serviceable cylinder that is straight and round within tolerances and fit for use with the same size piston it had in it to begin with, as would be the case when doing a normal re-ring and/or piston replacement. A ball hone is perfect for this, as it will follow the surface of the entire cylinder evenly regardless of whether it is perfectly round. A precision diamond hone run in a cylinder which is worn .001" would need to remove that much metal from the existing plating before it came into contact with the entire cylinder surface.

Ball hones are used by some of the best builders around, among them Ron Hamp, and as I said, I don't install rings without using one, even on a brand new bore job or a new engine, except for new cylinders from Yamaha. Scotch-Brite, etc., is simply inadequate.

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TDW

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Yeah I agree that the flex hone will do just fine for a routine rebuild when the bore is within the spec. Im not promoting the type of hone job that the video shows but since the first post of this thread mentioned the different selective sized pistons it seemed like this vid would help clear up some confusion about how to get the cylinder ready to use a slightly oversized piston.

I would guess this machine could do a light hone job, as well as oversizing the bore one or two thousandths but it really does seem like overkill if a flex type ball hone is available. Main thing is trying to keep the cylinder wall to piston clearance on the low side of the spec and ending up with a good surface finish-which a ball hone will do.

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TDW

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It can, but only to the extent that the cylinder is very nearly perfectly straight and round. Otherwise, it must remove the high spots to hit the low spots at all.

Obviously the cylinder bore condition and finish are very important for making a good seal and power. High and low spots, tapered and/or an out of round cylinder is going to be a potential problem for a good ring seal and piston life as well.

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