[stickyfox]

Thanks very much to both of you for some quite valuable information. If you're running with those numbers and happy with the way it's going that's good enough for me. My last bike was a Kawasaki (come to think of it the last two were), so engine noise does not bother me in the least. If anything I'm a little too desensitized to weird slapping noises.

I will definitely hold off on installing this piston until I make sure the clearance is what it should be.

Ace: your experience matches 100% with what I expected looking at the tables. My background is electrical, not mechanical, so I can't claim to be an expert, but it seems obvious that the piston, being trapped inside the motor and exposed only to combustion gases, will warm up quicker than the cylinder, which is wrapped in cooling fins and floating through the atmosphere at highway speeds (until it becomes welded to the piston anyway). And published figures are nearly useless unless you know the exact composition of the metal, but I think in this instance they are strong enough to arouse suspicion.

[ace.cafe]

Good luck with it!

The noises that will be heard the most will be the valves. They rattle because they run on solid tappets with hollow pushrods, and the valve lash clearance grows as it warms up.
They are supposed to make noise, so don't worry about it. Just make sure that they are adjusted properly when cold, and they will be fine.

[stickyfox]

Okay Ace, I've got one more question.

I got this piston out and fitted it in the bore, and I'm surprised. First thing I noticed is that the piston fits quite loosely in the bore. The second thing I noticed is that it's not nearly round at all! I talked to JE again and they said it's not round until it heats up, because it's designed with less diameter along the dimensions where it has greater thermal expansion.

So if you bore .006 over, where do you measure the piston and add .006?

[ace.cafe]

Okay Ace, I've got one more question.

I got this piston out and fitted it in the bore, and I'm surprised. First thing I noticed is that the piston fits quite loosely in the bore. The second thing I noticed is that it's not nearly round at all! I talked to JE again and they said it's not round until it heats up, because it's designed with less diameter along the dimensions where it has greater thermal expansion.

So if you bore .006 over, where do you measure the piston and add .006?

The piston usually has a specified place to take the measurement, from the mfr.
Generally, it is the largest diameter you can find on the skirt(below the rings) from front-to-back on the piston.

All modern pistons are built that way. The shape is called "cam", and the "cam of the piston" is how oval it is. There is also something called "barrel" which affects dimensions from top to bottom.
That's why the usual measurement place is the largest diameter from front-to-back, below the rings on the skirt.

Check with JE to see what they say about the location for measuremet. They should have an answer for you.

[stickyfox]

I've heard of this, but I've never seen it this pronounced. Of course, I never replaced the pistons on anything big before. All the Kawasaki motors I've rebuilt, the pistons feel like they actually match the shape of the bore.

But in this case we're talking about a big water pipe looking sleeve and a much larger bore.

(edit: come to think of it, most factory pistons don't have nearly as much metal on them; probably has something to do with it as the expansion would be a lot more uniform.)

My conclusion is that if you measure it the way JE says, I'm .004 over. If you measure 1/2" below the rings, I'm .010 over. But JE doesn't say to measure at the widest dimension you can find; they're 1/2" above the "hem."

Ace: your approach is more conservative than JE's, adding another half a thousandth to the required bore. I would rather have a little noise when the motor's cold than a whole lot of noise immediately followed by a permanent shutdown.

[ace.cafe]

I've heard of this, but I've never seen it this pronounced. Of course, I never replaced the pistons on anything big before. All the Kawasaki motors I've rebuilt, the pistons feel like they actually match the shape of the bore.

But in this case we're talking about a big water pipe looking sleeve and a much larger bore.

(edit: come to think of it, most factory pistons don't have nearly as much metal on them; probably has something to do with it as the expansion would be a lot more uniform.)

My conclusion is that if you measure it the way JE says, I'm .004 over. If you measure 1/2" below the rings, I'm .010 over. But JE doesn't say to measure at the widest dimension you can find; they're 1/2" above the "hem."

Ace: your approach is more conservative than JE's, adding another half a thousandth to the required bore. I would rather have a little noise when the motor's cold than a whole lot of noise immediately followed by a permanent shutdown.

Ok, well I'm not going to countermand what JE says about their widest expansion location for measuring the piston.
But, I agree a conservative approach with this clearance is safer.

Without an offset pin, it is more likely to make noise when cold, but we see others using .006" clearance without noise.
Even if it made a little noise, a forged piston in not likely to suffer any problem from that. Racers like 72westie use even wider clearances, and they always slap, and the piston runs fine.

[single]

Yeah,I don't know about bike motors running forged pistons but in race engines in cars custom forged pistons are sometimes noisy.A loud hollow sounding knock,about like you would expect.Mostly noticeable when cold,but usually there will be one or more that still knocks when warmed up.Can worry you if you aren't expecting it.

[stickyfox]

The tech I spoke to at JE also mentioned this. He said the noise characteristic of forged pistons is part of the reason they don't bother with an offset.