06 YZ450 piston & cylinder size questions

So my cylinder is done(per Kens Cycle Tech.). The top looks like the coating is pretty thin and you could feel the slightest ridge at the bottom from the pistons lowest point of the stroke.

My Yamaha manual says:

Cylinder tolerances - 3.7402” to 3.7406” Mine measures- 3.739”

Piston tolerances- 3.738” to 3.739” Mine measures- 3.736”

Piston to cylinder clearance- 0.0016” to 0.0026” With max @ .004” Mine is- .003”

I know I am pretty much at my limit as far as piston to cylinder clearance. But why are my parts worn out but still measuring smaller than the factory specs. I am going to get it replated through Millennium but I am curious as to what they would final bore it to.

It is an OEM piston.

Hi, Kingzero!!!

Maybe it's possible you simply got a factory cylinder/piston combo that was at the "low end" of the size spectrum? :lol:

Out of curiosity, what are you using to measure your cylinder, and where are you measuring at?

You probably know this already, but you need to measure in several places of your bore. I apologize if you've already accounted for that, but it does make a diffference...... :ride:

Also, what are you using to measure it with? Maybe an I.D. mic checked with an O.D, mic? Telescoping gages with an O.D. mic? Or possibly a dial bore gage? Maybe your measuring stuff isn't repeating well, or has a margin of error greater than, say, .0005" or so?

For our tight tolerance stuff at work (Boeing Commercial Jet Parts), we typically use a dial bore gage set with either a certified ring gage, or gage blocks. You can get a really good measurement with these setups. :thumbsup: We also have some air gages & other ways to measure, but the dial bore gages are "common"..... We have Mitutoyo Intrimiks, and Browne & Sharpe equivalents, but besides measuring at 3 points instead of 2 (as most cylinders are measured, to check for out of round), I was told they have a margin of error of .0004"! My point is you need to be able to make good measurments...... It's possible things may be "better" than ya think :lame: (or could be worse, also, unfortunately..... :( )


Edited by Diesel Goober

Unfortunately I don't have a bore gauge. I measured the top of the bore as far down as my Starrett calipers would go. They are notched out towards the back of the inside measurement tabs enough to let me get down to the ring contact area.....If you understand what im trying to say. I know that ideally that isn't the best place to measure but that’s the best I can do. I have a Mitutoyo micrometer but it only goes to 2" so I can't use that to double check.

I did however check my calipers against the 2" calibration post that came with my micrometer and it was right on. But it is always possible for it to change further down the stroke of the caliper.

Hmmm. I wonder if it might be possible to take your cylinder to, say, an automotive shop, or a machine shop, and get a "second opinion" on your measurment?

Please don't think I'm trying to be a puke, or somehow "elitist", or anything like that, Kingzero. But you really do need a better measurement for something as important as a bore & piston. I think the margin of error for most calipers is actually fairly high; like .001" or so, maybe? I would definitely try to get another measurement before I shelled out any $$$ on cylinder replating, or a piston, or whatever...... Like I said before, there's a chance your cylinder's size could be worse than we think here, but there's also a shot it might be better, ya know?

Side question for ya: Is your avatar the old "Big Daddy Roth Rat Fink" deal? :thumbsup::lol: I haven't seen one of those in years.....


Thats an inadequate measuring device for this application. A machine shop can measure it for you. But if your having it replated there is no need just send new piston and cylinder together when replating.

Thats an inadequate measuring device for this application. A machine shop can measure it for you. But if your having it replated there is no need just send new piston and cylinder together when replating.
Correct. Calipers will not give you any sort of accurate read on a cylinder. At the very least, you need a snap gauge and a micrometer. Most people will actually get more accurate results by using feeler gauges to measure piston clearance.

By the look of your readings, I suspect your caliper is off by .0025" or so, or you are using it incorrectly (and that would easy to do in this case, as it's an extremely awkward measurement to take with a caliper).

They won't bore it at all, in most cases. No oversize pistons are made other than big bore kits. The cylinder will be jigged up in a precision hone machine and honed with a rough diamond hone to a point where at least most of the old plating is removed. The bore will then be replated, and the cylinder precision honed to fit a new piston. Before proceeding with a re-plate, check the cost of just buying a new cylinder versus refinishing yours.

Good point, do you have any suggestions for the OP on where to purchase a new cylinder as I am in the market also.

I know the calipers were not the best tool for the job but it was all I had.

The cheapest I could find a new cylinder was through Stadiumyamaha.com for 248.

Millennium wants 190-250 depending on the damage to the cylinder, and they had one ready on the shelf for 215 if I wanted to use their change out program.

Yeah I meant honed for the final size, I don't know why I said bore.

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