cylinder boring

[Yamaniac]

I tore into the 80sg's motor today. once I got some of the carbon build-up off the pistons, I see "60" on the face of each piston. i am taking this to mean they are .060 over stock. Does this seem right... Why would evrything else be in mm and cm, and then this bore be .060".

After doing the math, that would amount to 2.8749606 inches or 73.024 mm (a .1524cm increase over stock bore of 71.5 mm). the piston measures about 2 13/16" across the face. I don't have any caliper to get an accurate measurement.

Just confused i guess. I would think that if it was bored out, it would be in some "even" form of mm, like 1mm or 2mm, etc.

Any thoughts or experience on this??

Thanks
Theron

[John]

I can tell you that the Wiesco 1179cc Big Bore is 74mm. I think the 1196cc kit is 75mm.

[Yamaniac]

John,

Am i right on the Wiseco kits to think that (74mm & 75mm) is the size of the cylinder bore, and that the piston itself would be about .040-.045mm (according to the book's piston clearance) less than that?

after doing more reading in the book, and more examining of the pistons, I am thinking that they are the 4th overbore from stock.. All the pistons have "3h7" on the face of them, but no indication on piston (or rings as the book says) as to what size of overbore it is. irritating. Guess I'm going to have to get some calipers to know for sure. bike only has 23658 on odometer, so that makes me question why it would have it's 'max' overbore, the Wiseco kits not withstanding.

Thanks
Theron

[Ken Talbot]

The '60' means it is 0.60mm oversize, which is 'second over'. This page from the fiche explains it:

[Incubus]

The place where I get my cylinders bored here is Austin has been in business doing bikes for a long time, ever since this bike was new and then some. The owner/machinist always suggests I go at least two steps up on an overbore. That helps make sure you hit specs without any trouble. The fact that it is two steps over is no surprise. My question is why a street bike like this would need a rebore at only 23,000 miles. It sounds like at some point something went very wrong in the top end.

Patrick

[Yamaniac]

Thanks Ken. The places I looked (book, yamaha website) the 2nd over was .50mm. Not to say your info is wrong, and I do appreciate you posting that fiche, but it does seem more logical that it would be .50mm because they appear to go up in increments of .25mm, up to 1.0mm as the 4th over. Of course, .1mm isn't a very big amount either (.004").

I ended up finding some digital calipers and the dang pistons are in line with the standard size. More measuring tomorrow when I have some better light and more sleep.

Thanks again for your help.
Theron

[Ken Talbot]

Now that you mention it, I can squint my eyes and make the '6' on the fiche look like a '5' real easily. Can you do the same with the '6' stamped on the piston?

[Yamaniac]

sorry for the unholy size, don't know how to resize it.

[Ken Talbot]

Well, there's not much doubt about it being a '6'. I'll have to pull some spare pistons out of the shed and see if maybe they have more markings on the underside that would indicate exactly what they are.

[GNEPIG]

.. shouldn't the real size be stamped in that circle in the middle?
i would trust your gauge to tell you what size it is, no one knows what the person was thinkin or drinkin when they wrote that number on the top of your piston.

[pggg]

3rd oversize piston from an XS11. Has yamaha 211073 stamped underneath

[Yamaniac]

If I trust the calipers, those are stock pistons (every one between 71.4 and 71.45mm). Why they all have 60 remains a mystery. Maybe they guy who put those pistons in turned 60 years old that day. I dunno. I do know they are gonna look good on the wall of the motorcycle room once the 1179 kit is in the bike.

Appreciate all the input guys.

[wingnut]

Balance

If you have one of the pistons out try weighting it. If its around 60 grams or 60 something I'd be willing to bet its a balanced engine and those numbers were put there by the guy doing the balance work. Or like you said he just turned 60 or he paid 60 dollars for them ....

[Yamaniac]

hadn't thought of that wingnut.
You'd think if somebody took the time to balance the motor, they would
have taken a little better care of the bike. not in very good shape asthetically.
Don't know yet on the mechanical. haven't torn it all the way down

[Do Lee]

Maybe a quality checker's ID mark.