TOPIC:

Got some rough measurements. My rule is in inches not millimeters so I had to mark and measure.
This is what I came up with.




The stock 903 piston has a 6mm crown and the wrist pin is at 30.64mm by measuring the marks with my caliper.


The new wiseco has a 10mm crown and a 35.40mm to center wrist pin.


The cast piston appears to be the same just taller in overall height.




10mm crown and 36mm wrist pin at center.
So I guess what I'm coming away with here is the 750 piston is a higher compression piston like the Wiseco's 10.25/1, the 903 is 8.5/1.
Still doesn't answer that nagging question. I guess I'll just have to wait.
Steve

The measured height of the piston crown means nothing.
You have to CC the cylinder with the piston at TDC to find the true compressed volume and then add the swept volume to that figure to work out the compression ratio.

Just a general discussion:
What I was getting at Steve, is that my Wiseco pistons, as measured from the C/L of the pin, very slightly puts the last .015"-.020" of the piston head, crown/dome deeper into the fixed dimension of the combustion chamber, on top of having a larger crown/dome volume.
From the top of the cylinder's deck at TDC, a little more of the piston fits into the combustion chamber area.
It is a larger total volume shoved into the combustion chamber then the stock ART piston.
As mentioned above by Zed1015, in order to determine that actual compression ratio, you'd need to sweep the combustion chamber and measure it's actual volume, (CCs), then compare that to the volume displacement of the bore and stroke and what effect the amount of "displacement" the new piston's crown/dome has on the total volume of bore and stroke displacement with that very piston at bottom dead center. What is lost at BDC is gained back at TDC
Kinda tricky.
In other words, imagine the piston having zero dome or crown... just a dead flat piston head.
Pull that piston all the way down to bottom dead center.
Fill the cylinder with a liquid from a flask that you can measure how much you put in the cylinder.
Now do the same test with a same bore piston but a very large dome.
The total swept volume will be less now.
That is a hard number and in fact is the swept volume of your domed piston head.
You could mathematically come up with a percentage loss. That would be a ratio too.
But that same dome volume is a positive displacement when the piston is at TDC and pushed into the combustion chamber.
It pushes deeper into the "fixed" volume of the combustion chamber. That makes it a higher compression.
Although not huge, the difference between 9:1 and 10.25:1, with respect to the combustion chamber vs bore and stroke, is a respectable one.

I'll have to try that before I pull my head down. I have stock 1015 pistons in there now. Thanks
Steve

MissionKZ I have had a Wiseco 1170 kit in the orange bike for 8 years and around 14k miles and is still running strong. APE did the boring. This bike will power wheelie as high as you want to go when I put my Mikuni RS36 flat slides on. Dont run those carbs any more as they are not fun to live with on a daily basis but they do make tons of power. I have a very reputable performance shop a few miles from where i live and would love to have them dyno it just never got around to it. I think it would be in the 110-115 HP range.

Many recommend to replace sleeve for 1015, if they have more than 40,000 km.
Confirm ?
Thanks.

Are you going from 903 to 1015?
Steve

RODZ wrote: Many recommend to replace sleeve for 1015, if they have more than 40,000 km.
Confirm ?
Thanks.

No! that is utter nonsense.
40,000 km is only 24,000 miles which is a very low mileage and the average engine should at least double or treble that figure before needing a re-bore.
Replacing a 1015cc (70mm bore) sleeve is only needed once it has been bored to its 73mm maximum (1105cc) although the liner spigot does gets very thin at that size and can develop cracks. they can also burn a little excess oil at that size due to the thin liner distorting under heavy load .
Many engine builders will change the liners after 72mm because of this.

swest wrote: Are you going from 903 to 1015?
Steve

Yes.
Doubt if 70 or 72 ( or more ... ) .
I am considering the various options / costs .

Meanwhile, I took another engine 900 , with 2 Weber 40 DCOE ! Not good for road use, but Amazing look
In Italy you ' eat ' good . And drink even better...

RODZ wrote:

swest wrote: Are you going from 903 to 1015?
Steve

Yes.
Doubt if 70 or 72 ( or more ... ) .
I am considering the various options / costs .

Meanwhile, I took another engine 900 , with 2 Weber 40 DCOE ! Not good for road use, but Amazing look
In Italy you ' eat ' good . And drink even better...

Stock 1015cc blocks and pistons seem to be rather abundant on eBay... I'd consider buying those used ones and new rings with a fresh hone and seeing if a good set was cheaper then going aftermarket pistons and rebore of the 900.
If all new, I think it's something like $400 to $700 by the time you are done with parts and labor,

The reason I asked is, when I bored my 903 barrels to 70mm, I only had 2.45mm thickness left. I had another oiling problem and the bores were marginal at best. I bought a low mile 1015 set up only to find I needed another 10.25/1 piston. I'm running the stock 1015 pistons and have for a year and a half. They are only 8.5/1 and there's a big difference in power and carboration. I finally have my new piston and am getting ready to install them. The 1015 barrels are a tight fit. I almost had to grind the block for it to fit in a few spots. The KZ 1000 barrels dissapate heat better than the 903' too. I can go out to 11 if I want to as well.
Steve

RODZ wrote:

swest wrote: Are you going from 903 to 1015?
Steve

Yes.
Doubt if 70 or 72 ( or more ... ) .
I am considering the various options / costs .

With the 900 block you can safely go out to 70mm (1015cc) and a maximum of 71mm (1045cc) ,
Again like taking the 1015 out to 1105 the liner spigot gets a little thin at 71mm.