Cagiva SuperCity 125. What piston....?

[cagivaman]

Hi all, im looking for a new piston for my SuperCity i've found a site that sells them but there are two options: Flat top and Dome top....
does anybody know what one i need?


Cheers


<SMC>

[prawny1]

It depends on what type you have fitted already, if you have a flat top and fit a domed piston the compresion will be increased, theoreticaly more power but will need running on super unleaded to prevent detonation.
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hellkat wrote:

Some people are only alive cos its against the law to kill them.

[cagivaman]

ok...So it'l be safe with a Dome top as long as i use top notch petrol....
any ideas on the increase in power?
also can i ask what detonation is? it sounds bad....

Cheers

SMC

[Kickstart]

Hi

No. Use a domed piston on a head designed for a flat top and I suspect the compression would be through the roof. Very much doubt you could get it to run on anything available without suffering instant detonation.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[prawny1]

When on my Ls in the uk i used high domed (competition) pistons in my dtr ran shite on standard unleaded, switched to super and it ran fine.

so using a standard domed piston (which will be a shallow curve) shouldn't reduce the head volume at tdc to much.

But as i said before it depends what is in there to begin with, it may well have domed pistons anyway.
____________________
hellkat wrote:

Some people are only alive cos its against the law to kill them.

[Kickstart]

Hi

The difference between normal unleaded and super unleaded is small, but if you had upped the compression enough to need the super unleaded then the chances are that using normal unleaded would rapidly cause it to burn a hole in the piston.

While it might work, I would seriously doubt it would without a fiddling round to get the compression back down to something sensible. And would certainly be a good idea to measure the compression before you even though about starting the engine.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[prawny1]

A two stroke motor doesn't get the maximum potential c.r. like a four stroke, Because the gasses only realy start to be compressed when the ports are covered so fitiing a domed piston doesn't increase the c.r. as much as you might think.

you may well find that the domed piston is for the mito and the flat top for the city, both using the same head and crank.
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hellkat wrote:

Some people are only alive cos its against the law to kill them.

[Kickstart]

Hi

It still makes a substantial difference. Factor in that the Cagiva already runs a high compression ratio and also (in some instances) requires 98 RON fuel as standard, even a small difference could be expensive.

Having killed pistons on an Aprilia 2 stroke using a 0.3mm base gasket on an engine set up for a 0.5mm base gasket (a small change in compression, but enough to burn a hole straight through the piston crown) I am a bit careful about this, and very dubious about saying to anyone to just give it a try.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[prawny1]

0.3mm on a base gasket won't kill a two stroke. thats like saying roboring your engine will make it burn out pistons, a 0.25mm rebore will increase comp more than a 0.2mm lower cylinder.
____________________
hellkat wrote:

Some people are only alive cos its against the law to kill them.

[Kickstart]

[prawny1]

my 1989 honda 400 runs a true 11.5-1 cr from factory and that runs 95 ron fuel (recomended),
8.3-1 would be low even for a car.
____________________
hellkat wrote:

Some people are only alive cos its against the law to kill them.

[Kickstart]

[Kickstart]

Hi

Had a word with a guy with more knowledge of the Cagiva engines. His reply:-

[cagiva gezzer]

As Kickstart, the piston that you should be running will depend on the cylinder type and head type.

Flats work where domes shoudl be used and give a little extra peak power and more revs, but less midrange.

Domes can't be used in place of flats.

https://i67.photobucket.com/albums/h294/supercitybity/Cagiva/DSC04730.jpg

https://i67.photobucket.com/albums/h294/supercitybity/Cagiva/sssssssss.jpg
You can see the cylinder number on the RH of the pic, above the circle date stamp. 73037 iirc.
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"because one stroke isnt enough and four strokes waste two"

[cagivaman]

ok cheers for the help guys




<SMC>