Domed piston in a flat piston bike?

[noobi]

Bought a new piston for my bike from Wossner, the piston kit is made for my bike, and the piston i got is a domed piston, my bike has only ever had flat top pistons in it.
The parts manual specifies a flat top piston,
The wossner parts catalogue specifies that its for my bike and its a domed piston...

left is a piston that was in the bike, right is the new wossner

[barty5]

thats going to up the compression and put some extra load on the bottom end hope its all good

[pete376403]

Then you're going to have a bit more compression. Check piston/head clearances carefully. Also pist /spark plug clearance (if it uses extended nose plug). Additional base gasket could be used to bring compression back, but that would marginally alter port timing.
Is the rest of the motor pretty stock or does it run right on the ragged edge?

[noobi]

Then you're going to have a bit more compression. Check piston/head clearances carefully. Also pist /spark plug clearance (if it uses extended nose plug). Additional base gasket could be used to bring compression back, but that would marginally alter port timing.
Is the rest of the motor pretty stock or does it run right on the ragged edge?

As far as i know the motor is bone stock, ill check all the clearances with the solder squish test before starting the bike, and i have extra base gaskets to raise the cylinder a bit,
but im thinking with the compression higher, that ill have to run 98 or higher, which is just a pain really.

[flyingcr250]

cut the top off with a hacksaw

[Reckless]

It may increase the compression Noobi but it depends if it decreases the Cc's of the engine at tdc. Their pin could be set a fraction higher (closer to the top edge) to compensate for the crown??? They may have decided a crowned piston gives better flow and back pressure response, which makes sense.
The only way to check is to get hold of a buret (spelling??) and cc the engine with both pistons at tdc. Not a big job but you need the right gear. Your dad might know, he seems pretty clued up about engines. It looks like it will from the pic, but who knows, the critical measurement is from the pin to the top edge, but that will only give you and indication its the volume change at tdc that will tell you for sure. Providing its the correct one for your bike.

[cheese]

It looks like the wrong piston. Id say that you've always been either running the wrong piston before hand, or they have sent you the wrong piston. I'd say that wossner fucked up.

[barty5]

should a just stuck with the wiseco

[barty5]

tend to disagree if you look real close the gaps between the port hole and the pin hole are about the same as with the gap to the skirt given the height difference over all there should be a noticeable visual difference in the spacing between the section i would have thought.

[IIIRII]

You 100% sure the factory piston if flat ? or are you assuming that because the first piston we took out was?
Is it an OEM piston ?
Everything said here so far is bang on, but you could actually calculate the volume difference in the head.
There would be nothing wrong with machining the new one flat, after some quick tghickness checks etc:
Measure the pin height first in case it is .2 0r .3mm higher, but...
You can calculate the volume in the new crown and we already know the squish clearance and bore, so we can work out head volume and work back from that.
EZ
Or is this going to turn into a discussion like how to work out piston the head clearance again ?
What happened last time ...
Dad was right ..
:P

[Reckless]

tend to disagree if you look real close the gaps between the port hole and the pin hole are about the same as with the gap to the skirt given the height difference over all there should be a noticeable visual difference in the spacing between the section i would have thought.

Yep certainly looks that way, but you know you can never make that judgement from a photo on the internet.
I was assumng nobbi had already done the standard piston bore clearance measurments and was indicating the next step so he can be sure of the changes that might occur to his engine and tuning, if he fits that piston.

[Starky307]

It may increase the compression Noobi but it depends if it decreases the Cc's of the engine at tdc. Their pin could be set a fraction higher (closer to the top edge) to compensate for the crown??? They may have decided a crowned piston gives better flow and back pressure response, which makes sense.
The only way to check is to get hold of a buret (spelling??) and cc the engine with both pistons at tdc. Not a big job but you need the right gear. Your dad might know, he seems pretty clued up about engines. It looks like it will from the pic, but who knows, the critical measurement is from the pin to the top edge, but that will only give you and indication its the volume change at tdc that will tell you for sure. Providing its the correct one for your bike.

Listen to what this guy has to say.

You 100% sure the factory piston if flat ? or are you assuming that because the first piston we took out was?
Is it an OEM piston ?
Everything said here so far is bang on, but you could actually calculate the volume difference in the head.
There would be nothing wrong with machining the new one flat, after some quick tghickness checks etc:
Measure the pin height first in case it is .2 0r .3mm higher, but...
You can calculate the volume in the new crown and we already know the squish clearance and bore, so we can work out head volume and work back from that.
EZ
Or is this going to turn into a discussion like how to work out piston the head clearance again ?
What happened last time ...
Dad was right ..
:P

I wouldn't recommend machining a new part to make it "fit".

I don't know the history of your bike but why don't you buy an original part for it?

[Reckless]

Dad was right ..
:P

Ah shit you posted while I was answering calls and typing mine up. I thought you'd be onto it!

Your dad might know, he seems pretty clued up about engines.

Good point too, we where assuming it was an OEM piston because noobi never said it wasn't or what it was. He did say it was "bone stock" though. Maybe Noobi should come see you instead of posting and getting all these opinions LOL!!

Btw I have a glass Buret (Somewhere???) and dial gauge if you need it or decide to go down that track. Cheers.

[SS90]

As far as i know the motor is bone stock, ill check all the clearances with the solder squish test before starting the bike, and i have extra base gaskets to raise the cylinder a bit,
but im thinking with the compression higher, that ill have to run 98 or higher, which is just a pain really.

There has been some good advise from a few guys here so far mate, and let's just concentrate on the important things.

A "solder squish test" is simply nothing more than a "piston to head clearance test"

You bike is likely to have a "squish head", but there is a difference in measuring a "squish clearance", and "head clearance".

Yes, putting a piece of solder through the plug hole will give you an acceptable indication of how much clearance there is between the piston and the head (in a non dynamic state....i.e "not revving", because, at say 8,000 RPM, the piston comes somewhat closer to the head than what you measure with this test.

Measuring "the squish" is slightly more involved, but what you are measuring is the WIDTH of the compressed piece if solder (as well as the compressed thickness), as the width, and compressed thickness, will give you an indication of the "squish angle" you are running.

I assure you that what squish angle you had before with the flat top piston (which, incidentally, no matter what some people may say, is actually better than a domed piston....) will now be totally different, and, if the cylinder and head where designed for one (or the other), mixing the two, even if you alter the compression ratios to suit, will lose power.

The shape of the top of your piston does more than control the amount of compression you have. It also controls the angle the compressed sir/fuel mixture enters the cylinder at BDC (crankcase scavenging), and this has an overall effect on your scavenge pattern, which, in a bike like this, is a well thought out thing.

Back to the compression.

If you don't know which piston is OEM, then I would go to the effort of finding out what the factory fitted.

Sure, you can cc the head, and with base gaskets alter the compression ratios (by ratio's, I mean the static (TDC volume, and the "corrected", or "trapped" compression, which is measured from the point of exhaust opening.

This is a far more accepted method than the TDC method alone, as cylinder compression on a two stroke is "dynamic" (i.e, they get more efficient as RPM increases, and the TDC only method does not take this into account)

Doing all this work sounds complicated, but I assure you it is worth the effort.

The important thing to note is that the "squish" test does NOT give you any idea of the actual compression ratio, only the distance between the head and piston when at a standstill.....if you start playing with piston heights, please cc the head and calculate the "corrected compression ratio"

You won't regret it!

[SS90]

There would be nothing wrong with machining the new one flat,

:P

Erm, I wouldn't recommend ever machining the top of a performance two stroke piston flat.... since the early eighties Japanese pistons have been as light as they can get (competition ones that is), and there is sweet FA material on the crown.

Remove any material from there and the chances of burning a hole in your piston go through the roof!