Squish

[noobi]

ok, new piston = 0.4mm of squish, too much?
the new piston is NOT a a genuine one, it is a 54mm wiseco cr125gp piston, thanks barty
and the squish with the old piston was 0.76mm, now it measures around 0.4mm. i did this by measureing the depth of the head with a depth mic, then the depth of the piston from the top of the bore at TDC, and took the head measurement away from the piston to top of bore = approx. 0.4mm
the only thing that was changed was the piston, the l/e bearing and the base gasket, the new ones equal .45mm, the old one was .5mm, so 0.05mm difference is explained, not the 0.3mm dieffernce there now.
i dont think theres a X parameter Reckless

[Rupe]

ok, new piston = 0.4mm of squish, too much?
the new piston is NOT a a genuine one, it is a 54mm wiseco cr125gp piston, thanks barty
and the squish with the old piston was 0.76mm, now it measures around 0.4mm. i did this by measureing the depth of the head with a depth mic, then the depth of the piston from the top of the bore at TDC, and took the head measurement away from the piston to top of bore = approx. 0.4mm
the only thing that was changed was the piston, the l/e bearing and the base gasket, the new ones equal .45mm, the old one was .5mm, so 0.05mm difference is explained, not the 0.3mm dieffernce there now.
i dont think theres a X parameter Reckless

They are not the same as KTM's (x parameter stuff), Linton would be able to answer all your questions mate, 033890080

[noobi]

They are not the same as KTM's (x parameter stuff), Linton would be able to answer all your questions mate, 033890080

chur. s10cr

[barty5]

shite with those small a measurements a very slight diff of where you call TDC will consantly change your measurements.

[Reckless]

I agree ring someone that knows.
But reducing your squish from .76mm to .4mm will increase your compression, heat etc etc, as your effectively pushing the piston higher into the head reducing the CC capacity of the head. Just like planing your head for more compression. This jump may be getting too fine a tolerance for your engine. There is factory tolerance built in as when the piston at full revs goes over TDC you don't want it smacking into the head.

Not sure you are measuring it correctly as well. In my karting days part of scrutineering was they put soldering wire down the plug hole till it hit the squish edge. Then you had to give the engine a crank to squash the soft solder wire with the piston. They then measure this with a micrometer. If your squish was illegal (to small) you where tech-ed out! I've also heard fo blue tack being used as well but you have to assemble the engine and let the piston squash whatever you put in there to find out what the actual squish is then add a factory tollerance allowance.

I'm not actually sure of the correct way to measure squish but as I understand it an engine at full revs throws past TDC ( I think????) so squish is very important on a two smoker.

You could get your head machined to increase the squish but then you have to have the bowl machined to bring compression back to standard. This is done by finding TDC with a dial gauge filling the head up with fluid with a Beret and measuring the CC differences.

You could use a thicker base gasket but that will change the port timing a little as well. Make sure this non standard piston has the correct boost port cutouts and skirt lengths etc as well. This is probably the cheapest way though?

You might find even The Gas gas dealer doesn't know this kind of thing??

To be honest getting a bit outta my league here most of the stuff we did was trial and error. Compression Vs heat Vs power Vs timing advance Vs longevity. My mates kart only lasted one meeting per piston it was fast but quite unreliable?

Your new piston must be taller why couldn't you get the proper one its a late model bike???

[barty5]

I agree ring someone that knows.
But reducing your squish from .76mm to .4mm will increase your compression, heat etc etc, as your effectively pushing the piston higher into the head reducing the CC capacity of the head. Just like planing your head for more compression. This jump may be getting too fine a tolerance for your engine. There is factory tolerance built in as when the piston at full revs goes over TDC you don't want it smacking into the head.

Not sure you are measuring it correctly as well. In my karting days part of scrutineering was they put soldering wire down the plug hole till it hit the squish edge. Then you had to give the engine a crank to squash the soft solder wire with the piston. They then measure this with a micrometer. If your squish was illegal (to small) you where tech-ed out! I've also heard fo blue tack being used as well but you have to assemble the engine and let the piston squash whatever you put in there to find out what the actual squish is then add a factory tollerance allowance.

I'm not actually sure of the correct way to measure squish but as I understand it an engine at full revs throws past TDC ( I think????) so squish is very important on a two smoker.

You could get your head machined to increase the squish but then you have to have the bowl machined to bring compression back to standard. This is done by finding TDC with a dial gauge filling the head up with fluid with a Beret and measuring the CC differences.

You could use a thicker base gasket but that will change the port timing a little as well. Make sure this non standard piston has the correct boost port cutouts and skirt lengths etc as well. This is probably the cheapest way though?

You might find even The Gas gas dealer doesn't know this kind of thing??

To be honest getting a bit outta my league here most of the stuff we did was trial and error. Compression Vs heat Vs power Vs timing advance Vs longevity. My mates kart only lasted one meeting per piston it was fast but quite unreliable?

Your new piston must be taller why couldn't you get the proper one its a late model bike???

it is the proper one gas gas use wiseco its just that it is also used in the cr125gp???

[noobi]

it is the proper one gas gas use wiseco its just that it is also used in the cr125gp???

yea, the wiseco website says that this piston is for my bike. and the reason i we didnt get a genuine one in a gasgas box, was because the gasgas one would have been twice as much as the wiseco one, although they are physicaly different, the old oen has holes in the side, the new one doesnt.
ill do the solder squish test, which is apparently the proper way to do it. what solder did you use in the kart motors to test squish Reckless?

[Reckless]

it is the proper one gas gas use wiseco its just that it is also used in the cr125gp???

Yeh Barty I don't doubt that what your supplying is correct by the book. But a piston that is supposed to be a replacement part that is physically different and doesn't have the windows for the boost ports sounds a bit strange?


yea, the wiseco website says that this piston is for my bike. and the reason i we didn't get a genuine one in a gasgas box, was because the gas-gas one would have been twice as much as the wiseco one, although they are physically different, the old one has holes in the side, the new one doesn't.
ill do the solder squish test, which is apparently the proper way to do it. what solder did you use in the kart motors to test squish Reckless?

We just used standard soldering iron solder Noobi it just has to be bigger than the dimension you are measuring so it will compress. The Standard roll the have at home here is 2mm so shouldn't be hard to get.

From the info given something is not quite right here. Either your measuring or the part??
The Wiseco could be quite alright and well within tolerance, even increase performance but if it where mine I'd like to know why and how?
The only thing I can think of is that the Wiseco is higher/taller but Flat topped instead of with a crown? Tightening the squish but leaving the compression the same??? Have you measured the piston itself is it really taller above the pin? or are your squish measurements a little inaccurate?

Maybe take the old and new back to Barty to have a look at and measure up for you.
or call the dealer and see if they use the 125gp Wieseco as a standard replacement for your bike and it has a history of being ok.
or email Wiseco and ask why the differences and why no windows in it.

Look I'm not going to start second guessing this situation and I'm starting to make up theories over the Internet which is no help to anyone.
Just do a bit more investigation to satisfy yourself.

[camchain]

Noobi, Years ago I regularly used some stuff made for measuring gaps like this (setting up shaft bearings on Huntly Power Stn construction). Damned if I can remember what it was called but it was blue, softish, came in strips and looked a bit like plasticine but not sticky at all, so assured of very accurate measurement. Engineering supply shop should have it or something like it. Your depth mic should do the trick though. I'm with Reckless, need to get it right. If piston you have has different crown maybe wiseco has correct measurement? I know you can measure combustion chamber volume with fluid but this obviously a bit tricky and who can be arsed doing that?

RE TDC When I did mine I found piston wouldn't sit perfectly at TDC due to magnet effect on generator. Piston slop/tilt in bore might have some effect as well if not measuring at centre of piston. BTW that weird hole in your OEM piston has had me thinking, would be interested to know what that's about. Don't think it would be related to boost ports if a blind hole, boost port governed by cut away on skirt?

[IIIRII]

With respect, The question is ,what would be considered the minimum piston to head clearance on this 125.
Having biult lots of serious race car engines, Im pretty sure I know how to measure piston to head clearance./
You dont need to plastigauge it ,given thats its very easy to measure with a depth mike.
We have never really biult a 2 stroke bike engine before, but have a good understanding off clearances etc:
so again is .4mm too close ?
Having to allow for rod stretch and bearing clearances, I would think it is too close.....
Cheers

[JoshH]

Have a look at this article on the gasgasriders website.

http://www.gasgasrider.org/html/measuring_squish.html

[Reckless]

Have a look at this article on the gasgasriders website.

http://www.gasgasrider.org/html/measuring_squish.html

Nice find Josh H! Perfect, a picture is worth a thousand words Eh.

[noobi]

Have a look at this article on the gasgasriders website.

http://www.gasgasrider.org/html/measuring_squish.html

yea, iv been around that site for a while. so i knew about that already. the guy who wrote it, Ron Black, who i have been emailing, said that 0.4mm is too close and that it should be around 0.8mm. so some more base gaskets i think is the solution

[vazza]

Fail. Means you cant ride at woody tomorrow! stink buzz ;(

[cheese]

Re the holes in teh piston, I got a OEM one for my old RM and then I got an Athena one and it had the holes in it. I found no real difference in power between them. I assumed that it was to spread oil better into the crank etc. I found be well wrong there.

[barty5]

Re the holes in teh piston, I got a OEM one for my old RM and then I got an Athena one and it had the holes in it. I found no real difference in power between them. I assumed that it was to spread oil better into the crank etc. I found be well wrong there.

I could be wrong also thought i read some place it was just to cut weight??? as they dont lead to any place and anything that went in would have to come back out same place???

[cheese]

Hmm maybe. 2 holes would be 5/8ths difference though. Mine you every little bit I guess....

[F5 Dave]

Firing solder down the hole to the edge of bore turning it over & retrieving it is easier & then repeat for 4 corners (well yeah a circle but. . .).

The site is good info I didn't bother to read it all again, but typically once you get the squish down to ideal (0.7 would be min for a 125, bit more for bigger) the compression tend to get a bit high. On several bikes (my GG included) I had to take some metal out to get com back to where it would be fine for the gas I ran. I don't take compression gauges as being accurate for ratio measuring, only for gauging change once ratio determined.

[camchain]

Pic worth 1k words alright. Hard to properly measure that any other way.
(Thanks IIIRI, 'plastigauge' back into part of memory where it used to be. Solder looks good as well though!)

Noobi, keep in mind more gaskets=more gasket crush to take into account, Reckless got about 0.05 squash with 0.75 gasket?. Ask that guy about the piston dent/hole. Seems unlikely, but maybe to keep a hot part of the bore away from piston? I know katooms sometimes have an issue where nip-ups happen at the mini power-valve exhaust ports.

[Reckless]

I could be wrong also thought i read some place it was just to cut weight??? as they dont lead to any place and anything that went in would have to come back out same place???


Hmm maybe. 2 holes would be 5/8ths difference though. Mine you every little bit I guess....


The piston windows do heaps!
As I understand it the piston windows are always in the back of the piston lining up with the inlet port. They let more fuel in as the piston travels. I probably put you guys wrong with calling them the wrong term boost ports?? which are in the barrell. My Ktm has two really trick ones that revolve as the power valve activates.
Back to Piston windows: As you can imagine as the piston travels up letting in fuel, if there is a window in the rear, the fuel starts entering as the window passes the inlet and doesn't have to wait for the bottom of the skirt to pass the inlet. This enables a larger volumn of fuel to enter and be pushed up the transfer ports as the piston fires and goes down again. I would imagine this is far better than shortening the skirt and making the piston less stable in the bore. Some pistons have them some don't. My Ktm has a section of the skirt cut higher but similar priciple. But you all know how a 2smoker works so I'll leave it there.

[barty5]

The piston windows do heaps!
As I understand it the piston windows are always in the back of the piston lining up with the inlet port. They let more fuel in as the piston travels. I probably put you guys wrong with calling them the wrong term boost ports?? which are in the barrell. My Ktm has two really trick ones that revolve as the power valve activates.
Back to Piston windows: As you can imagine as the piston travels up letting in fuel, if there is a window in the rear, the fuel starts entering as the window passes the inlet and doesn't have to wait for the bottom of the skirt to pass the inlet. This enables a larger volumn of fuel to enter and be pushed up the transfer ports as the piston fires and goes down again. I would imagine this is far better than shortening the skirt and making the piston less stable in the bore. Some pistons have them some don't. My Ktm has a section of the skirt cut higher but similar priciple. But you all know how a 2smoker works so I'll leave it there.

good point makes sence guess wiseco figure its not needed ans shorter skirts are ok im sure he'll get it sorted.

[noobi]

Back to Piston windows: As you can imagine as the piston travels up letting in fuel, if there is a window in the rear, the fuel starts entering as the window passes the inlet and doesn't have to wait for the bottom of the skirt to pass the inlet. This enables a larger volumn of fuel to enter and be pushed up the transfer ports as the piston fires and goes down again. I would imagine this is far better than shortening the skirt and making the piston less stable in the bore. Some pistons have them some don't. My Ktm has a section of the skirt cut higher but similar priciple. But you all know how a 2smoker works so I'll leave it there.

how does a blind hole increase fuel volume?
isnt a piston widow in the skirt? like this http://www.sip-scootershop.com/wwimages/b26a215b-7395-4725-b31f-618c42bd504b.jpg
i couldnt see how the blind hole in my piston could increase this fuel flow, this blind hole is above where the gudgeon pin goes ?

[Reckless]

how does a blind hole increase fuel volume?
isnt a piston widow in the skirt? like this http://www.sip-scootershop.com/wwimages/b26a215b-7395-4725-b31f-618c42bd504b.jpg
i couldnt see how the blind hole in my piston could increase this fuel flow, this blind hole is above where the gudgeon pin goes ?


Yes Noobi you are correct I assumed they where talking about windows in the skirt I prob didn't read it properly!

[IIIRII]

Sorted
Used an additional base gasket
Back to .80mm
Cheers
Guys

[barty5]

good to here its sorted