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Help ! What is this dome & what is it for? (YZ 250 03)

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Hello guys,

 

I was checking my top end today and a friend gave me some after market head he had around and I would like some info on what this dome shape could be for and what knowledgeable guys out there on heads, squish, domes etc could tell me by seeing it.

 

you have it compared to the stock one in the pics.

 

please see attached pictures.

 

It's for a yz 250 woods bike.

 

Any comment will be apreciated guys. I do not know anything about those things :(

 

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Edited by lephil

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That looks like a parabolic head design and I think it allows you to use a 12 degree piston. (Correct me if in wrong someone) I got one of these inserts with a vhm head last week. The insert I received was a one of a kind design from the factory but haven't got to try it yet.

Edit: at another look, and seeing my older bud racing head, the size of the recess in the head changes between acceleration or top speed or bottom end or top end can't remember which.

Edited by KTMjake22

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Your buddy can't tell you any power characteristics on it? To me that looks like a low to mid head. I believe a larger squish band percentage equals lower rpm power characteristics which the aftermarket head displays and has a larger band than stock. Do a google search of 2 stroke cylinder head theory and you will find a lot of info, mostly of 2 stroke street bike but the tech is the same. Also on the aftermarket head the transition from squish band to dome is fairly sharp which may also indicate that this head is for low rpm use. To be honest I would try it either way. Head o-rings are cheap and swapping a head out doesn't take much time at all. While ordering your parts for your top end just order a second set of o-rings, run it, and tell us what you find.

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Definitely a low-end oriented head. That squish band is HUGE, probably 70% of head area or more. I don't care for the sharp angles and cylindrical chamber top, but it probably was needed to get the volume where it needs to be. 

 

I would not just bolt it on and run it. I'd cc it to see if it changes compression, and I'd check the squish clearance on your bike. Make sure this thing doesn't kick compression thru the roof, or have some much squish clearance that it's ineffective. 

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I was thinking about checking the squish on this one and on the stock head. cant seems to find the right solder material around here.

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i've taken some measurements:

stock head =  20,5 ml
aftermarket head = 19 / 19,5 ml

 

it's a good aproximate as i did it with a seringe and without the plexi top.  i made sure water was flat and tried several times.

 

He was reporting a higher engine noise. maybe due to the higher compression?

 

Whith that kind of compression race gas is mandatory I suppose?

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i've made some squish tests on both heads and now need some electronic tool to take the measurements...

 

but i can see that the squish is smaller on the aftermarket head.

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