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Discussion Starter #1
Hello my first post here.
I have a '83 CR250R 68mm cylinder head that appears to be offset from looking at it from inside the cylinder.
I heard the cylinder on some Honda's may be offset is this what is going on with mine?
Thanks.
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Discussion Starter #2
Reason I'm asking about the offset head is I want to modify the '83 CR250R cylinder's mild 182°/126°porting with the piston 1.5mm in the hole to 191°/130° with 29.3 blowdown.
Do I need to machine the head dome to center in order to properly cc the head?
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That does not look like the right head. The recess normally starts at the full bore diameter. The recess (squish) then follows the curvature of the piston for a % of the bore to give the required engine characteristics. Time to remachine the head.
 

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Discussion Starter #4
That does not look like the right head. The recess normally starts at the full bore diameter. The recess (squish) then follows the curvature of the piston for a % of the bore to give the required engine characteristics. Time to remachine the head.
Thanks for the reply.
It is the correct head for 1983. There was only 2 years Honda put a 7 stud head on a CR250R. 1983 and 1984.
My '83 is non power valve , the '84 has a PV which has a slightly different head
I just read up on Gordon P Blairs newest 2 stroke book and offset combustion chambers are common. Cuts down detonation and makes faster dome flame speed. Also my spark plug is centered but has a offset lean .
  • Worst Case scenario I'll get a billet head made up.
 

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The offset heads are quite different to what you have. The offset head was to keep the plug away from the exhaust to try and keep the piston cool. The offset was quite considerable. From memory offset cylinders went out in the 1960.

The fact you have flats inside the combustion chamber and not part of the squish is not correct.

Easier to remachine what you have.
 

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Discussion Starter #7
The offset heads are quite different to what you have. The offset head was to keep the plug away from the exhaust to try and keep the piston cool. The offset was quite considerable. From memory offset cylinders went out in the 1960.

The fact you have flats inside the combustion chamber and not part of the squish is not correct.

Easier to remachine what you have.
I tried the head on a spare cylinder and same thing. So probably a manufacturing error like what Carlos from BDT told me a few hours ago on another site.
Lots of 1983 heads on ebay so might get another for a spare and see if any different. As I get further into this build I will get it cc'd and modified.

I checked the piston pin and it has no offset , centered in the middle. My '82 YZ125 pin is offset 3mm towards the exhaust.
I heard Honda put a offset in the CR250 cylinder somewhere. Was it in the intake manifold?
 

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So probably a manufacturing error like what Carlos from BDT told me a few hours ago on another site.
Hmm, Honda is normally pretty good.

Don't worry about the volume too much, higher compression, more peak power, lower, wider power band. More important is to get the squish correct. Head and piston parallel with about 0.8 mm gap.
 

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Looking at your picture, the plug looks like it leans to the right. The plug offset in engines is away from the exhaust so this looks wrong. Could it be that it is leaned over to miss the frame?
 

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Hmm, Honda is normally pretty good.

Don't worry about the volume too much, higher compression, more peak power, lower, wider power band. More important is to get the squish correct. Head and piston parallel with about 0.8 mm gap.
Yes from what I hear Honda is very consistent in their quality.
I heard the plug leans to the side for easy access. Some of the newer CR250 have it too.
I will have to get the head worked for a 68mm cylinder I have. I was told to flatten the edge in the squish band surrounding the dish. The squish can be adjusted with custom gaskets.
At TDC the piston crown edge is 1.5mm from the top of the cylinder deck. Should I mill the deck down to 0 whether I raise the ports or not?
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I always skim off the step in the head, match the shape of the the squish band to the bore and then set the squish with the correct thickness head gasket.

Put the head in a 4 jaw chuck, center it to the scribed line you make with head and barrel assembled, square it off, set the compound slide to the angle of the piston crown and machine the squish. If necessary, skim the whole head to get the squish to head flats at the diameter of the cylinder bore. You will invariably be increasing the CR, so I normally deepen the combustion chamber to compensate. Leave the plug depth the same and change the dome into a u shape. Leave the edges of the dome, squish interface as per original diameter.
 

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Discussion Starter #13
marsheng,
Here's an interesting point in Honda CR250R development
Here's my two 1983 cylinders. One has a open deck cylinder, the other has a semi closed deck....probably a upgrade later that year.
So I would think the bridged semi deck cylinder is what I should use when I machine the piston crown to squish and dome in the head.
Then there would be no chance of the piston striking the head.
I'll have to figure a way to bridge the open deck cylinder
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Yes the open deck cylinders work fine from what I heard too.
No chance of distortion in the cylinder bore when the head is torqued down .
With some research I found competitive engine builders would fill the lower half of the water jacket surrounding the cylinder with Devcon.
 
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