99 head on 94 cr250

[Hainesy4]

Let me start this out by stating that I am no engineer. I am a business analyst who likes to tinker with stuff. In a throttle sticking incident I broke the motor mount off of the original 94 head. A new one cost about $125.00. Background fact: the head is the same from 92 - 96 and from 99 - 01. The 97 and the 98 were different internal designs.

I have a 99 head (in new condition) that I am thinking about trying on my 94 cr250. It will bolt on and mount up but will it work? The 99 head is slightly taller so I have bought the taller studs. The motor mount is exactly the same and all the cooling ports match up exactly. One difference is that on the 99 head the spark plug goes in at a 90 degree vertical and on the 94 head the plug has slight angle to it. I doubt this is of much concern.

My concern comes from the inside shape comparison. The squish band and the cone shapes are different. The 94 has a deep cone shape that is about an inch and a half in diameter and the 99 has a more shallow cone shape that is about a 2 inch diameter. Sorry no thousands or good engineering knowledge here just rough analysis work using a ruler... The other point to note is that on the 94 from the squish band to where the cone shape starts it tapers down where as on the 99 there is no taper... it is level from the squish to the cone.

Has anybody tried this? Is there piston clearance or should I just scrap the idea and bolt on a new 94 head?

 

[76GMC1500]

I would worry most about the combustion chamber volume. You've got to keep your compression ratio reasonable.

 

[CRazy250]

that would increase compression and performance but make sure its not gonna hit the head.

 

[76GMC1500]

It isn't necessarily going to increase compression. Most Honda 250's ran in the 8.5:1 range. The 87 was the highest at 9.1:1. The right compression ratio depends on your ports.

 

[Hainesy4]

Thanks for the replys. From further inspection I don't think it has the clearance. I put some 2 stroke oil on the top of the piston while it was at the top of its stroke and laid the head on withought the gasket. When I took the head off some of the oil was on the head.

It was worth thinking about though.

 

[76GMC1500]

If you did it without the gasket, that is plenty of clearance. The less clearance you have, the less prone to detonation the motor is. I don't know exactly what the clearance is for a CR250, but in a Chevy 350, .035 to .040" is ideal. The bigger pistons require more clearance because the rods stretch more and they rock in the bore a little. If there was still some clearance without a gasket, you should have no problems with a gasket. For a more accurate measurement, put some clay on the crown of the piston.

 

[Hainesy4]

Thats a good idea. I will try the clay. So assuming that it would work. What would this do to the power of the bike? And would it cause more pressure on the bottom end causing eventual problems like a broken rod, crank, or bearing issues?

 

[DirtDawg]

To use the '99 head you might have to use a '99 piston. Top is shaped different on the newer pistons. '92-'96 has a dome shape then in '97 honda went to a flat top, not sure about the '99+. I was reading something about this awhile ago.
hth's

 

[Husky Texan]

97-01 heads require the use of the 97-01 piston. You are correct about the flat top piston. That's why the piston had a different part #.

 

[DirtDawg]

Husky Texan, thanks for confirming my thoughts. I have a '95 cr250 and thought about making the switch (head and piston), but could not find any info as to a performance gain, and I like the power of my '95 so much I'd hate to change it.

 

[76GMC1500]

The power is in the porting, not the piston.

 

[DirtDawg]

For the most part I agree but there is alot that can be done with the combustion chamber (cyl. head), shape, squish angle, etc. , The style of piston needs to match.
Modifying the head can have a pretty big effect on weather the engine needs higher octane (race fuel) or can get by on pump fuel for a given compression ratio.