Please read this and let me know asap. Head gasket Problem

[MagicJames]

I just had my 85 250r cylinder bored to .160 over, and the head gasket is smaller in diameter than the cylinder .... is this normal??
I dont have my book, and i dont want to break anything, please let me know asap!!

[MagicJames]

Oops i meant this ... 3 mm over these are the stats

Displacement: 258cc
Bore - Standard: 66.00mm
Bore - Actual: 69.00mm
Bore - Oversize: 3.00mm
Stroke: 72.00mm
Note: Pro-Lite
Note: Liquid cooled

[SWIGIN]

it'll probably have a tap or a ping due to a hot spot or contact the piston.....not a good thing at all

[Mosh]

I dont know if you are going for performance, or just a econmic rebuild.
But bare with me here. If your run a CR 250 Head gasket or the steel shimmed ATC headgasket, you definately want to test the set-up with this procedure.
They are thinner and could cuase detonation problems.
Either way, you probably want to call Cometic and get a headggasket for the larger bore. They make one that starts at 69.5mm.
So to answer your intial question, yes that samller headgasket may cause sealing issues.


Once you cylinder gets bored out that far, the squish changes in the head. The head will have a tighter squish band that will over hang in the cylinder.
So if you run the thinner head gasket, it will actually effect the squish clearence ,and it may get too tight, which will cause detonation.

The PROPER way to deal with this is have your head cut, to the same diameter to fit the larger bore cylinder.
As far as the gasket, Call Wiesco and Cometic and see if they offer a head gasket to fit that larger bore cylinder.

To check your squish, you have to re-use your old head gasket, bolt the jug on the engine, then tape a .100" thick piece of solder to your piston across the top center of the piston edge to edge.
Bolt your head on to spec with the old head gasket and leave the plug out.
Kick the engine over hard. It will squish the solder and measure your clearence.

Unbolt the head and remove the solder, then use a Micrometer to check the thickness of the solder you used, paying more attention to the outer edges.
Anything less than .045" will be a problem.

Here is a pic to help you with the test.
http://i100.photobucket.com/albums/m40/cosmicquads/Big%20bore%20Z/testcylinder-1.jpg

[MagicJames]

i just had the head decked and re-squished by Dave Moore. I think i might run the CR style head gasket. What year is preferred?

[Dirtcrasher]

3MM's on the stock sleeve?? I guess thats one thick stock sleeve............

[Mosh]

89 Cr head gasket is the one you want.
I used the Honda ones.
Never tried a aftermarket CR gasket, so I dont know if they are thin like the Honda ones.
And I do not know if the Honda gasket comes in a larger diameter like the 69mm you need.

[Billy Golightly]

Yeah I don't think they do. Commetic makes a 69.5mm head gasket thats about .040 thick, thats what I run.