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Author Topic: Is the combustion fire ring on a 78 cb750 supersport not a perfect circle?  (Read 1555 times)

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Offline bmikkalson

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Is the combustion fire ring on a 78 cb750 supersport not a perfect circle?
« on: February 29, 2012, 12:04:39 AM »
Took apart my 78 CB750,  And was wondering why the head gasket is miss shaped on the combustion o-ring.
Is it made this way, or should I be looking out for something?
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Offline trueblue

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Re: Is the combustion fire ring on a 78 cb750 supersport not a perfect circle?
« Reply #1 on: February 29, 2012, 01:22:32 AM »
It is quite common for the fire rings to be out of round slightly, I wouldn't worry too much.
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1979 CB650Z
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Offline bmikkalson

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Re: Is the combustion fire ring on a 78 cb750 supersport not a perfect circle?
« Reply #2 on: February 29, 2012, 10:37:32 AM »
Ok, I guess the gasket starts out as a perfect circle..
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Offline mrrch

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Re: Is the combustion fire ring on a 78 cb750 supersport not a perfect circle?
« Reply #3 on: February 29, 2012, 05:49:09 PM »
The head gasket is mis-shaped on the later models, not a perfect circle.
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Offline trueblue

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Re: Is the combustion fire ring on a 78 cb750 supersport not a perfect circle?
« Reply #4 on: March 01, 2012, 02:32:17 AM »
I should have asked this before, how far out of round is it, it is quite common for a head gasket to be a few mm out of round from new.
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1979 CB650Z
Nothing can be idiot proofed, the world keeps producing better idiots.
Electronic Guages for your SOHC 4

Offline Rgconner

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Re: Is the combustion fire ring on a 78 cb750 supersport not a perfect circle?
« Reply #5 on: March 01, 2012, 06:54:13 AM »
This thread is useless with pics.
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Offline mrrch

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Re: Is the combustion fire ring on a 78 cb750 supersport not a perfect circle?
« Reply #6 on: March 01, 2012, 02:16:41 PM »
Here is what a new OEM gasket looks like for your bike.
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my build

1977 CB750K WITH 1976 CB750F ENGINE

Offline lucky

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Re: Is the combustion fire ring on a 78 cb750 supersport not a perfect circle?
« Reply #7 on: March 01, 2012, 05:53:40 PM »
Quote from: bmikkalson on February 29, 2012, 12:04:39 AM
Took apart my 78 CB750,  And was wondering why the head gasket is miss shaped on the combustion o-ring.
Is it made this way, or should I be looking out for something?

It is not called a "fire ring" it is just a head gasket and the shape of it is so that when one of the valves that is very close to the edge of the cylinder wall. So when it is open and then you tighten down the cylinder head  the gasket crushes and creeps, then it will not hit the valve. Mostly 1977 and 1978 with the larger valves like the F model also.
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Offline trueblue

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Re: Is the combustion fire ring on a 78 cb750 supersport not a perfect circle?
« Reply #8 on: March 02, 2012, 04:06:44 AM »
Quote from: lucky on March 01, 2012, 05:53:40 PM
Quote from: bmikkalson on February 29, 2012, 12:04:39 AM
Took apart my 78 CB750,  And was wondering why the head gasket is miss shaped on the combustion o-ring.
Is it made this way, or should I be looking out for something?

It is not called a "fire ring" it is just a head gasket and the shape of it is so that when one of the valves that is very close to the edge of the cylinder wall. So when it is open and then you tighten down the cylinder head  the gasket crushes and creeps, then it will not hit the valve. Mostly 1977 and 1978 with the larger valves like the F model also.
FYI the ring around the combustion chamber in the head gasket is called a fire ring. 
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1979 CB650Z
Nothing can be idiot proofed, the world keeps producing better idiots.
Electronic Guages for your SOHC 4

Offline bmikkalson

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Re: Is the combustion fire ring on a 78 cb750 supersport not a perfect circle?
« Reply #9 on: March 02, 2012, 10:09:37 AM »
Thanks for the pic MRRCH, yes that what mine looks like. This is the 1st supersport I have had apart.
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