I checked to see what heads I was running and found they are mismatched.
the rear is a KV3L head and the front is a KV3H.
the compression on the KV3H is 110psi while the other is 120psi.
I know the KV3L is the sp head with is higher compression. So How do I tell if my cylinders are R or SE/SP? the Front cylinder has the number 2YR2L276 stamped on it. the rear I could not make out the number.

On the base of the cylinder ther is a circular stamp of either L1 or H1 I believe. L being SE/SP and H being R spec.
Have a look here:
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Jerry Lai
NSR250 [MC21SP]

Thanks Jezza, Just looked again and it looks like I have the L cylinder with L head for the rear, and the front has a L cylinder with a H head. That would explain the lower compression for the front cylinder.

That may be a good thing. The HRC recommended setup lists a 0.6mm base gasket for the front cylinder to prevent detonation. I cant remember off the top of my head, but I believe the difference between the H and L head is very close to 0.1mm. Someone may have put the H head on there to get the same effect.
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Paul Herr
'88 FZR4/GSXR/YZF Frankenbike
MY BIKE PICS

do you mean the thicker base gasket is used when doing the front head conversion? In stock form I wouldn't see a reason the front would detonate. I don't think my front head is a straight plug, I'll have to check when I get home.

The H head on the L cylinder will give lower comp as you have found.

As Paul says, this is a good thing and won't really cause any grief, however, an L head with a 0.6mm base gasket would possibly be a better choice as the comp would be a little higher so more mojo without runing into detonation troubles.

So are there a wide range of thicknesses for base and head gaskets for the NSR? are R and SE and SP gaskets all the same thickness?

See here: http://www.nsr250.net/marketplace/product_info.php?cPath=24&products_id=42
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Jerry Lai
NSR250 [MC21SP]

how thick is the OEM base gasket? how thick are head gaskets? From looking at the motors I have they look pretty thin because I can't even see one from the outside.

Sorry wrong link... I'm must be on drugs today or something
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Jerry Lai
NSR250 [MC21SP]