Just starting the re-build on Noodle and have just repalced the barrels with new standard Honda base gaskets. Just to check I put the front head on with the used (compressed) head gasket and took a reading at 3 points around the bore with solder and a micrometer. The average measurement works out to 1.18mm.

Now shoot me down gently if I'm wrong but this does seem a bit 'wide' to me. I've not yet checked the rear cylinder squish but wondered what you 'NSR experts' thought, and if it is too 'wide' can I do anything else apart from skim the heads? E.G... Thinner base gaskets...or none at all and just use gasket cement!
I have heard somebody talk about different base gaskets....where can I get them?

Thanks in advance.

Mike
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I don't mind dying...I just don't want to be there when it happens!

You should be able to get 0.6mm, 0.5mm and 0.4mm gaskets though the NSR-World market place or ask Andy.

I'm sure you will get a good response as to what people think a good squish is. I'd say bigger is better than smaller on an NSR motor. Tighten it up too much and you get a peaky (fast feeling) motor but that is otherwise worse off.

It's more important that what ever the squish is set to that they are the same on both cylinders. I like a relativity large squish.
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If I have to take the carbs off once more...

I set the squish at 0.8mm
Combustion volume was set at 12.8cc to the top of the spark plug hole.
Not too sure what a "relatively large squish" is, but would guess at 1.0mm?

I have not yet sorted out my jetting and got a dyno graph, so my words here mean nothing I guess.
But, I will get there.

Also:
http://www.nsr250.net/forums/viewtopic.php?t=11421

The fuel that you use will influence the squish you use .

Thanks all.
I'm definitely going the '.7 or .8mm' route. This is certainly not too tight on my engine with new crank/rods pistons etc. It was just that the squish that I have is so large.
I've ordered some thinner base gaskets from Marketplace and will do some experimenting when they arrive.
If I still can't get below 1mm then I think that I'll have to have the heads skimmed...there is a small lip on both heads to allow for this without having to go into the combustion areas.


Mike
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Mate, I got from 1mm+ down to .76/.77mm on the heads using just wet n dry in literally minutes. Mind you, I don't yet know if that was a good thing or not...still seems overkill for an engineer to do it. Probably takes as long to tool up as it does to sand it down.

Certainly less time than lapping valves on a bore stroke.

Spot on!
I'll give that a go. Got a large piece of plate glass.

Mike
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When we were running pump fuel , we used 1.1mm squish . The heads have been reprofilled and the squish band retuned and the bike pulled strongly .
When we ran race fuel we used 0.8mm with different profiles again .
Race fuel you bump up the compression to gain midrange , pump fuel you run less compression etc to rev higher to make your hp .
If you help me computerwise i can send you some pics of my old pump fuel 250 heads .

It all depends what you want from the bike and what you intend to do with it , there is free hp waiting for you when you tune the ports and heads . Best hp gain imo

For a 54 X 54.5 config 0.65mm is the mechanical limit , even then there will be a 'shadow' on the piston where it has almost touched the head on over run.
On our 98 PULP running 15:1 is no problem provided you sort the squish. There is more to it than the just the gap the width play a big part as well. Lately torriod heads are being touted as the new black as a 'dual angle' squish bands

There are some awful numbers being banded around here, so I will post these two excerpts from the NSR-WORLD English TT-F3 Setup Manual I am working on:

Pay particular attention to the head volume and the compression ratio.





The TT-F3 Setup Manual assumes fuel used will be AVGAS, pre-mixed at 30:1.

Some people need to go away, put the kettle on, and think about that for a moment, I reckon!
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Andy.
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Another old, but useful HRC diagram, posted here before:



*English translation by Honda/HRC.
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Andy.
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Andy , which information is worse - misinformed head volumes - squish size etc or no ballpark figures on a jet size to use ? Either way the motor will be damaged if incorrectly set .
Just highlight the "incorrect " info so that people can learn from a mistake . Surely thats the reason why people ask questions , to learn and make an informed decision .

I can't see the problem! There are HRC "ball park base settings", as you call them, in the 250 Tuning section, and the head volume info above is straight out of the HRC manual.

Are you telling me HRC didn't know what they were talking about?

Assemble a "stock" F-III motor "by the book", and it WILL make 71hp/72PS!

Assemble a pretty much stock motor, as per the service manual (i.e. STOCK) and it will make 60hp.*

*With airbox mod and wiresplice

We've said time and time again not to mess with the compression ratio. You will find numerous threads where we have "highlighted" bad information regarding high compression. NSR250's [a] don't need it, and [b] don't like it! To "highlight the incorrect info" in this thread would require highlighting pretty much all of it!!!!
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[edit] About a minute after posting this I realised that I had my brain on backwards.
But I will leave the post here so people can see what my (early Sunday morning) thinking was anyway.
[/edit]

In Andy's first post above it states that the compression ratio is 7.9 : 1
And item (4) in the same post states that the combustion volume should be 11.6cc +/- 0.1cc
ASSUMPTION: this volume is to the top of the spark plug hole? (TSPH)

Surely, if the combustion volume is 11.6cc (TSPH) on a 125cc cylinder, then
If we take the spark plug head filling volume to be 2.0cc, which is a totally rough calculation made by myself, then the REAL combustion volume will be pretty close to 9.6cc. I think I can remember being told once that spark plug volume for NSR250 is 2.4cc but it doesn't really matter too much for the purpose of this post.

So,.. Compression ratio = (124.8cc + 9.6cc) / 9.6cc = 14.0 : 1 (NOT 7.9 : 1 as stated by HRC ??)

Maybe I am missing something, please let me know if I am.
But from where I am standing, it seems as though HRC do NOT know what they are talking about. (tongue in cheek, of course)

For an MC18 in standard trim, I have measured the squish to be 1.1mm and the combustion volume to be 13.3cc (TSPH).
This gives a compression ratio of 12.2 : 1

I have modified my MC18 to have 0.8mm of squish and 12.8cc of combustion volume (TSPH)
This gives a compression ratio of 12.65 : 1
I am hoping that this will be OK for pump gas. I intend to use 95 octane or higher ULP.

I have no real data for the MC21, which is what is being discussed in this thread of course. However, it is going to be very similar I ASSUME.


Last edited by TuckerBag on Sat Feb 25, 2012 11:04 pm; edited 1 time in total

TuckerBag wrote:

So,.. Compression ratio = (124.8cc + 9.6cc) / 9.6cc = 14.0 : 1 (NOT 7.9 : 1 as stated by HRC ?? Maybe this is just a typo?)

Tucker,
7.9:1 compression ratio is according to how Japanese started measuring secondary compression a couple decades back,
also know as CCR ("corrected" compression ratio).

They do not use the displacement of the cylinder e.g 125cc for the nsr,
but they instead use the displacement of the cylinder just after the piston has blocked the exhaust port.

This might be a bit misleading for some people and on some occasions
and of course CCR is relevant to any alterations to the exhaust port e.g porting.

As a very rough estimate, you may find on some applications that CCR is almost half than the "traditionally" measured CR.


P.s I believe some people have now "discovered" a way on how to reveal more HRC info?
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