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NSR250 MC18 Mk1 and Mk2 Cylinder Barrel Comparison


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philup_7@hotmail.com

 
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NSR250 MC18 Mk1 and Mk2 Cylinder Barrel Comparison

Fri Sep 23, 2022 5:19 pm » Post: #1 » Download Post

Hi all

I have been given a spare MC18 Cylinder Barrel (which doesn’t happen very often). The main issue with this is that I don’t know if its from a Mk1 or Mk2 MC18 and the person who gave it me as no idea either!

If it’s a Mk1 then I can keep it as a spare for my MC18 R2J and if a Mk2 then my son can have it for his MC18 R5K.

I know the spares list show Different Part Numbers for Mk1/2s Cylinder Sets, for instance CYLINDER Set A for the:
- R2/4Js uses Part 06121-KV3-701;
- R5/6Ks uses Part06121-KV3-770;
and differing Part Nos are used for the other Cylinder Sets B, C.

My questions are:
- Is there anyways of finding out which Mark of MC18 this Barrel is from or am I just worrying too much LOL,?
- Are the differences between the Mk1 & Mk2 RC Valves as such, that they visibly don’t match or fit?

Note: I did put some spare RC Valves from a MC18 R5K in and they seem to fit ok, for example:
• rotation looks the same;
• Valves sit in their recess ok;
• R5K Right Hand RC Valve hits the same mark made by the original on the Exhaust Chamber;
• Comparing measurements between the original RC Valves and the R5K RC Valves appear to be approx. the same.


The only real difference I could spot, was that the left RC Valve had a slightly bigger chamfer on it as shown in attached photo.

This is very much not an exact science I have employed as I don’t have the necessary tools and I know seeming to be ok and actually being ok can be vastly two different worlds, but any help would be gratefully received as aways!

Many thanks in anticipation.

Philip
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RobH

 
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Fri Sep 23, 2022 8:04 pm » Post: #2 » Download Post

From my experience there's no difference
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philup_7@hotmail.com

 
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Sat Sep 24, 2022 4:02 pm » Post: #3 » Download Post

Hi Rob
That's interesting, many thanks for that much appreciated.
Philip
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Andy
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Wed Sep 28, 2022 5:37 pm » Post: #4 » Download Post

As Rob says, R2/4J & R5/6K barrel differences are negligable in the real world. You should always measure how tall they are, and the bore sizes (or letter A/B/C if OEM plating) though. Even with an MC18, if there's any variance in deck heights, I'd always fit the taller barrel to the front cylinder.
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philup_7@hotmail.com

 
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Wed Oct 12, 2022 3:02 pm » Post: #5 » Download Post

Hi Andy
Thanks very much for the info very much appreciated.
Never thought of measuring the Cylinder heights, must make a note of that in the manual (it’s no good me relying on memory these days I know it’s an age thing) as always concentrated just on the cylinder bore sizes.
Is there any particular reason to fitting the taller barrel to the front?
Apologies for the time it’s taken me to get back to you.
Take care
Phil
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Rothmans_SPud
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Wed Oct 12, 2022 5:58 pm » Post: #6 » Download Post

philup_7@hotmail.com wrote:Hi Andy
Is there any particular reason to fitting the taller barrel to the front?
Phil



Omg I’ve glad you asked the question,

Just out of idle curiosity I’m keen to know the answer as well
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Andy
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Fri Oct 14, 2022 12:04 pm » Post: #7 » Download Post

The NSR is very edgy, even in stock form, which is why they're so good out of the box, and why they don't respond well to things like compression and ignition timing changes. The later models more so, but I would still always fit a taller barrel on the front cylinder.

The issue is the spark plug location is less efficient on the lower cylinder, being more offset so as to increase clearance for the front wheel. This means the flame path is less efficient, and the cylinder head & piston more susceptible to detonation. A slight increase in compression, should you fit a shorter barrel (and note, replated barrels have often been "decked", making them undesireably and measureably shorter) can have a very short life if unchecked!

We had a (very quick) forum member here several years ago, who was racing an MC21, and destroying 4-5 barrels a weekend for 2 season, if I recall correctly. It took best part of a year to convince him to go back to standard compression, and as soon as he did, he won the F250 championship with 2 weekends (4 races) in hand!
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Rothmans_SPud
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Sat Oct 15, 2022 8:34 am » Post: #8 » Download Post

Thanks Andy - Makes a lot of sense

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philup_7@hotmail.com

 
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Mon Oct 17, 2022 9:38 am » Post: #9 » Download Post

Hi Andy
Many thanks for the info.

This is a whole new world to me (the world of 2T) and I am finding it very interesting. The old phrase comes to mind
"The more I learn the more I realise what I don't know".

As always many thanks for the explanation much appreciated.

Phil
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Andy
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Tue Oct 18, 2022 6:37 am » Post: #10 » Download Post

There are many out there who know a lot more about 2-strokes than myself, but as far as NSRs go, both GP and production models, I've been around them for 30 years now, and chosen to only interract at a technical level with others specifically knowledgable in the marque too. I.e. kept a disrance from the bullsh*tters!

There's always more to learn though, I'm sure! The most important thing is it's shared on the forum here, for all to benefit. Unfortunately a fractuous element of the community sought to dilute the knowledge pool in recent years, which has been detrimental to our community, but we're working hard to remain the best place for reliable, first-hand experience and information. Nothing you will read here is second-hand... everything advised NSR-WORLD has been proven/disproven in the real world by us.

I can say, with all confidence, this is the most reliable place for NSR information you will find outside of Japan. 😉
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