Hi everyone,

I am a newbie to this forum, but I have been following it with interest for a couple of weeks now. I have an R6K that I bought a couple of years ago as spares and have now decided to try and get it going. I have pulled the whole bike apart and found numerous things wrong, especially on the engine. I started out with the whole R5K/R6K crank bearing and big end bearing issue, but got lucky and purchased a basically pristine MC21 crank for next to nothing. I have to replace the engine casings in anyways, so I will be getting MC21 casings, 2T pump and GPS (will modify wiring on GPS for neutral light). Now, I have taken a good look at my barrels and RC valves and found out that they differ. I have pistons with no markings on, so I am supposed to have 'B' barrels. My one barrel is OK, marked 'B' and RC-Valve seems to be in good order, the other is marked 'C' and is very different, I suspect this is a MC16 barrel as it has a smaller exhaust port and RC-Valves. Can someone please confirm? See photo below, left one is the 'B' barrel with the larger port and RC-Valve (MC18?), right one is the one with the smaller exhaust port and RC-Valve, the one I suspect of being a MC16 barrel.

Thanks guys,

Jacques.

Hi Jacques, welcome to the forum.

The NSR250 has 3 barrel sizes with 3 matching piston sizes:
A barrel <--> B piston
B barrel <--> Blank piston (no mark)
C barrel <--> D piston

So it is correct that your 'B' barrel has a piston with no mark on it.

The left picture sure looks like an MC18 barrel to me. Although I cannot confirm 2/4j or 5/6k
I have no real idea what the right picture is of, but suspect that it is either MC16 or MC11. I'm sure someone else will identify it for you.

For what it is worth, the Honda manual states that the piston to bore clearance should be 0.035mm to 0.044mm with a service limit of 0.08mm, and that the piston OD is to be measured 15mm up from the bottom of the skirt on standard Honda pistons when working this out.

Also, this may be of some interest to you,..
I have blogged my restoration of an NSR250SP MC18 R6k.
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The NSR250 has 3 barrel sizes with 3 matching piston sizes:
A barrel <--> B piston
B barrel <--> Blank piston (no mark)
C barrel <--> D piston

@ Tuckerbug - Both my pistons are blank, yet both of them fit into either barrel (B or C). My pistons are basically new, if they have done 100km/60miles it would be a lot.

Also, this may be of some interest to you,..
I have blogged my restoration of an NSR250SP MC18 R6k.

@ Tuckerbug - I have actually read your whole blog, kind of the inspiration behind me pulling my one out of storage. Looking at it now, I wish I left it where it was...




Some new revelations on my two different barrels. Had an even closer inspection, and look at what I found on my supposedly MC18 'B' barrel... A damn hole... the previous idiot who 'raced' it obviously never adjusted the RC-valve correctly, and this is the result. See photo below.

My new question... How will my bike run if I get another MC16 barrel like the one on the left (photo above), and bolt that on. Also keeping in mind that my bottom end is straight out of a MC21. I will be using my standard R5K/R6K carbs, R5K/R6K exhausts and PGM2 CDi. I can get my hands on both MC16 and MC18 barrels, what the condition is, I dont know.

Jacques.

Never seen anything quite like that! Incredible! However, many older MC18 barrels have wear there, but I've never seen any actually worn through!

The barrel with the small RC Valve/exhaust ports is indeed MC16. (MC11 NS250R barrels are totally different to NSR barrels, and are finished in black. They also don't have RC Valves.)

Fitting 2x MC16 barrels to an MC18 will work, but likely see you with around 50hp. A pair of MC18 barrels is a much wiser choice, and it's worth doing the job properly.
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Never seen anything quite like that! Incredible! However, many older MC18 barrels have wear there, but I've never seen any actually worn through!

The most amazing thing is... It actually started. I started it up just before I pulled it apart, and it ran, with a fubar crank with no centre crank seal and this hole in the exhaust port. Even took it for a ride around the neighbourhood.

Fitting 2x MC16 barrels to an MC18 will work, but likely see you with around 50hp. A pair of MC18 barrels is a much wiser choice, and it's worth doing the job properly.

Couldn't agree more, I just have to lay my hands on a decent set of 'B' MC18 barrels somehow, or even a set of MC21 barrels and heads. Are the MC21 barrels also marked with 'A', 'B' and 'C'?

Thanks,

Jacques.