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NSR250 and NX5 FAQ

Question: What do I need to know about Cylinder Markings and Piston for a Top End Rebuild


Answer: There are different kits depending on the piston supplied and the pistons are sized to match the cylinder. The cylinders have a letter stamped on them.

A cylinder = B piston  B cylinder = no mark piston, C cylinder = D piston. The largest cylinder bore is A and the smallest is C. However, note that this is only for when cylinders are new or in good condition, so if the cylinder is worn, the corresponding piston will be a looser so a bigger piston might be advised. If the cross hatch honing of the cylinder is very feint then we suggest using the next size up piston. The differences in sizes are tiny (11 microns between largest and smallest) and for the most part not an issue so it is not a problem if a smaller piston is used in a larger cylinder or the other way round.


Cylinder A = 54.008mm – 54.011mm        use         Piston B = 53.966mm – 53.969mm

Cylinder B = 54.004mm – 54.008mm        use         Piston # = 53.962mm – 53.965mm

Cylinder C = 54.000mm – 54.004mm        use         Piston D = 53.958mm – 53.961mm


Piston to bore clearance = 0.039mm - 0.045mm

Top end Rebuild kits for MC18 here for MC21 here and for MC28 here

Question: Which model NSR is fitted with a close ratio gearbox?


Answer: There is no such thing on the production NSR! The transmissions are the same between R, SE & SP. However, each model (MC16/18/21/28) has different gear ratios.

Question: Can my R spec motor be fitted with a dry clutch?


Answer: Yes it can. It's quite straightforward but requires a lot of parts. The best plan would be to purchase an SE/SP engine that had seized and swap the complete transmission and clutch assembly, including the case covers.

Question: What are the differences between the R, SE & SP engines?


Answer:  The SE & SP engines are identical and use a dry clutch. The R engine uses a wet clutch.

The R engine also has different cylinders and heads to the SE/SP.

The R cylinders are cast with the letter "H" on the lower left edge, the SE/SP with the letter "L".

The R heads are cast with the "KV3H" mark, the SE/SP with the "KV3L" mark.

Question: Can the transmission be swapped between the MC21/28?


Answer: Yes. The MC28 has longer ratio 4th, 5th & 6th gears, but they are totally compatible between the two models provided they are both either wet clutch, or both dry clutch versions. The primary ratios are different though, so the MC21 Primary drive gear cannot be used with the MC28 primary driven gear, and vice versa.

Question: Are MC21 and MC28 engines interchangeable?


Answer: To a point yes. The major limiting differences between the two models are in the electrics. The flywheel, stator, pickups and gear position sensor are all matched to their respective CDI units. There are other differences such as gear ratios, but this will only affect the gearing, not the general working of the engine.

Also, early (1990) MC21 engines had a single 2 stroke oil feed to the R/H crank main bearing, which was then changed to a twin feed. This twin feed was then continued into the MC28.

Question: Can I fit an MC28 crank into my MC18?


Answer: No. The cranks are dimensionally different so will not fit.

Question: Can you supply Special clutch plates and harder springs?


Answer: Yes For those dry clutch motors which have had some tuning work, sometimes the stock components are not up to the task and clutch slip is a potential problem. To avoid this, the TYGA-Performance dry clutch kit will prevent such an occurrence. Extensively tested on our TYGA 300cc big bore engines it has proven its worth over and over again. This kit includes all parts needed to upgrade your clutch and get the power to the ground  (NHPS-0003)

Question: Do engine modifications make the engine unreliable?


Answer:  Depends on what you do really, and how you define unreliable. By 'blue-printing' the engine you in fact make the engine more reliable, smoother running and even get a little power into the bargain. This entails matching up of components, cylinder porting, head volumes, truing up the crankshaft etc. Giving the engine a better balance and a higher efficiency.

Tuning for power means making each component work harder than intended as stock. This is turn reduces the life span of each component, but doesn't necessary make the engine unreliable.

Question: Will fitting bigger carbs give me more power?


Answer: Not necessarily. The stock engine is well matched in all areas. Fitting bigger carbs gives the engine more 'potential' power at a different (higher) rpm, but other areas must be matched to these. Bigger than stock carbs suffer from lower velocity airflow at low rpm, so throttle response is affected.

Question Will removing my air filter give me more power?


Answer  It's not really the way to go, as it's the airbox lid that is restricting the flow, not the filter, which has a large surface area and flows very well. I also can't recommend it if the bike is used regularly on the street. The best plan is to fit a TYGA modified airbox lid, and keep the filter in place. BPCX-9010 and BPFX-9010

This modification offers greater top end power without any substantial midrange loss. It does however require carburettor jetting changes, otherwise the engine will run lean and could cause engine damage.

Question: What's the difference between SE & SP forks on the MC28?


Answer: The MC28SE forks (also MC21SP) are adjustable for preload and low speed rebound damping only, whereas the MC28SP forks' rebound adjuster knob also controls the low speed compression damping to a small amount.

The MC28 SP forks can be easily spotted by the bright blue anodized rebound adjuster knobs in the top of the fork cap.

Question: My crankshaft main bearings are noisy. Can I order new bearings?


Answer: Yes you will find them HERE

Question: What pistons are you using in your  Honda RS250s?

I’m still getting by on HRC OEM pistons but that can’t last forever, I’m a little reluctant to use the aftermarket pistons with the  piston ring location pins running over the rear boost port.


Anwser: We use VHM pistons in all our RS250's. NF5 and NX5. VHM have 14mm and 15mm piston pin types available.

Yes, they run the ring end gap in the rear boost port, but haven't had any problems with them at all. They're relatively inexpensive and easily available.

Back in '97 when I worked for Harc-Pro, we ran A kits on the 125s and 250, and they had this ring arrangement. Didn't have a single problem in the two seasons I was with the team, so I have no reservations using them. It also give you complete control of the sizing of the auxiliary transfer ports as you don't have to worry about the ring end gap falling into the port window.

The only thing I can say is that it's worth taking an extra close look at the timing edge of the boost port during a rebuild to see if the ring ends have caused any excessive wear. Saw a little on the A kits possible due to the ultra thin rings, but no problems so far with the VHM.


That’s good news, I have a set of VHM pistons for my NF5 and I needed someone like yourself to confirm the VHM pistons were safe to use with parts being so hard to find and all that.

So I can use VHM  pistons without doing anything fancy to my cylinders?

I have read some guys saying you need to dress the boost port but as my cylinders are already plated I don’t see how I can do much like that.

These VHM pistons look really well made.


On cylinder preparation, just make sure that there is no ragged Nikasil on the port where it meets the cylinder. It should be smooth so that there's nothing for the ring ends to catch on.

I use a "rubber" stone on the port window edges just to make sure that there's no way anything can get caught up. If you don't have the correct die grinding tools then you can wrap some #600 paper around the end of a drill or something and carefully dress it by hand. Doesn't take much time and this method is actually a recommendation in the HRC kit manual.

I would then further dress it with some #1000 paper to remove any previous sanding marks fro the #600. Don't over do it though. Just a few light strokes is enough in most cases.

WD40 makes a good lubricant if sanding.

VHM are a great company. I speak with them regularly and they are extremely knowledgeable. I know that the RS pistons went through a very rigorous development cycle prior to being on sale, so I don't think that you have any concerns there.


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