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Other Questions

Question (NSR Engine Rebuild,300 Kit)

 "Want to rebuild the engine completely.Need big bore kit, exhausts,reed valves, HRC carbu kit and whatever you recommend to get the engine as fast as possible with still a bit of reliability.

Want to run on avgas or racing fuel (years ago i used elf 124 octane, not sure what’s available over here) as i don’t trust the thing coming out from the pump on our island.

Already found a new crankshaft and dry clutch. My dyno will be installed in July so i still have some time to get all the parts."


Concerning the engine, the big bore kit, you have a choice of either using the NSR150SP heads (modified with new profile) or the VHM heads. To be perfectly honest, the VHM heads are the better choice as they have better cooling potential than the cast NSR150SP heads. But there is of course a price difference, so the choice is yours.

Fuel; Avgas 100LL can be sourced reasonably easily here in Thailand, with prices running around 75-80B/l here in the Chonburi area. Elf 124 is not available here as far as I know. I probably wouldn’t waste my time trying to find it.

If you run avgas 100LL then we can also provide inserts for the VHM heads with a smaller volume than usual to make the most of the higher octane fuel. These would need to be specially ordered, but it can be done if necessary. Running avgas with the standard volume heads would possibly lose power compared to running it on 95 unleaded out of the pump.

For carb parts, the standard MC21 carbs can be made to work very well without the need for the HRC kit. And on the subject of the HRC kit, it is long discontinued by HRC and difficult to find. There is another (replica) kit available in Japan, but I’ve never used it so not sure if I can recommend it. We can check price and availability if you are interested.

 Reed valves; as Paul says, coming later today. They’re a good addition and add a little mojo all through the range.

 Another good additional to a track NSR300 is proper race plugs. I recommend the following:

 Plug: NGK R7282-105

Cap: NGK TRS1225B

These are not readily available in Thailand and we don’t actually stock them for sale, but we can order if necessary. Never had an issue with these on either the NSR250 or 300. The standard BR9ECM plugs are not really the weapon of choice for a track bike.

If you are totally rebuilding the NSR, then there’s a few case mods that I recommend. One is to improve general flow in the cases, and another that works in conjunction with the big bore kit. You see, the big bore kit pistons are 59mm (std 54mm), and at BDC the (unmodified) pistons will hit the crankcase. As such we shorten the front skirt of the pistons to allow clearance. If the cases are modified then there’s no need to modify the piston, which is a good thing as their strength is not compromised in any way. But having said that, we’ve not had a problem with the shortened pistons, so again, the choice is yours.

If you want to modify the cases then let me know and I can try to advise. The basic mods are quite simple and don’t need special grinders and what have you.

On the clutch, I recommend using our clutch rebuild kit (NHPS-0003), or at least taking a look at what parts it uses. The standard dry clutch can fail quite quickly when the engine starts making good power. Just swapping the dry clutch springs for the wet clutch springs, and swapping the thin steel plate for a thick steel plate can make a huge difference on longevity.

 You haven’t actually mentioned expansion chambers. Do you have these already? We recommend our own TYGA pipes of course, but the 300 kit can be made to work with other makes (Harc-Pro, Jha or Endurance for example), but we have no setting data available for these, and can’t guarantee that there won’t be fitting issues.

 Anyway, the 300 kit adds a huge dollop of midrange torque over the 250, and peaks out at around 68hp, which is about 5-6hp more than a derestricted MC21 is capable of without engine modification. With this you get reliability. It’s possible to get into the mid 70hp range, but of course reliability starts to suffer as each component gets more stressed, so if you’re not looking to maintain it like a full on race bike then I suggest just the ‘standard’ big bore kit, with some crankcase mods.

Question (CrankShafts)

"I am just curious as to if you stock a performance crank shaft for my 1993 NSR 250 r MC21?

I have seen the rebuilt cranks on your page but they all seem to be stock, this is fine but i am really after a performance crank, one that I don't have to balance and is lighter."


We only sell OEM cranks. They are sold as they are delivered to us from Honda Japan.

We do not have a 'performance' crankshaft, and we do not sell rebuilt cranks.

The OEM cranks are very well assembled at the factory assembly line these days. The last OEM crank I checked had less than 0.01mm total runout, which is close to factory NSR500/250 Grand Prix racing spec. So they are actually one of the best assembled two stroke cranks available.

If you want to lighten something, take a look at the flywheel, primary drive/driven gears and the clutch assembly. There's way more weight here than necessary, and by lightening those areas you will also put less stress on the main bearings, so the crank assembly will also last longer and be more reliable.

Question (300 conversion, engine case modifications)

"For a 300 conversion we spoke about the cases needing to be altered.

Any instruction or images would be most help full as I have the tools now to get started on smoothing and grinding as required."


On the case mods for the big bore kit. See the 2 pictures attached. These pictures are ancient, but show you where you need to modify to allow clearance for the 59mm full length piston. Basically you need to trim the aluminium where the hatch lines are. Check by offering up the cylinder and sliding a piston down the bore to see if you have clearance or not. I aim for 0.5 – 1mm clearance.

Also, you can see in pic #2 where I’ve rounded the entries to the transfer slopes. This should be done on all the transfer entry slopes. Also grind a generous radius on all sharp edges. Don’t polish anything. Complete waste of time. Use a course grinding burr to remove the majority of material and then some #150 sandpaper (or similar) to smooth out any uneven surfaces.

Question (MC21 Clutch and Gearbox)

"I have several questions for you, please give me your unbiased opinion. I have a 21R wet clutch motor and (I think) that I am satisfied with its performance. Because I have never used a dry clutch motor I do not know what the difference is between the two. I will only ever use my 21 as a street bike as opposed to a track missile. Am I better off getting hold of a dry clutch SP motor for the better gearbox? or can I alter the ratios at a later stage to obtain a closer range of street gears on my wet motor? I hear a lot about F3 gear boxes and such like and am always on the lookout to improve on what I have."


There's always been a lot of talk about close ratio gearboxes, specials, SP gearboxes and the like, but the little know fact is that there is no such thing as an SP' close ratio box. All models of MC21 (R,SE,SP) have the same transmission ratio fitted.

The MC28 came with slightly wider spaced 4th, 5th & 6th gears, so the "trick" tranny for the MC28 was to fit the last three gears from the MC21. Simple as that!

There is however a special HRC first gear available. This is a higher ratio. The stock ratio is 13/37 = 2.846, and the HRC kit 1st is 15/36 = 2.400. HRC recommend that this special first, and the 4th, 5th & 6th gears from the MC21 are used when SP racing.

The Formula 3 transmission is a different thing altogether, and has long been discontinued by HRC. This was basically using the gear ratios from an RS250 NF5, but with special HRC mainshafts and shifter forks. Both wet and dry clutch versions were available. This gave the user a number of ratio options for each gear, so you could chose from an incredibly close box, to a wide box, depending on track conditions. The gear location dogs are undercut for a positive engagement of gears, unlike the stock NSR. This box can tend to be a little clunky, especially in the lower gears. The F3 gears cannot be mixed with the stock NSR gears as they use a completely different engagement design.

Concerning the differences between the wet and dry clutch models. To be honest, there's very little to gained from the dry clutch, although the noise it makes is a little special in itself :-) And it's quick and easy to change plates.

In power terms, the wet clutch will make slightly less hp due to the oil drag, but it does tend to last a lot longer than the dry clutch, as this can suffer from burned plates and springs going soggy due to heat fatigue.

Soooooo, by changing from an MC21 wet clutch R transmission, to an MC21 dry clutch SP transmission, the only things you'll gain is maybe one hp and a lot of clutch rattle (good at the traffic lights!). The tranny will stay the same.

We can build you a 300 motor with a dry clutch is you want one, but the beauty of the 300cc's extra gee gees means that the close box is not necessary, as it'll pull you out of trouble from revs a stock NSR would be choking at!

The major factor here would be the price, as we are no longer able to build up engines using all second hand components. The transmission would be used, but the crank and seals would have to have new parts fitted.

Question (Cylinder Heads)

"Do you know the difference between the KV3L head and the KV3H head?

High and low compression?"


There's a couple of minor differences between the R and SE/SP motors besides the obvious wet/dry clutch.

Although I don't know for sure, I suspect that the changes are to slip around the rules of SP race regs to give the NSR some help against the faster Yamahas.Anyway, the differences that I know of are only in the cylinders and heads.

The R spec cylinders are marked "H" and are 86.3mm tall. The SE/SP cylinders are marked "L" and are 86.5mm tall. The port timing seems to be the same between the R and SE/SP specs, but with production tolerances being what they are there tends to be small differences of around 0.3mm right across the board.

The R spec heads are marked KV3H and the SE/SP heads are marked KV3L. As close as I can measure, the bowl dimensions are the same between the two types, but the KV3H type has an extra 0.2mm on the gasket surface.

This would give the R and the SE/SP specs the same compression ratio as stock.

For racing, HRC specify the SP as the base machine and recommend fitting R spec cylinders while keeping SE/SP heads. This effectively reduces the squish clearance by 0.2mm and the combustion chamber volume by approximately 0.44cc from the base setting.

What all the above basically says is that the SE/SP (KV3L) heads are higher compression than the R (KV3H) heads, when fitted to a common cylinder.

Question (Cylinder Heads, Spark plugs)

"My engine man Steve has forwarded the info below. Does this help? NSR 250 MC28 engine.

Std Cyl base gaskets used, top of cyl's skimmed .65 to .7 mm, to make squish .75 mm. Calculated comp ratio up to 8.5 to 1, from 7.4 to 1 (std). Plug grade, NGK BR8ECM. In my opinion, this looks like an excessive temperature failure. Possible wrong grade of plug for the spec, and or too weak? There are no carbon deposits at all. The electrodes of both plugs are burned away, and both piston crowns have burned. Both pistons seized, but the `vertical' piston and cyl is worse. If this was a `pure' detonation problem, I would expect more damage to the lower cyl because of the offset plug location?"


Almost every NSR I have measured comes as stock with a 1.1mm~1.2mm squish clearance, so by cutting 0.7mm off the top of the cylinder would bring the squish down to 0.4~0.5mm. This is too small. However, your man Steve says that he got the squish to 0.75mm with the above machining, so is it possible that I'm presuming correct that you have SP spec cylinder (marked L1,L2 or L3) and R spec heads (marked KV3H)?

At a ratio of 7.4:1 just about any pump fuel will do, but when you get up to 8.5:1 then you're looking at needing something a little more sturdy. I set race engines at around the 8.5:1 figure for use with at least 95 octane fuel.

The plugs you're using could well be the problem. BR8ECM's will work OK on a stock setup in normal conditions, but Honda do fit a BR9ECM as standard. And with the higher compression, the internal pressure and temperature rise is quite considerable so the need for a 'colder' plug is necessary. The #8 you are using would (as you describe) just melt down. This would also be the reason for the lack of deposits within the combustion chamber, as things have just got too hot.

On an 8.5:1 engine I would go for a #10 or #10.5 plug. Your best choice for a street used bike would be BR10ECM.

In my opinion it is the plugs causing the problem. When you get it going again be sure to check the ignition timing though just to eliminate this, and don't forget to send me your jetting specs.

Question (RC Valves)

"Can you give me some tips re setting up the exhaust valves? How do you line up the marks on the valves themselves? Is it done cold/at idle or whatever?"


With the ignition switched on and the RUN/OFF switch to the RUN position, the valves will move to their closed position. Now turn the switch to OFF and pull apart the throttle position sensor (TPS) plugs. It's the whit three pin connector coming from the right hand side of the carbs, situated on the stay that also holds the air solenoids.

Turn the switch back to RUN, and the valves will move to their setting position. Adjust the cables to line up the holes in the pulley and the back plate.

Don't over tighten the cables or you could blow up the PGM! There should be a millimetre or so of slack in the cables.

Flick the switch back to OFF, reconnect the TPS plug and hit the RUN switch. Do this a couple of times to open and close the valves to check that they open to the perfect setting position.

Question (MC18  Adjusting RC Valve Cables)

"the cables on my 1988 MC18 don't look well adjusted - any details on setting them up?"


Here's the spin on setting up the RC valves on the '88 MC18.

The '89 and on NSR's can all have their valves adjusted without actually starting the machine, but the '88 MC18 needs to be running.

The first (and best) plan is to remove the pipes and shine a torch up into the exhaust port so that you can clearly see the valves. Now slacken off the cables (both of 'em, on each cylinder) so that you can wiggle the valve pulley by hand. Open the valve (pulley clockwise) until the valve is at least perfectly level with the top of the exhaust port. Now take a note of the position of the pointer on the pulley compared with the line marked on the back plate. If necessary, scribble on the back plate with a magic marker and scribe a new line. This'll help get it all lined up spot on with the motor running.

Right, put the pipes back on and leaving the valve cables loose, switch on the ignition. The RC valve servo will move to it's "lo" (closed) position.

Take up the slack on the front adjuster "A" (see pic, RC closed), so that the valve is in it's fully closed position. Leave 1mm or so of slack in the cable. Then take up the slack on cable "B", again leaving 1mm of slack in the cable. Do this on both cylinders.

Now start the machine up.

At idle (approx. 1250rpm), the valves will be in their fully closed position. Slowly rev the bike up to 2000rpm, and hold it there. The valves will now move to their fully open position (see pic Re open). Now, bit by bit you can dial the valve in to the perfect setting that you marked earlier. After every twiddle allow the engine to return to idle so that the valves close, and then return to the 2000rpm mark and twiddle again. Be sure to keep the 1mm slack in the cables at all times. If the valve should look to be turning slower than before it could be down to over tight cables, so slacken the cables off or you could blow the PGM!!

When you're sure that the valve opens consistently to the "fully open mark" at the 2000rpm set point, you're job is done.

Question (Differences MC21 and MC28 Subframes)

"Is there any difference with a MC21 or MC28 subframe ?"


There are slight differences but they are interchangeable. The main differences are:

MC21 has mounting lugs for the front seat cowl position whereas these are on the frame on the MC28.

The MC21 has further forward ( or back) pillion mount positions and they are about a mm different pitch so it is a squeeze to get the other pegs to fit on, but possible. Because of this, the 21 subframe on a 28 will mean the exhaust mounts off the pillion hangers will be in the wrong place.

There are a couple of other tabs ( exhaust mount stays on MC21 absent on MC28 etc.

Question (Compression, Piston Rings)

"I have had the engine together and apart several times now, and each time there didn't seem to be any damage to the pistons/cylinders. The bike runs, but I

have to push start it, and it seems a little lacking in the lower revs. I do'nt know the history of the bike, but it seemed to have had a fairly dodgy rebuild previous to my owning it (it has 22,000kms on the clock) and I guess I have sort of checked everything else. Then the latest thing was that I got to ride two other nsr's on the weekend (a friend of mine is looking at buying one) and both of them seemed to have *miles* more compression on kickstarting. So that is symptomatic of worn rings?"


You should try to get the bike compression tested. I'd be looking for around 120~125psi. Anything below this and you could be looking for some new parts.

If you fancy taking it apart again and measuring a few things then here's what to look for.

Remove the rings and one at a time, insert the rings into the bottom of the bore. Push them to about 10mm from the top of the bore with the piston (making sure they stay flat in the bore) and measure the ring end gap with a feeler. Stock is about 0.4mm, but anything over about 0.7mm needs changing. Rings can also lose their 'springiness' over time, especially if they've overheated. Note that the top and bottom rings are different and must be fitted in their correct groove and the right way up.

Now check the piston to bore clearance. This is done by pushing the piston into the bottom of the bore in it's correct orientation, inserting a feeler at the same time. Stock clearance is 0.039mm ~ 0.045mm. Top limit is around 0.65mm before things start getting noisy and losing power.

Take a careful look at the bore itself. Can you clearly see the honing marks? If these have worn away then you could be looking at replating, or replacing the cylinder.

A couple of other things that may be worth looking at are the battery, and the carbs. If the battery isn't too clever this can cause hard starting on the 21, and it'll feel gutless low down. Also, if the carbs are dirty and the pilot jets partially blocked this can cause low speed lean running and will also be hard to start.

One little trick that can be done while testing compression is to pour a teaspoon of 2T oil into the spark plug hole. This temporarily seals the rings and piston against the bore and if there was a problem with the rings then it will show up here by increased compression.

Question ( Lightening a NSR250)

"I sent you an e-mail with a few question on ways I can lighten my bike"


Regarding lightening your bike, what exactly did you have in mind? The chassis or the engine components?

The pipes themselves are a huge weight saving over stock.

Concerning the chassis, there's a couple of parts we make in aluminium to replace the stock steel parts, such as the meter stay and the upper/mirror stay. We can also make to order an aluminium subframe, but this isn't cheap!

I don't know what you intend to do with your bike, but fitting the GP race tail and associated subframe saves a chunk of weight. However, this conversion means no more passengers. Not a bad thing on an NSR!

For the engine, there's a fair bit of skimming gears and other parts that can be done, but this is really only for extreme racing conditions and the minimal gains in performance are more than offset by the reduced reliability.

As an example, the dry weight of a stock NSR is around 135Kgs. A race prepped NSR should be below 120kgs.

Question ( Oil leaking into gearbox)

"The two-stroke oil on my bike is leaking into the gearbox. Why?"


This is a very common problem on the NSR. If you pull off the oil injector pump and turn it upside down you'll see the drive shaft and an oil seal. This oil seal has more than likely popped out of it's housing and slipped down the shaft. An easy fix for this is to remove the shaft (it just pulls out), wash all the oil off the pump and seal with a degreaser. Smear a thin coating of liquid gasket onto the outer sealing face of the seal and pop it back into it's housing. Allow the sealant to go off and then replace the shaft and refit the assembly. This has worked for me several times with no ill effects.

Question ( RC valves)

"How do I set the RC Valves on my NSR?"   


Turn on the ignition key and turn the RUN/OFF switch to RUN. The valves will settle in the closed position. Now turn the switch to OFF and unplug the TPS connector. Turn the switch back to RUN and the valves will move to the setting position. Looking from the R/H side of the bike you'll see that there's a hole in the actuating pulley of the valves, and another in the plate bolted to the cylinder. These holes must align. Loosen off the adjusting screws off the RC Valve cables and adjust as necessary. There should be no tension in the cables, with about 1mm of side-to-side play. Repeat the above procedure until there is perfect alignment.


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