Sorry if this has been asked before but I am a fairly new owner of a 1988 NSR PGM1. The crank on the engine I have needs to be replaced sometime in the future. It still runs allright but the carb needs to be adjusted from time to time to stop fouling the plugs so frequently.

My question is whether it is possible to buy a rebuilt MC21 engine and use that in the MC18. What modifications would need to be made to make that work? Would there be any cutting involved or is it just a matter of matching up the correct wires? Basically what would be a best case scenario is if you could straight bolt everything in and use the carburators and everything from the previous engine. I really have no idea what the best option would be but any advice would be much appreciated.

I won't be performing any procedures myself and will be leaving this one up to the mechanic. Although he is not an NSR expert, he is an expert in 2 stroke engines so any information about possibilities of engine swaps or compatible parts would be great. I am in Japan so it seems parts are abundant just not for the MC18.

Thanks alot for any opinions!

I am in the process of rebuilding an MC21 engine to put into an MC18 R2J frame. Research on this site indicates there will be no problem fitting the MC21 engine into the MC18 frame. I am aware that the MC18 harness has wiring for a neutral switch whereas the MC21 engine will have a gear position sensor (GPS). I will be putting a blank plug in the GPS hole. The neutral light will not work. The wire splice for the MC18 is the simplest as there is no GPS. Not sure of other wiring issues like throttle position sensor (TPS) as i haven;t got there yet.

I don't see any problem with carbs and pipes either.

However, the crank on your MC18 can be rebuilt. New bearings and seals are available. Connecting rods and gudgeon pins are not though. Use the search feature on this site and you will find much information on rebuilding cranks.

Cylinders can be replated or new ones for an MC28 will fit and are available too.

Paying a mechanic's rate to rebuild an engine could be quite costly. You may want to take up the challenge and do it yourself. It's not too hard and you don't need much in the way of tools. But you will need time, patience and the desire to learn.

Or contact Tyga. They can help with a rebuilt engine. And have much experience in doing so.
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MC21R9N track
MC18R2J track
CBR954
CR250

You could use the neutral switch from an MC28 to replace the MC21 gear position switch.
Or just modify the 21 gps wiring loom to just use the neutral wire.

We put an MC21 engine into a PGM1 88 chassis after thinking the crank was toast. Your crank is rebuildable if your pins and rods look good to the trained eye. see here http://www.nsr250.net/forums/viewtopic.php?t=7806&highlight=

All parts available from **** in Australia apart from maybe the thrust washers. Wiseco do these or you can recoat the originals in silver if they are not too farked. Not a cheap process but nice to start with a sweet crank.

From memory we used the PGM1 flywheel and pickups to keep the ignition timing the same. You can keep the GPS in place and just hook up the neutral...easypeasy. I found the correct colour wire off the wiring diagram on this site somewhere.

Thanks for the responses. I'm going to talk to my mechanic and see what he thinks might be the direction to go with this. He has taken the engine apart before so he knows what needs to be replaced. I know the crank and cylinders for sure. Also the pistons were not in the greatest of condition. It sounded as though I would pretty much be looking at a complete overhaul. Since the parts are really hard to come by, I thought swapping engines might be the way to go.

MFDgame wrote:My question is whether it is possible to buy a rebuilt MC21 engine and use that in the MC18. What modifications would need to be made to make that work? Would there be any cutting involved or is it just a matter of matching up the correct wires? Basically what would be a best case scenario is if you could straight bolt everything in and use the carburators and everything from the previous engine. I really have no idea what the best option would be but any advice would be much appreciated

An mc21 engine will just bolt in and work, everything will line up.

There is a tiny difference between the mc18 and mc21 flywheel timing though they will both fit and work on either motor and the mc18 flywheel will fit an mc21 crank anyway.
As mentioned the mc21 has a gear postion sensor (GPS) but as the mc18 doesn't use one at all it's irrelevent, wire the neutral light up or swap the whole sensor and you're good to go.
Carbs, exhausts etc all fit and line up perfectly - the mc18 and mc21 crankcases are externally the same, barrels only differ in shape and a tiny bit of porting, not configuration.
Basically the mc18, mc21 and mc28 engines are mix and match, the only things you can't mix are mc21/28 barrels and mc18 heads (and vise versa) and early mc18 cranks with mc21/mc28 cases don't fit either.
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