Hi I just got a MC18 1989 which I'm gonna use only for racing. Is it possible to fit RS cylinder, head and pistons? Will it fit? wil it need a lot of other things as well if you start going this way?
Or should I just get a better exhaust system, but which one?
Best regards Torben, Denmark

Cylinders and heads can be made to work.

Pistons will not fit the NSR crank.

There are also problems with squish and volume using stock pistons in RS barrels.

TYGA are the best people to talk to about exhausts.
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If I have to take the carbs off once more...

I use 1989 NF5 cylinders and heads on my MC21. Like Stephen said, you can't use RS pistons, but there are lots of others that do work. You will have to make new brackets and adaptors to get the power valve pulleys to work. I dont know how Tyga pipes would perform with RS barrels (Matt????)Other than that, I don't know why more people dont do this. If done properly, it makes a beast out of a relatively gentle 250......and it is still a 250!

Deon's bike is strong ! This is a great conversion to do if you can find the parts .

RS barrels and heads are good. The stock NSR piston can be used, but the deck height must be set right.

The NSR pipes work but they don't realise the potential of the RS cylinders. On the conversion I did we started with NSR pipes. Power was great, but you could tell it wasn't 100% happy. RS pipes bodged to fit gavea great top end, but below about 9000 nothing was happening. Made a new set of pipes with similar tuned length but different header and divergent section and the bike became most user friendly. Very fast (76hp as measured in the good old US of A), and had midrange.

Thanks a lot everybody. Deon what exhaust do you run? How much should the squish be?

I ran '89 RS pipes on mine but found like Matt that it had no middle at all.

I then ran some Nikkons which worked on but ended up using the faithful Dog Fight SP's. I think they are one of the rev'est NSR pipes anyway and seemed to work quite well with the RS barrels.
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If I have to take the carbs off once more...

Your squish is determined by the fuel that you want to run . Race fuel can handle a tighter squish than pump fuel .

I'll run it on 98 led free pump fuel if possible.

Deon what exhaust do you run? How much should the squish be?

I thought my pipes were modified '89 RS, but based on Matt and Stephens experience with lack of middle, I cannot say for sure, mine has HEAPS of mid. They are labeled HRC and they are much fatter than NSR pipes. As far as the squish goes, I wont say that what I am running is good for anyone else. Most would say it's too little, but then the bike is only used for a short circuit (go kart) track and there is no over-rev at all. 73 HP at 10600 rpm. I think I can get more if I can get the bike to rev more, but believe me 73HP is quite wild for a 250.

The '89 pipes I had were quite slim with a longish tail section. Fitted the '21 though very well and the brackets almost lined up.
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If I have to take the carbs off once more...

I'm interested in this conversion, what carbs will need to be used on the engine with rs barrels fitted?

Cheers
Paul
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More **** two strokes than you can shake a **** stick at.

If I can get 73Hp and still have a reliable bike I'll be really happy . So what is standart NSR squish and what do you run Deon?
What about the 18" rearwheel should it be changed to 17" for more tyre options?

i used the standard mc18 carbs on mine

torbenstig , i set my squish at 1.00mm using 98 fuel
its reliable if done properly , but you will need a top end rebuild a lot more often

We've got to be careful here branding figures around, most of the UK people are using one dyno (Steve's).

On his dyno;
a stock wirespliced mc18 running on 95ron unleaded will make about 62bhp
a very well put togeather mc21 running on 97ron unleaded made 67bhp
a stock '91 RS250 nf5 running on 100LL avgas made 72bhp
a pretty much perfect late (PGM-II) F3 mc21 running 97ron unleaded made 74bhp

All those peaks will be over 11000rpm, the RS and F3 will be nearer 12000rpm.

Basically no 250 is going to make 73bhp at 10800 rpm on that dyno.

To make 70+bhp on an NSR bottom end you need a lot of things to be very right indeed.
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Please do not PM me technical questions, if you can't find it on the Forum start a thread