hi

couldnt find anything when searching but is it possible to change over an mc28 motor into a mc21 with the 21 electrics? i assume that everything else would be the same mechanically, would that be right?

has anyone done this before? if so please shed some light

thanks in advance

cheers

Yep, everything fits either way, 28 into 21 or 21 into 28.

Be aware the flywheels are different, 28 motor in a 21 is a downgrade as the timing is more retarded, swap the flywheel over if possible..
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thanks fontyyy

well my current motor has the 2t oil in gearbox problem, had to ride it yesterday as it was my only mode of transport, i did remove some of the contaminated oil and topped up with some gearoil on the way to work and did it again on the way back.

i've been told that i would get 2t oil in my crankcase, was just wondering what sort of problems could i have caused?

i also found these links from the site
http://www.nsr250.net/forums/viewtopic.php?p=7342
http://nsr250.net/forums/viewtopic.php?p=40663&highlight=stroke+oil%2A+gearbox#40663

on how to reseat the o-ring which i will try first, but i also have a mc28 motor which has been sitting for over 2 years which was taken from a crashed bike, what would i need to look over before fitting, from memory... i think it was rebuilt maybe a few thousands k's before the accident

Just take the pump off and push the seal back in to place, drain the transmission, and refil it with fresh oil. Job done!

Mountains and mole hills spring to mind at the moment!
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Andy.
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thanks andy

jobs all done, i thought there was another more sinister problem since my bike developed a whirring sound when gearing down so it got me worried

but i found that problem, only did a bandaid fix on it for the moment with a cable tie

but its still a good idea i reckon to sort out the other motor either way, just in case sh*t hits the fan one day, hopefully later rather than sooner

being able to ride the bike after fixing it for the first time today, it seems that i do have a more sinister problem

still having this whirring sound in gears and alot mor noticable when the bike is gearing down, even get it when coasting with the clutch in so i think its definately transmission related

with this spare motor i have, whats the best thing to do before putting it in? should i just give it a good clean and whack it in? or should i check other things?

i will need to flush and fill the transmission fluid, going to crack open the heads and see if theres rust should i just wipe em off or just leave it there?

its a dry clutch, would i need to clean it to ensure good operation?

anything else? i cant seem to think of anything at this stage, any help appreciated

If the current motor's running OK, why not just swap the transmissions? Saves taking the motor out.

If you remove the transmission from the spare motor first, you have an idea already then of what's involved before you pull the one in the bike apart. There's not a great deal of difference between the wet and dry setups, and the job should only be an hour or two for a first timer! Once the covers, clutches, and transmission oil pumps are off, both transmissions are only held in by six M6 bolts and come out as a complete unit. (Don't forget to take the front sprocket off though!!)
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just digging this one up chaps, am i right in thinking that 21 and 28 engines of the same spec (std, se,sp) are essentially the same engine?

If there are any differences between them could someone let me know what. I assume its just port design and cylinder head alterations etc? I'm not yet sure of the exact spec of the 28 engine fitted to my 21, but i'm almost certain its topend rebild time and will be expecting re plating.

Are new barrels still available from honda?

One last question, the 21 loom i have has had both splices carried out GPS set to 3rd gear, would putting the 21 flywheel onto the 28 engine over advance the timing for the 28 transfer timing. Hoping perhaps of of the racers has caried out the same mod and can offer advice.

Cheers
Brendan

Unless you've seized it, I highly doubt you'll need to replate either barrel... in fact, even if you have seized it, the barrels will often clean up!

For all intents and purposes, the MC21 and MC28 R motors are the same, and the MC21 and MC28 SE & SP motors are the same. Don't mix R head/barrel parts with SE/SP unless you know exactly what you are doing, and are handy with a variety of accurate measuring devices!

21 and 28 transmissions are different, but interchangeable.

If you put an MC28 motor in an MC21 frame with MC21 wiring harness and ignition, simply use the MC21 flywheel, and vise versa.

Barrels are available from Honda still... if you really want to pay about £450 each for them!
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excellent cheers for the info, not planning on buying new barrels just checking.

The gear box ratio's were altered to over come the '28 lack of zip wasn't it?
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I think at one point in its development, Honda considered asking jeff350lc and nsr.wayne to build a special 1, 2, 3, 3, 3, 3 box specifically for that very reason!
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StephenRC45 wrote:The gear box ratio's were altered to over come the '28 lack of zip wasn't it?

That's the story.
A '28 has got a more spread out gearbox and a lower final drive, they could have achieved almost identical results by just running 15/43 gearing and retaining the entire mc21 box.

If you run a '28 box, to get as near as possible to '21 top speed gearing you want to run 15/41.
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That will give it more torque, right? lol
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