Help!! What are the differences and interchangeability of the various MC18 Cranks: 2J, 4J and 5K?? What are the pros and cons of each?

I should have been more clear. Trying to piece together a motor as the one that came in my 88 R2J has issues. The crank, clutch and one head are scrap. The barrels and crankcases are not great being full of rust from the rusted out coolant feed pipe. I have been looking at a couple of dry clutch engines which I guess are 6k. Am I correct that there is no interchangeability between the r2j and 6k cranks? Are any of the component parts interchangeable such as flywheel, crank pins, webs, rods etc. Are the seals and bearings the same? I am hoping to get pieces together to come up with at least one fully re-built crank.

There's a search option which is very useful for common questions. I searched using the words crank difference and got this posting http://nsr250.net/forums/viewtopic.php?t=12902&highlight=crank+difference
I'm sure Andy will correct me if I've got it wrong but 5K onwards cranks are interchangeable. So 2J/4J cranks are not compatible with 5K onwards
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Thanks Rob, that's a good start.

RobH wrote:
I'm sure Andy will correct me if I've got it wrong but 5K onwards cranks are interchangeable. So 2J/4J cranks are not compatible with 5K onwards

The 90-97 (MC21/MC28) cranks can be machined to fit the 1989 MC18 R5/6K, but the 89 crank can't be used in the later models.

The 87 & 88 cranks have the separate centre seal (similar to the HRC RS250 NF5). The 87 and 88 seals are not the same, and at the moment there is no solution for a 1987 MC16 centre seal failure, however we have successfully used T2's pattern seal for the 88 R2/4J.

The 89-97 cranks have the centre seal integrated into the centre main bearing, and is unquestionably a weak point that I expect is being exacerbated by ethanol content in modern fuels.
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Andy.
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Thanks Andy. So having a choice between an R2J bottom end on the one hand and a 5K or 6K on the other, which is better?

My impression is that now that the centre seal is available for the R2J and R4J crank, that seal is more durable than the integrated seal in the 5K and later cranks, so that might be the better option if I am going to re-build the crank anyway. Is that right?

How about the electronics? Does the PGM have to match the crank (I read the placement of the pulse generators is different)? If so, is the PGM II any better than the PGM I for a racer/track bike?

I cannot confirm that the reproduction 1988 centre seal is as durable as the original Honda seal, only that we have yet to experience any problem with them. I wouldn't like to guess whether a new reproduction seal on a rebuilt crank will last the same/any longer than a new Koyo seal on a rebuilt 89~97 crank. No-one's going to know for a year or two yet!

I assure you that you will not be able to feel the difference between an 88 PGM and an 89 PGM-II, and probably wouldn't even be able to measure it. Technically, the PGM-II is better, but the difference is negligible (and may even be negative) without 1989 carbs, flywheel, air correctors, barrels, heads, crank/crankcases, exhausts etc. etc.
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Andy.
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Thanks Andy. Looks like I will be one of the guinea pigs! will report in due course.