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Hello everyone!

(I posted this in the Facebook group of Dommie owners (https://www.facebook.com/groups/nx650/?post_id=3081173991920785) as well, but would like to double the clarification here, as many people with wide knowledgebase might not follow both the FB group and this forum.)

Due to several simultaneous unfortunate coincidences, I have an engine rebuild (yet to be determined, whether smaller or larger scale) at hand. Overall, the cause of rebuild is running low on oil for some amount of time plus dust getting past the air filter due to the failure ohv either snorkel seal and manifold seal that clips on carburettor inlet, or the seal that seals the gap between the airbox case and its cover. So far I've dissassembled the cylinder head, cylinder (damaged, needs reboring), piston, piston pin and rings, and checked the radial play on the conrod's lower end (and felt no play there), next, need to check oil pump for damage, but it seems very unlikely that I might get around splitting the crankcase, possibly disassembling the crank for signs of damage, and at the same time check inside the gearbox, as the contaminated oil and later lack of lubrication might have caused problems there as well. (Can't bare to even think about the amount of work and money to be needed for this :))

But to the point, my first question is about different generations' engine parts' numbers. I gathered the info on cmsnl.com, but the pattern is the same on various parts' sites.

In particular, the part numbers for the piston, piston rings, valve seals and cylinder head differ between the (1) '88-'95 and (2) '96-'99 ('02/'03) model years. That is despite to my knowledge, which I thought (possibly due to a noviceness) was common that the engines and their parts are widely known as interchangeable throughout the model years, leaving aside the major differences like kick-starter mech. and the smog-reducing pump (or whatever's its correct name).

Are the abovementioned parts actually the same, or are there minute differences in design, is there some manufacturing bureaucracy/different manufacturer involved or what is the cause of part nr. differences? There was a suggestion on FB, that the main cause might be small improvements in design, which have no effect on the overall interchangeability of parts.

Secondly, why is there a 0,1mm difference in valve seal inner diameters between the "brown" and "green" valve seals. Are they different for inlet and exh. valves?

Thanks for your input!
Kaupo
 

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There should be no problem with interchangeability in engine components in general.
HOWEVER, the exception is the Oil pump. This is the biggest problem with the 650 RFVC engines and has been responsible for many of them prematurely ending up scrapped or even worse made into a Café Racer.
There were several revisions of the oil pump and all of them were trying to fix the problem of wet sumping where all the oil ends up in the crankcase, the original pump was an 8mm rotor and that was subsequently changed to a 12mm rotor in an attempt to correct the oiling issue. The best pump for longevity and performance is the XR650L pump from the US.
There are plenty of NX650 Enthusiasts around, although not many original NX650's any longer.
Here is a link to a german page that covers it in some detail.
 
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