RC03 motor in my 82 Nighthawk. Not original motor?

[fizzlebottom]

I've had my '82 CB650 Nighthawk for 10 years and have done a lot of work on it. All the while, I knew that these bikes originally came with black painted motors, which mine is not. I figured there was a minor run of unpainted motors due to either assembly line changes or process changes. It has all of the right parts, took a top end rebuild perfectly, and runs on VB44C carbs which tells me the cylinder head is correct.

I never paid attention to the motor's serial # since everything has gone smoothly. But based on what I read, RC03 was the older designation (79/80?) and All Nighthawks were RC08. Mine is an RC03.

So what's the deal? I recognize there probably isn't robust documentation around these but one can that something exists somewhere. Was this some early '82 motor before they were painted black? My clutch cover is the only component painted black, so I suppose it is possible that my motor is some salvaged mishmash of parts.

Insight?

[Volker_P]

Welcome here!

All the RC03 and RC05 frames came with RC03-E engines. The cylinder head was different from 1981 on to match the wider spacing between the CV carbs.
To my knowledge the RC08 came only with the black RC08-E engine that all had the CV carbs with the screwed idle jets (VB44C).
An finally there is the rare police version RG01 with the (silver) RG01-E engine.

I weakly remember that there was something about the RC08-E black engine clutch lever so possibly the clutch cover was swapped back to the silver engine to make the original 1982 SC clutch cable fit.
I also remember that some transmission gears are different between RC03-E and RC08-E engines.
Probably there are some more tiny details that I am not aware of or that I do not remember any more.

So I am sorry this is all I can provide.
If you really want to find out more details comparing part numbers at partzilla.com may help a bit.

[fizzlebottom]

That confirms my suspicions. I realized how much of a frankenstein motor I had (past tense on purpose, will explain). The engine was an '81 RC-03 but the clutch cover and starter motor covers were both black.

I ended up taking a risk and buying an '82 RC08 motor off eBay. The risk was that there were no compression numbers, but my old one was throwing a lot of glitter into the oil filter and making enough of a racket that I was pretty certain of imminent demise.

The new(er) motor arrived and it went in without a hitch. Both the ignition cover and alternator covers are spotless, and the rest of the motor is just dusty. No obvious leaks yet or anything. Fresh oil, new filter and pan gasket, and it fired right up. It sounds divine compared to the old one. Took it out for a good spin and everything is dandy. Going to change the oil again this weekend just to make sure I'm all set.

I ended up tearing the top end off the old motor and found that the cam chain guide decided to give up. The side walls aren't there anymore, and the chain started removing bits from the cases & cylinder. I haven't opened it up anymore to see if crankshaft bearings are wrecked, but I honestly don't have any desire to rebuild this one. The new motor is great.

[Volker_P]

Good to hear you did not wreck your engine and you were lucky with your ebay replacement.
There are a few reports on cam chain guides with missing side walls in this forum but I think this is no typical issue.

You may check out the cam chain tensioner for correct position: viewtopic.php?f=4&t=74
If placed in wrong position it will not really be able to keep tension to the chain. I have no proof for a direct correlation of tensioner misalignment and cam chain guide wall damage yet as most people are that reasonable not to ignore the horrible metallic noise of the cam chain tensioner scraping and grinding at the primary shaft. Here an example of the grinding traces on the tensioner: viewtopic.php?f=4&t=8433
I saw this several times, it would be interesting to see how your cam chain tensioner looks like.

If you are worried about the crankshaft bearings have a look inside the oil gallery below the crankshaft. There is a screw plug on the right side, the left side plug is just plugged in and fixed by a nose inside the alternator cover. If it is still clean inside this channel, the oil filter overpressure valve did not yet open and probably the bearings and the camshaft gallery oil nozzles are not plugged with metal filings.
Anyway I also would just keep the engine for possible parts and not try a rebuild now.