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Discussion Starter · #1 ·
So I went to change my engine oil last night, and while draining the old oil, some chewed up pieces of a bearing came out with it. So needless to say, this engine (a 250cc with 28k in a 1992 Nighthawk) is in need of bottom end work.

I'm curious if the cb350 engine will bolt in and if it will wire up easily. The 250, though reliable and spunky, just doesn't have to power I want. Ive also heard that the 350 doesn't have that much more power though, and people who have gone from a 250 to 350 haven't noticed a big difference.

This was my first bike, and she's sentimental. If I can keep the Nighthawk theme going with a more powerful engine, that'd be awesome.

So, anyone know how interchangeable these small engines are? Can anyone able speak to the differences in power between the smaller Honda engines?
 

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I can't speak to the power of these engines, but what fits the frame will be more like the CM185, CM200, CM250's we got in the states. They're not performance engines by any means (and some are 6 volt electrics) and with their 2 valves per cylinder they just can't breathe very hard. Some came with two carbs, but I understand that just changed the torque curve a bit.

Have you changed sprocket sizes?
That can really change a bikes road manners to better suit your riding style.

I've always wondered what it'd take to modify one of these 250 engines to emulate a CM400 engine in scale. Money I suppose, as its just another tool to be used... But seriously even this 250 engine built with proportional specs to a CM400 would only produce in the neighborhood of 25 horsepower. That's only 5-7 additional over stock. This is where people suggest a faster bike for better cost effective performance, but I'm not that guy.

There may be another engine to bolt in, that has more power, that wasn't available in North America, anyone?
 

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Discussion Starter · #3 ·
I was actually planning on ordering a set this weekend that would change up the ratio to my advantage.

If what you said about the CM's is the case, I think that, inevitably, I just need a bigger bike. Worst case scenario, ill rebuild this engine and keep this bike as a spare bike to tootle around town on.
 

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My slow phone finally loaded your photo, are you sure that's bearing material? (Roller bearings on these cranks) If you are able to dimple it with a center punch, might it be too soft for that?

Any odd engine noises?
 

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Swapping the engine to a 350 would be difficult due to the electrics once the mounting issues are solved like chain alignment, etc.
The chunks in the picture appears to be aluminum, have you tried a magnet on them? Reminds me of the slugs on the ends of cables.
 

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Discussion Starter · #6 ·
I work at a machine shop, I tested those pieces, they are steel. They look like and measure similarly to rollors in a typical bearing. They're just so knarled up, its hard to see. I cant say for sure of course till I split the case where they came from. I am meticulous with the care of my engines though, and these guys were not in the crankcase last time, so they showed up within the last 1000miles. I haven't heard and new or odd noises. But now as I type this, I remember while riding the bike on a 200 mile trip this last Saturday, i thought I felt a slightly unusual vibration at higher rpms. Enough to stand out, I could feel it resonating in my feet.

The engine still runs, I rode it yesterday on a short trip before I realized what was happening. I pulled the engine this morning, hopefully I'll have time to get into it and see whats going on this week. But yeah, from what I'm hearing and researching, itsounds like a rebuild of this engine will be most practical.
 

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Discussion Starter · #9 ·
Rather, I should say I really think they are rollors. I work with bearings daily and these are identical to some I see... Though they are so chewed up. Not sure what else they could be.
 

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Discussion Starter · #12 ·
My only thought is it might be shift fork guide pin, but fische doesn't list them

https://www.cmsnl.com/honda-cb250-nighthawk-1993-p-usa_model7209/partslist/E++17.html#results

Why not try and find another nighthawk engine to toss in?
I don't think a larger would fit without a lot of cutting and welding. The 350 is a lot taller from what I remember. And the carbs would get in the way... Or the frame would be in their way...
Yeah, ill either rebuild this engine or buy a used motor on eBay. Depending on what's cheaper, a bottom end rebuild or a used engine. Ive found 250's on eBay anywhere from $400-$900.
 

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I've got 2 in need of rebuild, a '92 and a '93.
It may be advantageous to start a group rebuild thread. We could share parts sources and overcome obstacles while documenting the process.
Anyone in? No timeline yet.
 

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Discussion Starter · #15 ·
I've got 2 in need of rebuild, a '92 and a '93.
It may be advantageous to start a group rebuild thread. We could share parts sources and overcome obstacles while documenting the process.
Anyone in? No timeline yet.
Looks like ill probably be buying a used engine to throw in my bike, which means ill have my current one to rebuild. I'm in.
 

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Are those bits magnetic ?

If rollers, from where ?

Look about maybe the size of the starter clutch rollers, but they live in an oil free chamber on these ball raced main and rollered big end engines ?

Too small for big end rollers ? sorry, should read "too big for big end rollers" )

Nice magnetic sump plug, worth all there expense !! but the bits aint stuck to it !
 

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Discussion Starter · #20 ·
Are those bits magnetic ?

If rollers, from where ?

Look about maybe the size of the starter clutch rollers, but they live in an oil free chamber on these ball raced main and rollered big end engines ?

Too small for big end rollers ?

Nice magnetic sump plug, worth all there expense !! but the bits aint stuck to it !
They are magnetic, they're just not sticking to the magnet in that picture. So they do look like the rollers from the starter clutch, ill pull that assembly apart tonight and see. I'm curious how they could make it into the oil, like you said, it looks like a sealed area. But the crank bearings are supposed to balls, not rollers, so.... Here's to hoping lol

In the pic below, it shows rollers (part 12) in the starter setup
 

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