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Project Alan's CB250 Nighthawk

Discussion in 'Your 250cc Projects' started by Alan f., Jul 18, 2018.

  1. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    So I picked this up almost 2 years ago now. Spent quite a bit of time on it and stalled out again. I'm working in a walk up basement workshop so in the end I'll reassemble everything somewhere else.

    For $200 USD I started out with a 93 CB250 Nighthawk, which in the US only came with 16/18 spoked wheels and drum brakes front and rear. This alone was enough to relegate these bikes to closed course learning and most of them never saw street duty at all.
    my plan was to adapt the rear alloy wheel from a US spec CB750 Nighthawk. I've had this wheel around a while, had sanded and repainted it, new bearings and new tire, then it sat unused and neglected.
    CB750Nighthawk_0858.jpg

    First order of business is to adapt the 15mm axle to the 17MM wheel bearings, it's easier for me than cutting the swingarm slots to accept the 17mm axle, which would be way too long anyway. Since the bearings were new there was no sense in swapping them out to fit the axle either. So I turned some steel sleeves with a 1mm wall and a flange on the end from stock I had on hand, not stainless but it hasn't rusted in a damp basement for 6 years either so it's a good alloy choice. Starting out with narrower stock would have been easier, but the stock was free... I'm no machinist, and my machines are best classified as toys, but I'm confident I can get this done.


    axlesleeve.jpg
    axlesleeve2ndview.jpg
    axlesleeve3rdview.jpg
     
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  2. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    Next I needed to get the wheel centered in the swingarm. (Centered yes, just like the stock wheel was. Not all rear wheels are actually centered in their swingarms to be in the center of the bike though, so look out for this if you ever get into this situation) Unfortunately to center this wheel I needed to take off a little material from the drive side, specifically the area that the seal that rides around the wheel spacer is driven into. So I'd have to figure out a new way to replace the seal.
    With a little consideration I figured a thin spacer with an o-ring would do the job, this area is only sealed from the elements to protect the spacer on the sprocket carrier and it's sealed bearing, not all that critical but necessary.
    This is what I came up with:


    [​IMG]


    It's a bit thinner than the stock seal:
    [​IMG]


    But it fits and with a little grease behind it, does the job without binding.
    [​IMG]


    [​IMG]


    [​IMG]


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    [​IMG]


    [​IMG]


    [​IMG]


    [​IMG]
     
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  3. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    On to the Brake side of the hub.


    One simple spacer over here to fill the gap. However it was late and I didn't feel like rounding a square piece of stock to get the right spacer length, so I made a two-piece spacer for this side.


    brakesidespacers.jpg


    brakesidespacers2.jpg


    brakesidespacers3.jpg


    brakesidespacers4.jpg


    Looks like plenty of room for the brake stay and actuator rods.


    I had to mill down the sprocket carrier to restore chain line center, and while I was at it I shortened the sprocket studs so they won't contact the swingarm.


    I soaked them in plain 5% Acetic Acid White Vinegar (safe enough to drink) for a few days, the acid will affect the rust but not the steel, unless you're very patient. Then I zinc electroplated them with the solution left over from some experimenting last winter. They came out looking more galvanized than I'd hoped (and there is still some rust seen inside the nut threads) but I can always throw them into the vibratory polisher with some corn cob to shine them back up when I replate the rest of the hardware.


    sprocketstudsandnuts.jpg


    Here is a shot with the rear wheel mounted, stock seat set in place, and that CB350 tank in place (too bad the seat gap is so bad) and the engine mounted.


    mockedupwithenginere.jpg
     

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  4. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    With the rear wheel adapted it's time for me to think about the front end. I'm planning to use a set of forks from the same US spec CB750 Nighthawk, they're about the same length as the 250 forks but they're 41mm and have a single disk brake on the left side.

    To Adapt the 750NH steering stem to the 250 frame I'm using the bearings from the 750NH and making bearing cups to adapt to the 250's frame. The steering stem is long enough to do the job unmodified if I play my cards right.


    I finished up the upper steering bearing cup first, I'm making them from aluminum to keep weight down, but they're pretty chunky. It's probably overkill but all stressed areas are minimum .300" thick. I'll clean up the finish on both after the bottom cup is done and all dimensions are finalized.


    [​IMG]


    [​IMG]


    [​IMG]


    [​IMG]
     
  5. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    The bottom bearing cup looks so much like the upper that I didn't bother taking any photos of it on the bench.
    I finished it up earlier tonight and the 750 triples bolted up flawlessly to the 250 frame using the tapered roller bearing set from the 750.
    Here's a few pics:

    attachment.php?attachmentid=211697&stc=1.jpg

    attachment.php?attachmentid=211705&stc=1.jpg

    attachment.php?attachmentid=211713&stc=1.jpg

    attachment.php?attachmentid=211721&stc=1.jpg
     
  6. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    I was getting tired of that detached back half of the frame getting underfoot so it's time to get it back in place. Since I'm working in a basement shop welding is probably not gonna fly so the plan is to make some slugs, square it up, drill some thru holes in the frame and drill and tap some holes in the slugs, then just bolt that sucker back on. I'm also planning to cut some tabs from this section of frame as well. The slugs are made from stainless steel round bar (not sure of the grade), 4.75" x .710" with a quarter inch center bore (.230" wall) and I rounded the ends, they're a bit heavy but fit inside the frame tubing nice and tight and shouldn't rust permanently in place in case I want to do a frame loop later on. I also unbolted some parts from the rear section, removed the helmet lock (no key anyway) and the seat latch cable. I removed the seat latch, cleaned and re-greased it before bolting it back in place. Without the cable actuation for the seat latch it's just a matter of reaching under the rear of the seat and pulling the latch open with the loop that the cable used to attach to, simple.


    While my drill batteries charge here are some new pics:

    attachment.php?attachmentid=212562&stc=1.jpg attachment.php?attachmentid=212570&stc=1.jpg attachment.php?attachmentid=212578&stc=1.jpg
     
  7. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    I measured the stock shocks against the CB700SC shocks I'm using, they're only one inch over (13" eye to eye) not two as previously stated.

    [​IMG]
    Here's a look at the fit of the steering stem with my bearing cups added with the stock fuel tank in place.


    [​IMG]


    Here's an overall view of where i'm at today.


    [​IMG]


    [​IMG]
    And a shot of the frame slugs joining the cut rear frame to the front half, all bolted up.
     
  8. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    I got all of my fork parts cleaned up and ready to reassemble, they'd been apart and stored for several years and I'd never really cleaned them up. I used a big aluminum foil pan from a dollar-store and poured in half an inch of simple green then 2 more inches of tap water. I put in everything but the fork tubes and lowers and let them soak for 15 minutes before I started scrubbing them one at a time with green scotchbrite, rinsing with fresh tap water and towel drying each part. I really don't want anything to rust. Then I did the lowers, and then the fork tubes themselves. Its a bit difficult drying out the inside of the fork tubes, I stuffed a twisted rag in there but they still weren't as dry as I'd have hoped, so I sprayed them inside with wd-40 and left it at that. I'll let them air dry for a few days before getting back to them.
    I picked up a set of cartridge emulators from an eBay-US seller, they're made by V-Twin Mfg and they're listed for Harleys with 41mm Showa forks. They measure 33mm in diameter, just a bit too small to fit correctly on these damper rods, so I'll be making up some adaptor rings to fit them properly. For about one third the price of Racetech emulators, i'll make them work.


    20171104001054resize.jpg


    20171104001617resize.jpg
     
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  9. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    These cartridge emulators fit my damper rods like this:

    20171104001722resize.jpg

    So I needed to make some adaptor rings to help them fit better:

    c25AdaptorbushingforVTw.jpg
    20171110214717resize.jpg
    20171110214915resize.jpg

    Plenty of room for the springs to sit on top of the emulators too:

    20171110215050resize.jpg

    Racetech says to start the adjustment at 2 turns past where spring tension begins, I've asked a few friends and read a bit on the web and everyone says it's a little stiff for mixed street riding so I set them at 1.5 turns to start with.

    20171111122324resize.jpg

    The nylon lock of the check nut is fully seated on the adjustment screw, and the nut is tight against the valve body. It took several tries to get them the same, lots of room for error here.

    Next up is to drill out the oil holes in the stock damper rods
     
  10. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    modified the fork damper rods per the Racetech directions by adding a third 5/16" hole.


    [​IMG]
    Then I assembled the forks and filled with Dextron ATF, it's cheap and I had it handy.
    [​IMG][​IMG][​IMG][​IMG][​IMG]
    The front wheel is from a 1987 CBR600F, 17" x 2.5", same axle, same caliper, same rotor...
    The 750 forks are 1/2" longer than the 250's stockers.
    Front tire is 110-80-17 (23.92" diameter) and rear is 130-70-17 (24.16" diameter)
     
  11. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    Finished grinding the tabs from the frame tail and mount up an old CB750K rear fender. attachment.php?attachmentid=229730&stc=1.jpg
     
  12. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    And that's where it sits to this very day. I'm interested in keeping the stock seat but was thinking of chopping the back of it off. I hate to cut up a perfectly good stock seat though so I've been thinking about it for quite a while now.
     
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  13. Murdo

    Murdo The Good Doctor Staff Member Premium Member Ride and Events Crew Contributing Member

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    Wow, some nice work there Alan. This should be a real 'head turner' when finished. :thumb_ups:
     
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  14. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    Thanks Murdo.
    If I can figure out what to do with that back end I'll sure feel better about the whole thing. I've picked up a small case similar to a Pelican brand case,
    it's about 22 x 15 x 9cm
    https://m.harborfreight.com/1800-weatherproof-protective-case-9-316-in-63518.html
    I'm thinking of a sheetmetal rear rack to mount it on that will also blend with the back of that seat.
    Sort of a mini top case if you will. But I'm not sure how that'll blend with the tail light, may have to hang the case foreward above the edge of the seat to get a visible tail light mounted to that fender.

    I'll play with some photos and see what I come up with.
     
  15. Wozza

    Wozza Active Member

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    Nice work ..for the seat have a look at the BMCO series of bike builds

    he has some great ideas and tech's and his videos are easy to watch as he doesn't waffle on :)
     
  16. Andych

    Andych Well-Known Member Premium Member

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    I agree with @Murdo ... this is a really nice project. Lots of detail as well. I am looking forward to how it all turns out.
    It will be interesting to see how the front damping will turn out as there will be a lot less weight on them that the original bike.
     
  17. Alan f.

    Alan f. US Spec CB250 Nighthawk 1993

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    Thanks Wozza and Andy, the stock preload spacers in these for ks are huge, they're at the lower left in the shot above where all the fork parts are spread out on the table. My first move will be to remove those and replace them with PVC spacers at -13mm increments until I'm happy with it, as they are now those forks caps were very difficult to install with that amount of preload compression. I'll stay with the stock seat, there are plenty around and covers are available, I will experiment with different foam though.

    I've given a look at the case I mentioned with the bike, it looks like I can make it work.

    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
     
  18. Murdo

    Murdo The Good Doctor Staff Member Premium Member Ride and Events Crew Contributing Member

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    Neat little case. Looks good.
     
  19. Linkin

    Linkin The Apprentice Premium Member Dirty Wheel Club Contributing Member

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    Nice bike & work you've done there... well above the standard of work that most people who 'cafe' or 'bobber' their bike do.
     
  20. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    Nice work so far, looking forward to seeing it progress
     

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