Nighthawk CB750 rear wheel swap part 4 - the grind

Preparing to remove 2.38mm from each side

I got some time to myself and decided to get the cb750c up on the center stand to remove the swing arm. (It's getting harder and harder to hike the old girl up in my old age, so I needed my son to help me)  I figured I was done riding for the season anyway.

Working as methodically as possible and keeping parts together so I could remember to put them back properly at some point I got the swing arm out and was disappointed to discover the nighthawk swingarm was 4.76mm wider than the cb750c swingarm with the dust caps attached.

Carefully measuring both swing arms, I calculated that I need to remove approximately 2.38mm from each side of the swing arm pivot to make the NH swinger fit.  This is not a lot and I'm hoping I can still use the oem dust seals (new seals already in hand) pushed in a little further after grinding the pivoting area of the swingarm.

I used my calipers to mark the swing arm and my air grinder to slowly remove enough metal to hit my 2.38mm marks.  I had to change grinding discs 3 times when the grinder seemed to only heat up the metal without removing materiel.

The swingarm's internal spacers (that goes between the bearings) will need the same amount of shortening but I'll check again after narrowing and installing the bearings. I'm removing the material slowly and fitting it periodically to make sure I don't remove too much metal. Since I don't have easy access to a machinist or a mill, I'm going to try and be as even as possible to keep the edges square(ish)

I also received my carbon fiber tube in 16mm OD x 15mm ID tube that I hope to use to sleeve the larger frame mounting hole for the 15mm pivot bolt ($12).  I'm fairly certain this is the only option to get a snug fit in the frame *IF* you are moving to a narrower bolt.

Checking my work

The Nighthawk pivot bolt is too short, but the cb pivot bolt is too thick. I need to take one of the following options:

• find a suitable swingarm pivot bolt sized 15mm X 343mm
• bore out the spacers from the Nighthawk swingarm to accept the 16mm cb pivot bolt
• modify the original swing arm bolt down to 15mm

After spending a few hours researching a way out of my dilemma, I discovered that the online machine shops like emachineshop.com are probably all too expensive for me (not sure about the other guys but a $225 setup charge in addition to the actual machining is more than I'm willing to pay).  

I was also unsuccessful in trying to find a replacement bolt that I could just purchase on ebay (unfortunately, there is no handy list of swingarm pivot bolt and rear wheel axle dimensions by motorcycle model).  I did find some aluminum pipe on onlinemetals.com with the following details:

• .5" Nom Schedule 40  6061 part 1219 (.622" ID X .84" OD) about $12 after shipping

Unfortunately the ID is too tight (.622 vs .629) to work without boring out a little. I should have converted to mm instead of comparing the thousandths number before ordering because 15.79mm vs 16mm clearly doesn't fit ($12 out the window)

Another option that came to mind over the week is to buy 2 different sizes of carbon fiber tube (21mmOD X19mmID and 19mmOD X 16mmID) and epoxy them together making a single thickwall spacer, but before I do this, I'm taking the pivot spacers to a machine shop that someone at work told me about. He said they made engine mounts for his friend, so they should be open to taking a small simple job from a "walk in" like me.

I found the shop and told them I wanted to either bore out the NH tubes or reduce the cb pivot bolt to fit in the tube. They said reducing the bolt would be easier and it will probably cost about $80 and take 3 weeks due to their backlog so I left it with them along with my contact info and the only thing left to do while I wait is to see what else I can work on.

EDIT: just coming back to post an update here. April 2020. If you're thinking you might want rear-sets in the future, the early 80s cb750F and 900F rear-set brackets bolt up to the cb750c frame but if you decide to go in this direction, you will need the longer swing-arm axle from either of those donor bikes because it needs to be longer to mount the brackets.  There is no point to machine the cb750c swing arm axle(bolt) only to need to machine another axle when you later think you might want rearsets.  My loss is your gain.

After removing enough of the swingarm pivot material, I was able to slide the NH swingarm into place without the bearings and mount up the wheel to check for alignment and clearance.  The sprockets look like they will line up perfectly and the wheel is centered as expected since I'm moving the wheel along with the swingarm and OE Nighthawk axle spacers.

Checking wheel center (ignore that jumble of temporary wiring)

Thinking ahead
Here's a list of things I plan to research for future work:

• 525 pitch chain vs 520 pitch chain
• Gearing (sprocket sizes)
• Steering head bearing now or wait for front end swap
• Building a brat style seat from scratch
• Jetting changes for pod filters
• New shocks

The idea of swapping from the original 530 chain down to a 520 pitch is attractive to me for the simple idea that I could save weight and possibly gain a tiny bit more power but I am also aware of the trade-off of availability of parts for a chain conversion vs. ease of future maintenance. 

Moving to a 520 pitch chain means custom aluminum sprockets in the rear which are hard to find and wear out faster.  Even though the lighter components can translate more power to the rear wheel, it's not 4 or 5 HP more but closer to maybe 1.5 HP.  On the other hand, 525 chains seem to be more expensive with significantly cheaper steel rear sprockets.  It looks like going 520 will be roughly $50-100 more money. Since I'm losing almost 17 pounds just from swapping the wheel, I could leave a little weight on the table with the steel sprocket and 525 chain.

Since this will be a street bike, there's no need to wring out the last bit of HP and the trade-off seems to me that a stock rear sprocket for easy sourcing and durable steel construction and a 525 chain should be the right path to take compared to using a 520 pitch aluminum sprocket with anodizing.

I originally thought the front sprocket would need to be a custom item from sprocketspecialists, but when I received my sprocket, I noticed that it's a standard Nighhawk 750 sprocket.  Cheaper off the shelf is always preferred over custom parts (my loss this time, live and learn). While I will still be dropping to a smaller, lighter chain, I will not go all the way to a 520 and stay in the size that originally came with the Nighthawk and use off the shelf (readily available) parts.

NH OEM is 15/38T for a 2.53 gear ratio
CB OEM is 18/43T for a 2.39 gear ratio

The gearing calculator says a 40 tooth rear sprocket should give me roughly 10% more torque over the old CB setup (using a 15T front sprocket), which means the bike will rev higher after the change. I need to weigh my options since I don't want to burn a ton more gas than before but I also want a little extra kick. Hopefully, the 34mpg I was seeing won't be too much worse, especially with the increased jets sizes for the POD project that is in the works.

Edit: I got my numbers mixed up and ordered 14/43T for a nearly 22.2% bump in torque. I'm going to order 15/38 to match the original Nighthawk gearing which will be approximately 5% increase in Torque over the original gearing.