Topic: It's a chopper, baby. My CB750K0 rebuild (warning LOTS of pics)

[mystic_1]

So I figured it was about time that I began a build thread, my project is finally moving forward with enough momentum that I feel it's worth documenting it here.  I've been working on this for a few years now on and off. so I have to play some catch-up with posts in order to reach where I am today.  I apologize in advance for the long picture-heavy posts.



My bike is a 1970 CB750 K0 die cast model.  When I acquired it, it had been subjected to the classic 70's "semi chopper" treatment that was so common on these bikes at the time.  The forks are 6 inch over Forking by Frank tubes.  Because the frame has not been raked, and the rear has been lowered, the bike sits at an angle.  The seat is a Corbin king and queen seat.  The tank is a fiberglass tank by the Fibre-mold company.  As purchased it had a stock front wheel and a 16" drop spoke conversion in the rear, complete with nasty 70's square-section slick tire.  The bike had struts when I bought it.  It's my understanding that dealers were doing this sort of work commonly back in the day.

     



A year or so after buying the bike, I decided that it was REALLY in need of a rebuild due to severe engine wear.  At that time I fixed several problems including a trashed cam, cam chain, and tensioner, badly worn shift forks, worn clutch, and a lot of overall corrosion.  I installed a stock tank because the fiberglass tank was tiny at only two gallons, and part of that was unusable due to the angle it sits at.  I also bought some better-condition K1 side covers because the original K0 covers had missing mounting pins and would fly off on the highway   Later, replaced the thrashed 4-1 system with pipes from a DOHC model.



After the rebuild, the motor was dead reliable and gave me years of trouble-free miles.  In the first several years of ownership I went through the carbs and electrics from stem to stern, removing corrosion and restoring functionality.  Over the years, the bike morphed and mutated somewhat as I replaced, swapped, and upgraded parts.  Wife got her own bike (first CB350 twin then upgraded to VT600VLX) and we rode many miles together.  After the initial bout of work to put everything in good order, little work was required.

     



Note, you can click any of the pics in this thread and then click "All Sizes" for a larger view.

mystic_1

[mystic_1]

Let me preface the following by saying that my bike already had the classic chain damage to the crankcase when I bought it, but it had been welded up and didn't leak.

So, fast forward to a few years ago, and I had an incident wherein (I believe) some debris got pulled through the chain and the final drive sprocket.  The chain came into contact with the "case protector" and bent the #$%* out of it.  It also pushed inward on the crankcase enough to cave it in by about half an inch.  The inner surface of the crankcase came into contact with one of the transmission gears and got ground to a fine powder which I later found in the oil  The chain did not break, nothing siezed, and while I heard the noise of the incident and saw oil dripping from the area when I stopped, the bike continued to run just fine.  Here's a pic of the damage, note the gap between the crankcase halves, in front of the final drive shaft.  Note, the chain marks were already present when purchased.



I must admit that I JB-welded a metal plate over the damaged area and kept riding it for the remainder of the season.  It didn't leak and the bike continued to run flawlessly however I found more metal shavings at oil change time so I knew that continuing to ride it was a bad idea.

So, time for another rebuild


   


mystic_1

[mystic_1]

On disassembly I also discovered that the outer final drive bearing's outer race had broken, liberating a piece that traced a wild path through the motor (I was able to follow the marks on the inside of the crankcase) until it was eventually crunched in between piston #4's lower skirt and the crankshaft lobe, on the downstroke.  Time for new pistons too  The fragment eventually came to rest in the lower sump, I found it inside one of the lower crankcase's bottom inspection/drain caps.  Note pics of mangled "case protector).  #4 piston is junk and there's some minor damage to the bottom of the #4 cylinder sleeve, but that's salvagable.  Fortunately there was no damage to the rest of the engine's internals.










mystic_1

[mystic_1]

My overall plan for this rebuild is to do a stock rebuild on the motor, do a propper chop job on the frame, and give the bike an overall makeover while staying true to it's heritage as a K0 CB750.  Only a few of the K0 signature parts were still present when I got heh bike, such as the instruments, side covers and oil tank.  The headlight ears, airbox, etc were long gone by that time.  Still, I will not get too radical with the redesign.  I will talk more about the triple tree and instruments in a later post but suffice it to say for now that I won't be using the K0 units.  

I'm totally an ametuer when it comes to this sort of work.  By day I am the IT manager of a small testing and calibration company.  I'm also a software developer and systems designer.  That said, I grew up working on cars with my dad and brother, so I know which end of the torque wrench to hit the nails with  The 750 was my first bike, but since owning it I've rebuild a couple of other bikes including a 1973 CB350 twin (the wife's first bike),  a couple of CM400T Honda Hawks, and a 1981 DOHC CB750.  I've also put a KZ550 back on the road (carb rebuild).  I've done a wide range of work on cars as well.  Over the years I've become the defacto go-to guy for maintenance on my friends' and families bikes.  Motorcycles and garage work are my escapism from cyberspace






All of this current work is occurring in my modest 1-car garage.  I rent a storage space where I keep all the spare bike parts.  I have a good collection of hand tools but I'm limited when it comes to machining.  I get by on most things with the drill press, grinder, ghetto mini-chop saw, etc.  Have a small 20g compressor under the bench and a Lincoln 4200HD MIG welder with gas.  Here's a pic of the workspace.




mystic_1

[mystic_1]

During this rebuild, I decided that I was going to do several things I wish I'd done the first time around, such as new primary chains, mild work on the ports (casting cleanup mostly), oil pump rebuild, polished engine fin tips, etc.  I also decided that it was time to do something about the bike's overall geometry.  As a semi-chop there are several problems.  First, steering trail was increased so steering was slowed and heavier.  The frame was tilted up by about at a 5 degree angle, raising the center of gravity.  As a result of the extended forks and the tilted frame, the steering has a fair amount of 'flop' to it.  This, combined with the increased  trail (currently 5.3 inches instead of stock 3.75 inches) and my short inseam length, makes low-speed maneuvering a bit of a challenge although I am very used to it after all these years.




I will, as part of this rebuild, be raking the frame in order to level the chassis.  My plan is to stretch the downtubes by approximately four inches.  At the same time, I will be addressing some other issues that the frame has, it's had a hard life!  PO cut off and moved the seat hinges when the seat was installed, front hinge was re-fabbed BADLY and had broken again by the time I bought the bike.  Right side centerstand tab had been cut off.  Kickstand mount had been moved slightly.  PO's welding skills left much to be desired.  Airbox mounts were missing.  I'll be taking care of all these issues, and more, as I rake the frame.  




I've spent a lot of time researching steering geometry theory, reading about mods other people have done, and toying with lots of "what if" scenarios.  Because I'm lowering the rear as well, my current plan results in a whopping 7.1" trail and a 38 degree steering angle rather than the stock 27 degrees, but results in a level frame.  I'm not thrilled about the seven inch trail but it does reduce my seat height quite a bit, a good thing since my inseam is only 30 inches and seat height is currently over 32 inches!  I may opt to stretch the backbone a couple of inches which would allow me to run less steering head angle and would therefore bring the trail down just a bit, but that's a lot more work and I don't want the thing to look like a mile-high OCC chopper.  If raked triple trees were easy to get for these bikes still, 5 degrees or so would solve my geometry issues but they are hard to come by nowadays.

       


mystic_1

[mystic_1]

I'm also making some other cosmetic changes.  Sissy bar was replaced with a NOS one.  Seat will be modded and re-covered, or replaced with a similar unit.  Fenders are being replace by HD Wide-Glide fenders front and rear.  Shocks are new MDI 11.5 inch chrome units.  Wheels will be Invader 5 spoke mags.  Fibre-mold tank is going back on, and I have new K0 side covers from LPM Replica plastics.  New paint is planned all around, metallic blue and black is the color scheme.  All visible fasteners will be replaced with either chrome or polished stainless allen or acorn fasteners as appropriate.  I've mocked up the frame mods using a damaged title-less frame I had in storage.










Some of the new parts.  I've recently acquired a 5-spoke front Invader so won't be running the 10-spoke wheel you'll see in some of the pics, but will be using the drilled rotors.

       

       

       





mystic_1

[mystic_1]

For the motor, I'm again going with a stock rebuild.  Performance mods don't make too much sense on this bike due to it's chassis geometry and it's riding style.  This is not a canyon-carver.  I started by eBaying a numberless replacement crankcasecase.  This is how they would have fixed this motor back in the 70's.  




I will keep the damaged K0 case for possible future repair, but I have my doubts as it's pretty bad off.  It had already been broken and welded in that area before I ever owned the bike, and the new cracks are pretty extensive.

I've sourced stock bore ART pistons, rings, wristpins and clips, all new internal chains and tensioners, crank, rod and transmission bearings, heavy-duty APE studs, all new external Allen fasteners, and of course new gaskets. The engine has already been completely disassembled, cleaned, and inspected.  I have a cache of 750 parts for spares.  Many small internal bits like locating dowels, transmission snaprings, and so on are also being replaced with NOS parts.







mystic_1

[mystic_1]

Due to life being a #$%*, this project has been dragging for some time.  Because money has been tight, sourcing all the parts to do the job "properly" has taken some time, and my time in the garage had been quite irregular until recently, but I have now (I think) collected all the bits and pieces and have a little more time to do the work these days, so I'm finally making progress.


   

   



Don't forget these critical tools:





For the first several posts of this thread, I'll start going into the work I've already accomplished while I was acquiring parts.  I'll try to keep the posts in somewhat chronological order but I skipped around from one area to another, so I'll consolidate information from time to time.

As of today I'm working on bottom-end reassembly of the motor.  Chassis work has been on hold due to the cold weather this winter but now that spring has sprung I'll be getting back to that soon.  



mystic_1

[mystic_1]

In closing for now, let me say that all the work I've done wouldn't have been possible without my wife and kids, who have been very supportive and infinitely patient as I labor on this project.  My wife has never once asked me if I should just get a new bike, she totally gets it.  My son Andrew is also involved in the build.  When I got the bike, he was just a wee tot.  Now he's 19 and helps me when he is in town (lives with his mother, my ex).  May younger son Karac is more an egghead than a wrenchmonkey, but contributes in other ways.  Everyone chips in with polishing and scrubbing parts from time to time.   Wife has even let me use the kitchen oven to dry parts, and enthusiastically encouraged me to take over the family room table to rebuild the motor this winter/spring.

Andrew:


   

Wife on her VLX:




I'd also like to send a "shout-out" to mkramer1121, who sold me a number of the NOS parts including the primary and cam chain gear, to dpender836 from whom I have purchased parts a number of times, Hondaman for his priceless book that I wish I had years ago, and to all of you members of SOHC4.net, who truly understand my obsession


More to come...

mystic_1

[andy750]

Wow Mystic what an epic first "project" post  - truly excellent detailing and what a great history behind your bike. I am signing up for this rebuild. If you have any plans to get ride of the 10-spoke invader wheel (since you have the 5-point one now) let me know .

Good luck
Andy

[754]

Cool project.. used to see lots of bikes like that..
 looks like you have had lots of fun and good times on that bike..

 I will keep an eye on this one..

[mystic_1]

Thanks for the comments guys.  I've been holding back so there's a lot I have to post to get us up do date.  Documenting this work has been part of the fun this time around for me.

Oh, and I'll be hanging on to the 10 spoke Invader, and also the extra 5 spoke 18" spool Invader I have, at least until I'm certain the 5 spoke disc brake Invader will work out.

754, Hondaman says it all with the inscription in my signed copy of his book.  "Miles of smiles."

mystic_1

[mystic_1]

So as I mentioned earlier, this rebuild has been dragging along for a few years now due to both budgetary and scheduluing reasons.  Recently however I have been inspired by Ron (MCRider) as far as setting aside a few hours per week to work on the bike.  He works on Sunday mornings, my time isn't quite as regimented but I figured that if he can manage a few hours per week, so can I.  That, combined with some life changes in the last year or so have given the project new momentum

As of today, the bike is completely stripped down to the frame and the motor has been completely disassembled.  




All parts were cleaned, bagged, and tagged during disassembly.  Related parts were stored together in plastic containers, which were then organized onto shelves in the garage.












I'll skip the motor disassembly for now and cover it in detail in a later post.  Here's a preview.

 

 

 

During the disassembly process I began making lists of parts that needed replacement, parts that I meerly wanted to replace, and parts that would be re-used.  Any damage was noted and a choice was made as to repair or replace each item.

After disassembly I started acquiring the parts that I knew I'd be needing.  A lot of the parts were purchased on eBay, but I also got parts from other forum members, at local swap-meets, and from various vendors we're all familiar with here on SOHC4.net.  Too much to show here all at once, see future posts for pics of the new and "old but new to me" stuff.


mystic_1

[mystic_1]

Let me take a moment to talk about the style of this bike.

As previously stated, this bike was given the "semi-chopper" treatment years ago, long before I bought it.  In fact, by the time I got my hands on it, it had been ridden hard and was in the late stages of Terminal Rust disease.  It looked to me as if the bike sat for a few years before I bought it but I have no idea of the history from before I bought it since the deal was done through a third party.

I do not really intend to change what the bike is, so to speak.  It's been a chopper for a long time and that's what it will remain.  As a semi-chopper it was never really all that impressive compared to the many custom-built and aftermarket framed choppers that were built using CB750 motors, and it will never compete in the same league as those fine machines.  On the other hand, many many semi-choppers were built back in the day as dealers cashed in on the chopper craze.  This bike, to me, represents a sort of homage to that trend and as such I want to leave it as such. 

Therefore I will not be making radical transformations to the bike, but will be refreshing everything, addressing some of the engineering problems, and improving the styling a bit (I hope).

Moving forward, I'll start by covering the chassis mods that I have planned, focusing on a particular subsystem in each post.  I'll then talk about the frame mods I'm doing.  After that I'll go into the motor disassembly and rebuild, which will bring us up to where I am today.


mystic_1

[mystic_1]

Sheet Metal / Body


Lots of pics in this post, again click any of them for larger versions.




When I bought it the front had been "bobbed" rather poorly, and the rear was both bent and badly corroded.  Back when I did the first rebuild I replaced the rear fender with something I basterdized out of a couple of different fenders.  This time around I want to do something better.  

   


After trolling various swap meets with my tape measure, I decided on a pair of Harley Davidson FXWG fenders from model years around 80 to 86.  Got 'em cheap, just needed to scrub off the surface rust from the seller leaving them in the rain at the swap meets a few too many times.

   

   

The front fender is 3.5 inches wide, and fits the stock CB750 front fender brace pretty darn well.  After drilling out the rivets and removing the stock fender, the FXWG front fender drops right on.  Two of the mounting holes are a bit off, I could have re-drilled the fender bracket but instead I opted to weld the fender holes closed and drill new ones.  Worked out quite nicely.

   

   




In the rear we again have an FXWG fender.  This had the shape that I wanted and the style of taillight that I was looking for.  It is 7 inches wide, whereas the space between the frame was about 6 and 3/4 inches.  This fender took a bit more work to make fit.  I had to squeeze it a bit in the front to clear the frame rails, and I used various hammers and dollies to shape the fender in this area to achieve the gaps and clearances I wanted.  Fitting this fender also required removal of the top rear frame cross member.  I'll talk more about this when I talk about the frame.

   

   



At the front of the rear fender, I found that the new fender dovetails quite nicely with the stock plastic inner fender.  This allows me to use the stock front lower mounting tabs on the inside of the frame.  In the final arrangement I will fasten the plastic inner fender to the leading edge of the metal fender with small fasteners.  I trimmed the front of the new fender to meet the frame rails nicely.  I might extend them a bit further to achieve a nicer line.

   




For the main fender mounts at the rear of the top frame rails, I carefully marked out and drilled new holes in the appropriate locations.  I made a template or jig out of some steel bar stock and indexed off of some of the pre-drilled holes in the fender.  This ensured accurate alignment of the holes.  I then welded fender washers to the inside of the fender, aligned with the holes.  I also welded shut some unneeded holes.  I have not yet decided if I will weld nuts to the inside of the washers or if I will use loose nuts and lock washers.

   

   

   


In the rear we have a stock FXWG tail light assembly.  Found some nice chrome button head allen fasteners at Ace hardware.  I plan to convert the light to LED.  Have not yet decided on a rear turn signal solution but those will be LED also.  Sorry, don't have a better pic of the actual tail light at the moment.




For a gas tank, I plan to repaint and remount my Fibre-mold fiberglass gas tank.  I stopped using this years ago because of it's limited fuel capacity but I do love how it looks on the bike.  I'll come up with something different to use when I take longer trips.  I have a couple of spare stock tanks to bash around with for that purpose.  The gas tank will be the subject of it's own post in the future.

   

   





Side covers are courtest of LPM Plastics.  These are nice repros of the stock side covers, my only complaint is that the lack the tabs to mount the screen behind the vents.  I'll be using epoxy I think to mount some small mounting studs to take care of this.  Again, this will be the subject of some future post of it's own.

   

   




More to come....

mystic_1

[MCRider]

Just posting to get updates. Haven't read what's written so far. Carry on!

[BIGKATDADDY]

really,really sweet!like the chopper look.love your project.the trike im sittin on is a 66 corvair that was triked out.i got some pics just trying to figure out how to post em on my profile.

[mystic_1]

really,really sweet!like the chopper look.love your project.the trike im sittin on is a 66 corvair that was triked out.i got some pics just trying to figure out how to post em on my profile.

Thanks for the kind words.  I hadn't realized it had been quite this long since I posted an update.  Will do so soon, work has been proceeding on the bike I just haven't taken the time to post up the pics and updates.

I just buttoned up the bottom end of the motor this weekend and am ready to start with top-end assembly.  Still have a fair amount of finishing work to do on the chassis.

mystic_1

[mystic_1]

So, I've been feeling a bit down lately about the slow progress on my build.  In an effort to boost my morale/gumption, I decided to do a dry-fit on the motor to preview how it will look when I'm done.  At this point the bottom end is more or less complete but I still have the whole of top-end assembly to do.  The top-end parts are just set into place so I can get a preview.

Standing back and looking at it all, I'm now feeling better about things but still wish I was moving quicker.

Here she be:























I'll go into more detail in a future post.

mystic_1

[MCRider]

Hey there, lookin good. I've mocked mine up several times too, some just for inspiration but some times useful. The mockup allows you to think and maybe see what may be missing and need work or procurement.

I was talked out of polishing my fins, but i really like it, may still do it. For me just my cylinders are black, head is case color.

Was tossing around what to do with the oil filter cover, black may be it.
My HD primarty chains are in the states, but may be another week to get them to me. That's my major holdup.

Carry on!

[mickey6]

you seem to be making great progress.
Just curious what soft wear did you use for your plans?

[MCRider]

I've got the same engine fins too. Somewhere there is a points cover with fins tilted to match the jugs, but they are very rare.

I had the same tappet covers, like em, but going with some H Abe's I found that have radial fins for a change.

[Joel]

My 400F came to me with the same modifications yours had.  It's not my style so I've removed them.    The engine looks fantastic.  I like the black fins with silver tips.  I'll be doing that to mine if I get the chance to rebuild the engine.

[BIGKATDADDY]

darn good looking engine.

[mystic_1]

Thanks for the comments guys.  I'm pretty pleased with the polished fin tips, that's something I've always wanted to do.  Like lots of other things, the key was in the preparation. 

mickey6, believe it or not, all of the tech drawings above were done using the drawing tools in Microsoft Word.  All of them were done at a 1"=10" scale so they are reasonably accurate.  Certainly not CAD quality but acceptable for what I was trying to accomplish.

For the frame drawings, I took a scan of the frame dimension drawing from the workshop manual, pasted it into a Word document as a JPG, traced over all of the lines with the drawing tools, then deleted the JPG.  This left me with a duplicate of the frame drawing that I could manipulate on a line-by-line basis.  This enabled me to group drawing objects comprising the front of the frame and then move/rotate them as necessary.

For some of the chassis parts, I drew them up from scratch based on measurements of the parts.  For some things like the rear fender, seat, side covers, and tank, I used a combination of the two approaches mentioned above.  First I took some pics with a camera and traced over them, then I manipulated the resulting lines based on actual measurements.  This corrected for the inaccuracies in the pics due to angles and perspective.

Once I had all the components drawn it was fairly straight-forward to drag-and-drop them into place, rotating them to fit as necessary.  From there I was able to play Mr Potato Head with things to determine how much I wanted to mod the frame, and was able to get an estimate of the final rake and trail figures based on the shorter rear shocks and the modified frame downtubes.  It was also a great help in making some aesthetic choices.

I've attached a copy of the Word document with the stock frame diagram, traced over with the line-drawing tools, and also a copy of the word document with the stock suspension components depicted.  These were the basis for all of above diagrams and also for my current avatar pic.  Also attached is a JPG version of the dimension drawing I used to trace over to produce these.

cheers
mystic_1