Topic: CB750 K8 Build

[firebane]

I thought I had started a thread on this but I can't seem to find it so I figure I'll start it now.

I bought this bike in November of 2015 and was hoping to get it on the road in a timely fashion but unfortunately between work and life its been taking a bit longer than normal. In the time I have owned it it has gone through a lot of updates and been cleaned up a lot.

It is still a work in progress and recently I discovered that the swing arm was lengthened by at least 3-3.5" so that is the current battle in the bike.

Day 1 of bike being home:


Cut the frame and pulled the valve cover as this thing was being held on by gobs of orange rtv and was leaking everywhere. Also discovered a few tappet nuts were missing and fixed that all up.


Pulled off the ugly exhaust that the bike came with (I have a 4-1 replacing this now)


Pulled the stock bars and put some renthal ultra lows on


Not sure what the purpose of this was but it really makes the back of this bike look horrible


Discovered with the help of some awesome forum members the swing was extended by 3-3.5" and following this found everything related to the rear brake drum setup was also extended. With this extra 3-3.5" of extension putting this bike onto its center stand was a living miserable of a suck job to the point I hurt my back the first time.


New wiring harness attached and needing tidied up.


Swing arm yanked out and cut that atrocious weld job off the back of the bike


Current state of bike.. Front forks have been cleaned and painted black removing the horrible blue (fork dust covers to be installed later). Hoping once I replace the swing arm and all hardware that I will then continue on with the wiring and getting the bike running. At this point I'm not expecting this to happen till probably spring and be able to ride for next season.

[madmtnmotors]

With this extra 3-3.5" of extension putting this bike onto its center stand was a living miserable of a suck job to the point I hurt my back the first time.

That pretty much describes putting a stock CB750 on the centerstand as well! The motion to lift it onto the centerstand is somewhat unnatural. I will face the side of the bike, left hand on the handle bar, right hand on the grab rail or under the seat, just behind the upper shock mount. I then place the ball of my left foot on the centerstand lever. Most of the force is applied downward with my left foot with some lifting force with my right hand and a little pulling force with my left hand. 

[SOHC4 Cafe Racer Fan]

That swingarm looked all kinds of wrong.  Even if dragging is your goal, THAT is not the way to do it.
I'm glad to hear you are rectifying the errors of the PO.

[firebane]

That swingarm looked all kinds of wrong.  Even if dragging is your goal, THAT is not the way to do it.
I'm glad to hear you are rectifying the errors of the PO.

Its difficult finding these bikes in good shape anymore and I'm pretty finicky and like things to be right. Even if I chose to sell this bike right now I would feel all sorts of wrong knowing I sold a bike with sketchy parts on it.

So the hunt right now is on for a new swing arm and either another frame to repair this one or I'm going to just rebuild the back end of this thing.

[Desert-SOHC]

That pretty much describes putting a stock CB750 on the centerstand as well! The motion to lift it onto the centerstand is somewhat unnatural. I will face the side of the bike, left hand on the handle bar, right hand on the grab rail or under the seat, just behind the upper shock mount. I then place the ball of my left foot on the centerstand lever. Most of the force is applied downward with my left foot with some lifting force with my right hand and a little pulling force with my left hand. 

Mine is lowered and I roll the rear tire up on a 2x4 and a 1x4....takes no effort.

[Retro Rocket]

Make sure you get a K7/K8 swingarm to replace that one, they are wider on the left hand side than the early K models {10mm} ....

[calj737]

Send a PM to Bill Benton, likely he will have a good quality spare swing arm.

[firebane]

Found a swing arm on ebay for $50 shipped to my door and have bronze bushings on the way too.

I know the bolt on the drum side for the brake stay is shouldered, but is the head of that bolt special? I have looked around and it looks like a normal bolt

[Retro Rocket]

Found a swing arm on ebay for $50 shipped to my door and have bronze bushings on the way too.

I know the bolt on the drum side for the brake stay is shouldered, but is the head of that bolt special? I have looked around and it looks like a normal bolt

You should ask here first mate, you'll probably be able to get everything you need from the forum, and at a very reasonable price...
Was the swingarm you bought specifically a K7/K8 arm ...?

[firebane]

Found a swing arm on ebay for $50 shipped to my door and have bronze bushings on the way too.

I know the bolt on the drum side for the brake stay is shouldered, but is the head of that bolt special? I have looked around and it looks like a normal bolt

You should ask here first mate, you'll probably be able to get everything you need from the forum, and at a very reasonable price...
Was the swingarm you bought specifically a K7/K8 arm ...?

It is specific. Issue is that I'm Canadian so shipping plus exchange rate generally kills me. So finding one for $50 shipped on Ebay was a great price. I asked someone on here and they quoted me a great price for the arm but $60 for shipping.

[Retro Rocket]

Found a swing arm on ebay for $50 shipped to my door and have bronze bushings on the way too.

I know the bolt on the drum side for the brake stay is shouldered, but is the head of that bolt special? I have looked around and it looks like a normal bolt

You should ask here first mate, you'll probably be able to get everything you need from the forum, and at a very reasonable price...
Was the swingarm you bought specifically a K7/K8 arm ...?

It is specific. Issue is that I'm Canadian so shipping plus exchange rate generally kills me. So finding one for $50 shipped on Ebay was a great price. I asked someone on here and they quoted me a great price for the arm but $60 for shipping.

Cool, good result...

[firebane]

So I've had some time lately and was able to get that swing arm from ebay. The final measurement of extension is exactly 3" longer over stock!!

Swingarm that came with the bike is on the left and the proper one is on the right:


Showing the difference in length between the two

[firebane]

So here is the next conundrum in my story. I recently picked up a hoop to tie up the back end of my bike that required hacking off a lot of crap from the previous owner. Unfortunately in doing this you really find out how bad things are and now I'm at a point where I have to decide what I want to do.

My first choice is to weld the hoop on and put some new plate over both sides to tie everything together. My second choice is completely getting rid of the shock stud and all applicable metal and either installing something like cognito motos shock stud kit (https://cognitomoto.com/products/shock-mounting-kit) or coming up with my own means.

Here are some pics of just the left side as I haven't started on the right till I decide what I want to do or where I go. I will also be the first to admit I may have cut out a bit more than needed on this side and if I do the right side I have a better idea of how to do things.

[calj737]

Cut the stock gusset off clean to the triangular brace, weld in a new plate with a stud inserted, weld on your new hoop and tie everything in together. I'd encourage you to weld a slug into the existing frame and slip your hoop over it. Leave a 1/16" gape between your hoop and the frame, and weld it up tight. Stronger and cleaner.

[firebane]

Cut the stock gusset off clean to the triangular brace, weld in a new plate with a stud inserted, weld on your new hoop and tie everything in together. I'd encourage you to weld a slug into the existing frame and slip your hoop over it. Leave a 1/16" gape between your hoop and the frame, and weld it up tight. Stronger and cleaner.

You share my thoughts. But that then leaves one concern.... When I put in a new stud how does one know where to place it so it's as close to stick as possible? I have seen drawings showing measurements but most of the references to help me are gone on the bike.

Maybe I will seek help with another post asking for references

[calj737]

Just measure the location of the stock unit before cutting it off. Even if your new stud is off by 1/4", it won't matter. Being in the proper plane relative to the swing arm shock mount does matter.

[70CB750]

Following

[firebane]

Stage 1... Complete. Offending metal is gone. Thanks to harisluv I got the hoop and slugs put in but not yet welded yet. Also in place is the new and proper swing arm that isn't extended by 3".  I took an original seat pan and now found out well its far too long so a custom seat will be play now lol! I now have a cb750f complete seat, cb750k pan and a seat from a unknown bike.

Next up will be getting some metal and redoing shock mounts and I'm sure that will be fun.
 

[firebane]

So figure it was time to add some updates since I've had some time to work on the bike again. I have figured out a secure method of reattaching the shocks to the frame and it seems that it should work well. Here are some pics of the progress.

What you see is a piece of 5/8 rod which is the diameter of the shock bushing hole, a few collars, and some bushings. I don't have access to high end equipment so all of this is being done with a grinder with a grinding wheel and a cut off wheel with a small 110v welder.

I haven't decided how I want to secure the shock yet... I'm debating one of two means. First means is drilling the center of the rod and using either a m6 or m8 socket cap bolt with washer or use the 5/8 rod and thread it for a flanged acorn nut... thoughts? I also realize the right shock is leaning WAY out. I'm verifying side to side by placing a square on the rod being lifted by the jack and squaring from that to the side of the shock.

I am also taking feedback if you guys think the left shock can be brought in just a tad or if perhaps where it mounts to the frame needs to be moved back a bit. Once I have everything figured out to secure the shock I will be getting some 14g or 16g steel to make a plate for welding to the frame and putting the sleeve inside to hold everything snug.

Side profile:


3/4 Rear View:


Underside View:


Rear View:

[calj737]

Your shocks should be plumb from bottom to top. A forward lean angle is fine, but they can't be splayed outward as you have them.

Thread the end of the rod and secure it with a proper nut. As for the 14-16ga steel as a mounting plate, thats way too thin and weak. 1/4" minimum. I appreciate that a 110v welder won't weld that up straight away, but you have a couple of choices: have someone else weld that aspect of the frame properly, OR, if you can't then-

Use some 3/8" plate. Bevel the outside and inside edge so the portion in contact with the frame is about 1/8". Then weld a single thin pass on the outside, and again on the inside. Then repeat with a "cover pass". The 1/8" contact portion is a "landing area" and the bevel allows you to get enough weld in with appropriate penetration to fuse the gusset to the frame.

Shock mounts take significant stress. This is NOT a place to DIY and wing-it. Got to be safe, and strong.

[firebane]

I appreciate the feed back but the original steel for shock mounts is about 14g.. maybe closer to 13g but I don't have a great piece to test off of. The piece I did have came in around 0.06xx inches. I realize you can go thicker but putting 1/2 inch of steel on the back is crazy.

My plan is to use either 14g or 16g and fold it over like the original piece and then weld that to the bike.

I know about the shocks I wrote about that above lol.

[calj737]

What is "crazy" is trying to shortcut on structurally important aspects of a vehicle that your life depends upon with a 110v welder. If you rent. Competent and experienced welder, then you should leave those modifications and repairtls to someone who is.

Your choices of materials are completely wrong and that's an indication that your lack of experience calls out for good advice. But it's your bike and your neck. I hope you're safe and that shoukd you ever have a failure that neither you nor anyone around you is injured.

Owning a welding machine and knowing how to operate it doesn't make you a welder or fabricator. It just makes you handy and dangerous.

[firebane]

What is "crazy" is trying to shortcut on structurally important aspects of a vehicle that your life depends upon with a 110v welder. If you rent. Competent and experienced welder, then you should leave those modifications and repairtls to someone who is.

Your choices of materials are completely wrong and that's an indication that your lack of experience calls out for good advice. But it's your bike and your neck. I hope you're safe and that shoukd you ever have a failure that neither you nor anyone around you is injured.

Owning a welding machine and knowing how to operate it doesn't make you a welder or fabricator. It just makes you handy and dangerous.

I never said I was going to do the final welding on this as I know my limitations. I can do everything up to that part and ensure its welded correctly and properly.

I'm not sure how your saying my choice of materials is wrong? I'm using the exact same materials that the original pieces were made of so I'm not sure why you are making that assumption?

[540nova]

Personally, I would have left the stock mounts, and worked around it.


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[slikwilli420]

Interested to see where this goes as I may have to redo some shock mounts. The stock material is sheet metal and is pretty substantial. I usually agree with Cal, but I cannot see how using 1/4" plate would be even remotely necessary. Many bikes over the years have used sheet metal in that gauge range for shock and motor/swingarm mounts. Its all in the prep and actual welding.

[calj737]

Reconstructing 40 year old manufacturing techniques has little practicality. If this approach were still prudent, it would still be in use. These frames are notoriously flimsy and the metal weld used were bulk manufacturing techniques. Simple as that.

s for 1/4" or greater plate, as I said, bevel it and then you get full penetration of a 2x edge against a thin tube. The purpose of using thicker than original plate is to provide stronger load bearing support against the remainder of stock thin tubing.

As I said, your bike, your neck. In my opinion these types of choices are exactly what future owners point out when abasing POs and their hack jobs. I guess I would rather have something very strong than maybe strong enough. And I build to "better than" stock specifications. Guess I'm funny that way.

[firebane]

Reconstructing 40 year old manufacturing techniques has little practicality. If this approach were still prudent, it would still be in use. These frames are notoriously flimsy and the metal weld used were bulk manufacturing techniques. Simple as that.

s for 1/4" or greater plate, as I said, bevel it and then you get full penetration of a 2x edge against a thin tube. The purpose of using thicker than original plate is to provide stronger load bearing support against the remainder of stock thin tubing.

As I said, your bike, your neck. In my opinion these types of choices are exactly what future owners point out when abasing POs and their hack jobs. I guess I would rather have something very strong than maybe strong enough. And I build to "better than" stock specifications. Guess I'm funny that way.

I'm not here to create an argument or have one and I am listening to your valid points. But if a 40 year old bike with 40 year welds on 14-16g sheet metal is used for shock mounts and it was that much of a safety concern we would see a lot more 40 year old bikes with broken shock mount plates or shock mount issues. When generally the only shock mount issues I've seen is the thread bolt part gets broken off due to rust or over tightening.

Like I said earlier adding 1/2" of steel to support the shocks when there isn't a shred of structural support on the bike made from 1/4" just seems silly. You would be adding 1" of extra material onto the back of the bike.

If you can show me any instances where stock setup has failed in the shock area I would be glad to see it but I would say that it was engineered well and as long as its done the same way again it should be right back to where it is.

I will heed your warnings and your feedback and thoughts and take it into consideration.

[calj737]

I never told you to use 1/2", I said 1/4"-3/8". And the reason is simple, it's about the ratio of metal to metal when welding thin tubing to thicker metal. I'll give you an example, then I'll give you my "rationale" for my position.

Your tubing is 0.060. If you use 1/4" plate, bevel 1/16" from each edge. That leaves 1/8" landing area. That is 2x the thickness of the tubing. When you weld something that is 0.060 thick, you can not get any penetration without burning through. Using 1/4" (or more) allows heavy penetration against the plate, with a tie-in to the tubing. This is very strong and allows the plate to support the tubing.

Welding 0.060 wall to 0.060 sheet gives you nothing to tie to. That weld will be very weak. In fact, depending upon how it's done, it will either be cold, or crack adjacent, and likely fail any structural stress test. The original was a stamped gusset welded on an assembly line and tied into the rails where you have cut it.

I literally just did this same modification 3 weeks ago for a friend of my son, on his KZ400. When I showed him the sample pieces (welding my approach versus the stock gussets) after saw cuts and stress tests, he was shocked (pardon the pun) at the weakness in the gussets. Had I known this topic would surface, I would have taken pictures as they would now be quite informative to the point.

Now, why do I harp on this? A few years back I was fortunate enough to be introduced to a local guy who for 28+ years has been building spec race car tube chassis's for NASCAR, SCCA, etc. The guy is a prominent builder. He was kind enough to make a subframe for me because a mutual friend took me to him. It was he, that explained the importance of suspension gussets and the flaw in the stamped gussets. So I follow his professional advice on frame mods, bracing, and material thicknesses without question as he is the expert.

I'll say it one more time; it's your bike, do as you wish and as you are comfortable. When I see something that I strongly believe is not prudent or sound, I'm going to speak up. Good luck with your project.

[754]

I could be wrong here but generally welding a 4x thicker piece to another can create a place for stresses to concentrate, and that can lead to breakage.
 That is why they use doublers and sometimes layers.
 Look at the way the sidestand mount is welded to a 750 frame...do they just weld a heavy piece on ? No they double it up and weld the thick piece to one or both of the reinforcement pieces.
 I have not read thru all of this, but would like to see these 1/4 mounts compared to the stock boxed in section.
 What I should mention if I were to relocate shock mounts, if possible I would machine the two mounts out of one piece of bar, and part the material most of the way thru..where they would get cut off . Then install the whole assemble if possible, keeping everything in plane.. Then tack or finish weld..then cut the centre part out.

[firebane]

Someone decided to want my bike as is so I sold the 750 and that saga ends but a new saga begins as I now own a 76 cb550f