Topic: 1971 CB750 with GSXR Front End

[GNGSXR]

Time to start a build thread.

I have lots of experience with removing stock parts and putting new parts on, basic bolt on performance modifications, routine maintenance, and some slight vehicle alterations from stock.  I have never done something like this CB750 build. With that said being, I am completely open to criticism, advice, help or anything of the sort from everyone on the forum. I have gathered a lot of information from this site and other places. I will try to post/refer to other build threads to help consolidate information to help people who want to do something similar to their bikes.

My original plan was to ride this �71 CB750 for a few months before the snow started falling because it is mechanically/electrically perfect. I wanted to get that classic motorcycle feel for a while. I ended up acquiring a lot of the GSXR pieces very quickly at a really low cost so I decided to start tearing the bike down as soon as possible. Also the stock bike, even in good condition, handles like a bull with one leg shorter than the other so I immediately wanted to see if I can improve it by what purists would call �butchering� it.

Here is the bike the way I got it. It shows 51,xxx miles. Frame/motor are �71 but some members on this site informed me that some of the other pieces on the bike are from a �76.

Goal for this winter:
�Complete �06 GSXR front end including rims/dual disc brakes.  I will upgrade the brake lines to steel braided and I have not yet decided what I will do to the controls, and speedometer cable.  Gauges will most likely be some Dime City 2.5�.
�Rear GSXR rim with a 180 tire. Again�..no sure how I will make this work but this will be an on going thread. I want to use the stock swing arm. I like the look of it and I am not a fan of these bikes having massive aluminum swing arms from a newer donor bike. The small width of the arm with a tire that clearly is too big for the small amount of horsepower just looks amazing to me to a certain extent. UPDATE. Decided to use a CBR rim with max size of 160mm. Not as ridiculous as a 180. See below.
�Rear disc brake from already mentioned donor bike.

If I can still feed my girlfriend and I after those goals (assuming she hasn�t left me yet) then I will strip the bike down for powder coating/paint of the frame/other parts and rebuild the engine/carbs/electrical system if necessary. I plan on riding this bike from Colorado-California-Alaska and back so the engine build will mostly be stock. 

I will be selling most of the parts that are removed on the bike. I will most likely start selling them in a month or so.

[GNGSXR]

Here are all the 2006 GSXR parts I picked up off craigslist. All of it was from one ad and one trip.



The rear swing arm will not be on the CB. I purchased it for the rear axle and brake setup.

wow sorry for the massive picture. I'll resize them from now on.

[GNGSXR]

Front end tear down.

[spazz1984]

Why don't u want to use the swingarm? 

Sent from my SPH-L710 using Tapatalk 2

[GNGSXR]

I don't like the look of a big massive swing arm on a bike with a small diamter tubular frame.

[spazz1984]

I don't know this is my 73 I think with that wide rim will look funny

Sent from my SPH-L710 using Tapatalk 2

[GNGSXR]

Problem #1  GSXR Stem is too short for the CB750 frame
This thread http://forums.sohc4.net/index.php?topic=27159.0 explains the stem diameter issues which is a simple fix by contacting https://www.allballsracing.com/. They will provide you with the correct bearings to match your donor bikes stem to the frame of your choice. FunJimmy's build shows that it is a direct bolt on scenario FOR A CB550.

The problem for a CB750 though is that frame head length is longer than a CB550. Yes the bearings are sized correctly from allballs but the GSXR stem length is about a ½ inch short to work in the CB750  frame.  Allballs warns you of this in bold letters on their website as a disclaimer and I completely ignored it.  I could make some modifications to at least get a few threads showing for the upper nut but I would not be comfortable with that since the GSXR stem uses two nuts under the top triple.

I have found one small company that is in the process of producing a custom stem to the required length for a CB750 frame. It has the same diameter as the stock GSXR stem but it makes up the ½ inch that is needed to work in the frame. It is not advertised on their main website yet but his facebook page mentions it. He also has some killer custom hubs to use spoked wheels with the gsxr front end and some other trick pieces.
http://cognitomoto.com/collections/cognito-moto-designs/products/gsx-r-fork-on-honda-cb750-frame-conversion-stem

Solution #1
I decided to use the stock CB750 stem and then order stock 71 cb750 steering bearings from allballs.

I don’t have any pictures of this process but I’ll explain it the best I can. The stock CB750 stem is pressed and welded on the bottom of the triple. You need to grind this weld off then it is pressed out using a 20 ton hydraulic press. The GSXR stem has a collar on its base and needs to be pressed out as well through the bottom of the triple. That’s it to get the stems free from their triples.

Problem #2 Stem diameters

My GSXR stem is 30.25mm at its base. My CB750 stem was ~29mm. This will not allow for a tight fit in the GSXR triple.

Solution #2
Industrial Cafe converts the CB750 stem into the GSXR lower triple by stretching the CB stem at the base to create the appropriate tight fit in the gsxr triple. http://forums.sohc4.net/index.php?topic=63784.0

Another solution is here. http://www.dotheton.com/forum/index.php?topic=34387.0 He modifies the upper head with a dremmel to get more of the GSXR stem to reveal its threads. I don’t recommend this but he seems capable of pulling it off.

We added 6 bead welds to the base of my  CB750 stem. This was done to increase the stems girth.

Sorry for the blurry pic. I was drinking.

We then used a lathe to get the stem to our desired diameter.

Reusing same excuse for blurry pic.

We then cut a collar out of some steel to fit on the base of CB stem and welded it on. This isn’t really necessary but we decided to add it on to have a similar design to the GSXR stem.

CB750 Stem on left.


Side by side full length. They aren’t lined up perfectly but you can still see the length difference.



Then it was just a matter of pressing the modified CB stem into the GSXR triple.
Here is the bottom of it pressed in.


Finished product. I need to do a bit of sanding on the base near the triple then wait for the new bearings to come in.

[754]

Perhaps you need stretched and widened stock arm for this build.. Like so did this in the eighties

[GNGSXR]

Perhaps you need stretched and widened stock arm for this build.. Like so did this in the eighties

do you have any information on it other than the picture?

[754]

If you weld, you should have full contact.
 Alternate way.
 V the end of stem weld on a flange to hold stem from pulling thru...then machine adaptor sleeve to press fit on stem to take up size difference in the tree area.

 I stretched that arm..very simple if you do not run shocks, a bit more work to have working pivots . I can give more info later ..got to go offline a few hours..

[spazz1984]

For the steering stem I used a cbr929 and add only about 10mm plug to it

Sent from my SPH-L710 using Tapatalk 2

[GNGSXR]

The front end is on hold for now while I wait for a few little odds and ends. I may mock it up this weekend using the old races in the steering head. I removed the center stand and other pieces on the back and test fitted the '06 GSXR wheel with 180mm tire. I just used a long screw driver as an axle because the GSXR axle and stock CBaxle will not fit in the swing arm due to various reasons.


The stock GSXR axle is too wide for the swing arm.


I might be able to reuse the stock axle (i dont want too) but the rim is maxing out the swing arm length.





A little difficult to see but the tire is touching the swing arm.


Right Side tire against the swing arm.


This is obviously a very crude mock up. The tire could be adjusted slightly but regardless this will not work without some modifications. I do have another set of GSXR rims of the same year with a taller profile DOT Race tire mounted that I will test fit. I think a 160mm on this rim might work. The 160mm will give me roughly .39 inches on each side.

I still need to figure out the rear disc setup and sprocket which will then lead to a front offset sprocket.

I cam across this build from dimecity. http://www.dimecitycycles.com/dccdiatribe/2011/07/recently-finished-resto-mod-cb750-sohc/
Anyone have any information on it? The build is almost identical to what I am doing and i have no issues with copying what someone else did. I am not trying to reinvent the wheel with my build. I'd really like to identify the rear rim they used. 

[spazz1984]

The offset is about 5/8 to 3/4 but that depends on how wide of a rim u have

Sent from my SPH-L710 using Tapatalk 2

[spazz1984]

I used a 3/8 offset front sprocket from cycle x n offseted the rear end to make it work

Sent from my SPH-L710 using Tapatalk 2

[754]

Easiest thing to do with the swingarm is grind out a section where the tire will hit itshould be avpble to go at least 1/4 inch into the thicker part of the arm. Then weld a small, patch in.using same thicness material as swing arm.  Easy..
 To use stock axle, make stepped spacers that fit into the bigger bearings and fit your axle size..

[GNGSXR]

I'll be using this thread to sort out my ideas which can also be viewed as random rambling especially when the IPAs start to take over. Yes it is 11:21 am my time right now.

After doing some searching I discovered that the later model CB750s were equipped with slightly longer and wider swing arms. http://www.sohc4.net/cb750-timeline/ CB750 timeline shows 1976 is when it received a "stronger" swing arm but I can't seem to find any measurements on it to verify if it is truly longer and wider.

Found the information I wanted to know. http://forums.sohc4.net/index.php?topic=34880.0
The above link has pictures of a 74, ,77, and 80 or 81 DOHC swing arm side by side with measurements. FYI for anyone that doesnt know this you can use google to search within a specific site. The search function embedded on this site is a little flaky at times or I might not be using it correctly. Google instructions http://www.google.com/insidesearch/tipstricks/all.html

I also realized that the CB550 and DOHC CB750 swing arms are tubular which in my opinion would look better on my bike.

New goal is to swap a DOHC CB750 swing arm onto my bike. If anyone knows of one for sale let me know!
http://forums.sohc4.net/index.php?topic=46599.0;all Industrial Cafe has already done the hard work for me. I just hope it will give me the clearance I need for the disc brake and larger rim/tire combination I will be using. This will stretch my wheelbase by about 2.4" +or- a bit depending on axle placement. I was worried how this would look but it goes well on Industrial Cafe's bike. 

[GNGSXR]

Test fitting most of the front end. I know this is getting to be a popular swap but i still feel it is a gorgeous retro-modern setup regardless of how many people are doing it.



The side to side lock out isn't great. The right side is stopped by the horn bracket which is ok for now but the left fork hits the tank on a full left turn. I'll figure out some new lower triple stops later.

This is the upper stock gsxr triple with the CB750 stem. I will either cut off the ignition slot and smooth out the triple or get a custom piece. There is also a gap between the triple and cb750 stem.

[754]

You need to fill that gap.. Machined spacer is best, or even shim stock is better than the problems that wil arise.
 If the lower part of rhe stem is only fitting on those 4 weld strips, and there is movement up top, as you ride it wilm beat the parts up bad over a fairly short time.
 Wish my camera worked, would make a drawing of the stem conversion.. Easier to see it.
 But basically stretch the lower part of stem like you did.. Minus the weld strips.  So weld extension on..then machine back to stock diameter. Then fit a sleeve over that part that goes inside lower tree, and another sleeve for the top ..and done.  Tight, proper fit.

[GNGSXR]

there are 6 weld strips on the base creating a very tight fit in the triple but i am still not comfortable with it. I'll probably add more material to it along with a shim at the top or get a custom GSXR stem with the stock diameter but stretched to the length i need.

[GNGSXR]

Bike has been delayed mostly due to my own indecisiveness with parts and how I want the bike to function when complete. Here is a little update.

I ended up using a '76 CB550 swing arm. The 550 is on the right

Overall the cb550 swing arm has the same dimensions as the CB750 but the 550 is slightly longer. I switched swing arms because the tubular steel looks better and flows with the rest of the frame. The stock stamped steel swing arm doesn't fit the look of the stock tubular frame. I also wanted a slightly longer swing arm since the stock '71 CB750 swing arms are too short in my opinion.

I picked up an entire 1996 CBR600 F3 for dirt cheap. Fairings and air box pieces are in the shed.

I am using the F3 rear rim on the cb550 swing arm. The tire is a 170 but the stock size for this rim is a 160 which I will eventually switch to.
The main benefit of using this F3 rim is the axle diameter is identical to the stock CB axle. I will need to offset the front sprocket, switch to a 530 chain, machine the rear hub carrier, determine/make axle spacers, and figure out a rear caliper mount.



I ended up ordering this custom stem from an awesome company. The company specializes in these honda-gsxr conversions but also carry a lot of other parts for these bikes. Stem link http://cognitomoto.com/collections/gsxr-conversion-parts/products/copy-of-copy-of-copy-of-gsx-r-fork-on-cb750-frame-conversion-stem

I decided not to go with the modified stock CB stem. The stem from Cognito is exactly the same as the stock GSXR stem but lengthened to match the CB stem length to fit properly in the CB frame.
DISCLAIMER: The following pictures are not the final installation of the stem. The pictures show two washers sittng on top of the bottom race. I only ended up using one washer to get the correct spacing at the base of the stem and to get a few more threads exposed at the top.


I forgot to take another set of pictures showing one washer and a few more threads exposed at the top but trust me when I say it all fits perfectly now.

Here are the Allballsracing part numbers I used for the stem bearings.


I installed steel braided brake lines on the front. I opted to use two lines from the master cylinder instead of a T fitting or the double banjo on one of the calipers. You can also see the black headlight mounts that i was going to use but I've decided to go a different direction.


I also mounted the steering damper and used zip ties to mock it up. I am not sure what headlight assembly i will be using. I would like to keep all mounting off the forks to "show off" as much of the gold tubes as possible which means the headlight will need to be a bottom center mount. I'll need to make a bracket to accomodate the damper and headlight.

Picture of the bike (prior to me mounting the steel braided brake lines).


I was uneasy about having two different style rims on the bike. Luckily though the giant front rotors hide most of the front rim so I will leave it as is.

Other than the stuff I have mentioned above, I will need to pull the trigger on some electronic gauges and wire in the GSXR clip on controls. The controls and LED lighting are making me lean towards creating a wire harness from scratch.

[Tews19]

The stance looks really good. Great work and keep updates coming!!!..

[Bigsam302]

GNGSXR,

I love your build!  I've wanted to do the exact same swingarm swap on my CB750.  Are you concerned with the weight and HP over the CB550?  The 750 swingarm is a lot beefier than the 550.  Are you worried about it flexing?

I agree, the tubular swingarm does flow a lot better!

[Simple]

Cool project, nice GN.

[ohiocaferacer]

Nice project!!

[GNGSXR]

GNGSXR,

I love your build!  I've wanted to do the exact same swingarm swap on my CB750.  Are you concerned with the weight and HP over the CB550?  The 750 swingarm is a lot beefier than the 550.  Are you worried about it flexing?

I agree, the tubular swingarm does flow a lot better!

The DOHC CB750 switched to a tubular swing arm as well. http://forums.sohc4.net/index.php?topic=34880.0 This link shows photos of a '74, '77 and DOHC swing arm side by side. There is more metal near the pivot point on the DOHC swinger compared to the CB550 but the DOHC is a substantially heavier bike. You can also see the same difference in the amount of "meat" near the pivot point on my side by side picture too. hmmmmmm....
I wish i would of weighed the bike prior to stripping all the stock parts off. The stock front rotors alone were at least 2-3 times heavier than the GSXR rotors that are on it now. The rims are A LOT lighter too especially at the rear. I wouldn't be surprised if this bike weighs 30-50 lbs less than stock once I am done with everything.
The nice thing about this swap is the CB550 swing arm bolts into my CB750 frame with no modification at all. The DOHC swing arm would of required me to grind off a bit from it's left side to get it to mount in the frame.

Am I worried about flexing? Not really. The significant difference in metal near the pivot point does worry me a bit but I am hoping these swing arms weren't designed by Honda with that small of a strength tolerance difference. If anyone out there can prove me right or wrong then please do!
I am reducing the weight significantly at the back end of the bike compared to stock which in turn reduces the stress experienced at the pivot point. Yes there is a hp difference but the weight reduction to hp increase on the CB550 swing arm shouldn't be enough to experience a negative impact on the swing arm but I am shooting from the hip here. I'll have real world experiences soon enough.

I do have access to CAD. I am a mechanical engineer during normal business hours. Once work slows down I could model the different swing arms and do a finite element analysis stock vs CB550. That may be more work than I have the motivation for though.
Does anyone know if someone has already fully modeled our bikes in CAD? That would make it a lot easier to answer stuff like this and I'd be more than happy to run a model on it.