Author Topic: My 1975 cb 750 Build (Read 17659 times)

PPG9944 · « **on:** September 28, 2014, 05:02:26 PM »

Hello,

I've been lurking around the site for a little bit asking questions here and there but I figured i would start a thread and hopefully use it to get answers to my questions.

So the story starts with this pile that I picked up for $300 bucks

The Previous owner was apparently storing the bike for his buddy who was hiding it from his wife. Seemed kinda sketchy but he had the title so whatever, off I went stoked on this.

The bike looked pretty clean but the engine was seized and I noticed a few other quirks.

I'm pretty sure the gas tank was from a CB DOHC as the mounting point was just welded to the frame.

Nevertheless I bought the bike to learn some stuff on so I moved on. I tried soaking the cylinders in trans fluid and acetone in hopes of releasing seized piston but to no avail so I wound up pulling the engine and discovered this.

I wound up having to use a Jaw puller to release the piston from the cylinder wall. I made a few posts about doing an engine rebuild but long story short is I wound up getting a cylinder block from a 71CB 750 that had 64mm pistons in it. So the cylinder block and valve head is currently being worked on. I had to outsource that stuff as I have no idea what I'm doing.

Hit up the swap meet and found this sweet super sport tank for good money so I'm going to wind up going with it

My mind is pretty set on dual disc brakes on the front as well as a different look to the front end. So a friend of mine has an old VFR750 that he's been parting out for a while so we made a deal and off I went to see if it would fit.

Sidenote: A CB750 will fit into a Nissan Altima if the front wheel is taken off.

Upon disassembling the front end we realized that it was probably a good call that the front end was getting changed anyways. I didn't get any pictures of the nasty front forks but here is a picture of what was left of the bearings.

To my surprise the VFR750 front end looks like it will pretty much bolt up to the stock neck tube. According to the allballs site the bearings are even the same size. That's a win!

I want to be able to use the Stock spoke wheel with the VFR front end but I have to figure out how to get it all mounted. I have no idea what I'm doing but i got the spoke wheel mocked up using the bearings from the VFR wheel. I'm not sure what to use for the inner wheel collar. I was hoping some other honda wheel has the hub collar be the same length as the CB but in 20mm diameter. Also if anyone could shed some light on how i can achieve dual discs on the CB hub to fit the front end correctly that would be greatly appreciated. The stock CB rotors seemed to be about 295mm in diameter and the VFR rotors are 275mm in diameter.

The idea of this build will to go with a cafe racer brat vibe I guess. I've got a few ideas on the aesthetics of the bike but I really want the mechanical facet of the bike to be solid first.

I'm trying to do as much as I can myself or with the help of some friends but I'll be asking a lot of questions on how to accomplish rebuilding this bike into something cool. This project will take a long time but I'll update my progress periodically.

budman · « **Reply #1 on:** September 28, 2014, 06:57:08 PM »

Looks good so far. I'm right there with you. I just started my first CB750 build a couple of weeks ago, except my bike was in way worse shape. This site has some great info for sure. I've already used a lot of tips I've found here.

PPG9944 · « **Reply #2 on:** September 28, 2014, 07:05:20 PM »

The best thing would be if all the parts listed online had dimensions listed as well haha

PPG9944 · « **Reply #3 on:** September 29, 2014, 04:33:57 PM »

I've seen the cognito moto stuff before. It looks good but probably out of my price range. And I just started doing some research on electronic speedometers. I was going to go that route regardless. I think I might have some luck being able to get some spacers made. Just have to find someone that has access to a lathe

PPG9944 · « **Reply #4 on:** September 29, 2014, 08:12:34 PM »

Also, does anyone know where I can get a piece of tube to slip over the 20mm axle around the chicagoland area?

PPG9944 · « **Reply #5 on:** October 02, 2014, 06:44:47 PM »

I managed to pull out a spacer I had from another wheel that will work. Need to figure out how to get that spacer shortened up bearings and stuff in. Also Searching for brake rotors that are 275 or 276mm in diameter with the same bolt pattern as the stock cb750 hub

ekpent · « **Reply #6 on:** October 02, 2014, 08:19:52 PM »

Nothing major but the gas tank is more of a 77-78 cb750K not an SS. Looks like you got some interesting stuff going on with your mods, be fun to see the finished product.

PPG9944 · « **Reply #7 on:** October 02, 2014, 08:21:59 PM »

The gas tank was a swap meet find. I've also been informed that cb550 rotors are about the same diameter as the rotors that were originally on the VFR. This will be a long road but I'm excited to see the end result as well.

PPG9944 · « **Reply #8 on:** October 19, 2014, 02:27:44 PM »

Update. Picked up another brake disc at the swap meet.

Pulled a wheel spacer out of my crashed magna front wheel and cut to size

The wheel mocked up with dual discs. Still need to figure out how to seal the exposed bearing on the side that's showing. I have to figure out how to run an electronic speedometer since the speedo plate will no longer fit. Need to get some longer bolts to be able to bolt both of the discs together. Figure out the spacing of the front wheel. Then send the rotors out to get cut down to size, thinned and drilled.

Still waiting on my cylinder block and valve head to get finished. Once I get that back I'm going to figure out how to reassemble the engine.

PPG9944 · « **Reply #9 on:** October 19, 2014, 04:50:41 PM »

Quote from: calj737 on October 19, 2014, 04:32:34 PM

For your electronic speedo, Devin at CognitoMoto turned me onto a pretty slick install.

Under the left side cover, attach the magnet to one of the front sprocket bolts (he drilled the bolt head a touch, then JB Welded it). Then, on the underside of the cover, theres a flat bracket on the outboard side. Drill a small hole and pass the pickup through it. Then as the sprocket rotates, it passes the pickup sensor. Run the sensor wire along with the Oil Pressure/Neutral Switch wire into the loom, then forward to your gauge.

Clean, neat and tidy. No exposed wire. You then adjust your WHEEL DIAMETER scale to meet your needs. I did the same on my 550 with MotoGadget electronic gauge. I road-matched with my car, adjusted the DIAMETER as I went until it perfectly matched the displayed speed on my car's electronic cruise control.

That is awesome! Thanks for that tip, I definitely thought there was going to have to be a sensor on the front forks somewhere.

Tews19 · « **Reply #10 on:** October 20, 2014, 04:48:37 PM »

Ppg- send me a pm with your mailing info. I may have some seals to use for your bearings. Brand new in bag still. They may work they may not but doesn't hurt to try them. I have no need and yours for free.

PPG9944 · « **Reply #11 on:** October 20, 2014, 06:01:30 PM »

Quote from: Tews19 on October 20, 2014, 04:48:37 PM

Ppg- send me a pm with your mailing info. I may have some seals to use for your bearings. Brand new in bag still. They may work they may not but doesn't hurt to try them. I have no need and yours for free.

PM sent. I definitely appreciate the gesture! Thank you!

PPG9944 · « **Reply #12 on:** November 22, 2014, 11:12:51 AM »

Been busy lately trying to get the front wheel centered up. Managed to take a trip over to Godffrey's today to pick up new brake rotors that will fit my application. Stoked to get these things on the wheels. Totally worth getting lost in wisconsin haha. Hopefully going to be able to press the steering stem out tomorrow to get the lower bearing race off. Then I'll be able to replace the bearings and continue on with the front end.

SOHC4 Cafe Racer Fan · « **Reply #13 on:** November 22, 2014, 05:17:45 PM »

Very nice. I have a pair on my 550. The thinning is a great weight saver.

Tews19 · « **Reply #14 on:** November 22, 2014, 05:22:12 PM »

Hey PPG its been a while since I have actually read threads but I forgot to mail you the seals. Can you send me your info and I will send out first thing Monday.

PPG9944 · « **Reply #15 on:** November 22, 2014, 05:24:44 PM »

Tews. I sent you a PM and I needed thinner and lighter rotors to fit the VFR brakes. Should hopefully have the front end sorted out soon.

PPG9944 · « **Reply #16 on:** November 30, 2014, 12:48:45 PM »

Made some progress. Had a bit of a mishap with the steering stem. Tried pressing it out and wound up messing up the threads. No matter took it to a machine shop here and they were able to help me out. Didn't take any pics because i was covered with grease. The top dust cap with the lock nut sits a little high but I was able to just bend a bit of aluminum to reduce the amount of dust and dirt that can get up in the top steering bearing. Also been playing around with PVC spacers to get the wheel centered. Got some progress pics up, stopping here for now cuz I want to watch some of the bulls game this afternoon. The wheel is almost centered, and once i get it where i like it, I will transfer my measurements to some metal tubing to make my spacers.

Here is the left caliper

Here is the right caliper, almost there.

I'll probably continue to mess around with that a little bit more later but I'm feeling confident this will work.

PPG9944 · « **Reply #17 on:** December 09, 2014, 06:51:11 PM »

Got the spacers made and wheel is centered and fits into the brake brackets. Just need to put the seals on. I feel like this is the first bit of progress that I've been able to make on this thing. I also realized that the 7 inch headlight bucket is going to look way too big on this front end. Going to have to keep an eye out for some cool headlights at future swap meets.

PPG9944 · « **Reply #18 on:** December 17, 2014, 01:49:40 PM »

So things have been slow going. I've got some caliper seals in the mail. Should be here today, then ill be able to rebuild the brakes. So while waiting for that I decided to spend some time on the headlight. I have this crashed 2001 Magna that I've been borrowing parts from and the headlight bucket and bracket were good. The glass is trashed. I wanna get an LED or H4 bulb setup anyway. I don't have fancy tools or loads of scrap metals lying around but i did have a bracket that used to hold the windshield and the bolt spacing was the same as the bracket. Lucked out for sure. Cut off one of the ends, drilled some holes and was able to mount the headlight bucket up. And I'm pleased with it for now. I gotta call my engine guy, he's had my cylinder block and valve head for months. Kinda want to get cracking on rebuilding this engine.

PPG9944 · « **Reply #19 on:** December 18, 2014, 06:45:34 PM »

Got the brakes off and apart. Getting the pistons out of the cylinders was an adventure for sure. Didn't realize how enthusiastically those things come out under pressure.

Those pistons don't look too bad, but what do i know. I'll clean these up.

Got everything ready for some more cleaning. Thinking I'll devote some time saturday morning to a more thorough cleaning before i reassemble everything.

Debating whether I want to paint the calipers. I think I'm going to leave them for now and revisit the idea once it warms up though. Would probably go with black on the calipers.

PPG9944 · « **Reply #20 on:** December 19, 2014, 04:53:24 AM »

I will do that. The pistons look pretty good aside from 2 marks on one of them that I made. The marks are at the very top of the piston so i don't think that will be an issue.

PPG9944 · « **Reply #21 on:** December 20, 2014, 02:19:47 PM »

Questions for the Pros.

I cleaned the brakes up put new seals in and put them back together I noticed on the one piston you can see and hear a bit of brake fluid coming out just a little and when you release the lever it retracts back.

I'm not sure if it's this piston but I scored one of them up with the channel locks.

Is that going to be a problem?

Also I noticed that the pistons dont move at the same rate when pumping the lever. Is that normal? Is it something to do with one piston being the leading piston and the second one being a secondary?

That's the only piston that does it too

Riceman · « **Reply #22 on:** December 20, 2014, 02:27:06 PM »

Probably shouldn't over extend the pistons like that...
just doesn't seem good

PPG9944 · « **Reply #23 on:** December 20, 2014, 03:04:10 PM »

I took out one of the pads to see what was happening. I dont know if i would call it a leak or not but a little brake fluid pools up around the groove where the piston goes in

PPG9944 · « **Reply #24 on:** January 04, 2015, 05:51:14 PM »

Few updates.

Rebuilt the brakes AGAIN, i bought #$%*ty seals initially. Went and got the pricey honda seals from the dealer put brakes back together and no leaks. Lesson Learned.

Got the front wheel centered on the axle and lined up with the calipers as well

Got a headlight lens from a dude that has a honey hole of parts near where I live. Will most likely be going back to him for some odds and ends. He said he has mint exhausts. We shall see.

Scored an oil cooler adapter plate with an oil cooler off ebay for like 20 bucks.

And I got the carbs put back together. One of the floats is messed up, ill probably have to grab a new one. Oh well.

The front end looks pretty mean so far.

Need to do some more research but up next I am going to have to give the engine guy a call to see where my valve head and cylinder block are at. I'm kinda itchin to start putting the engine back together. Also need to look into possibly taking the swing arm apart to replace the bushings. Not sure if i need to or not but we shall see. Im still debating looking into electronic ignition systems, the Acewell speedometers ,breadbox air filters and upgraded rear suspension bits. Any advice?

News:

Author Topic: My 1975 cb 750 Build (Read 17659 times)