1981 CB750K - Repair/Tune-up

[cdaiscool]

Shit shit shit shit. I just noticed in those pictures that the lip near the top is beveled. I just confirmed this with a visual inspection outside looking at it. Pulled the tape off protecting the outside from the paint, and yep - it's beveled. Is this normal?

I can get a caliper and measure precisely how bad it is, if needed. But it's definitely, 100%, 2 different sizes.

Need to know if I have to order one asap, or if it is normal or what.

[TOOLS1]

Yes the lip is supposed to be beveled. That piston is right at the edge of useability. However it should be all right as long as the bad spot sticks past the seal.
TOOLS

[shinyribs]

Looks like progress!Keep it up! I'm with Tools on the piston. Try it and if it leaks then you know you need a new one.Worth a shot.You already own that one!

[cdaiscool]

Awesome.

I'm going to put the piston in Tuesday, and we'll see what happens. I have a GRSEC meeting Monday after work, so I am not sure if I'll get anything done then. Maybe put the caliper together if I get the kit. But I won't be doing any actual bike work.

Tuesday, I can get to the bike and work on it. Going to pull out the one shim and check what size it is currently so I can get a slightly larger one. And put the master cylinder on.

Hopefully everything comes in by Tuesday!

[cdaiscool]

Got my brake master cylinder in, and my speed bleeders. Paint on the calipers looks decent enough as well.

Hopefully the rebuild kit comes in tomorrow, so I can rebuild and mount my brake stuff! Also looking for/hoping  the valve cover gasket comes in, so I can do all that tomorrow. Ordered up my gaskets today, they said Thursday was going to be a long shot. :( But a guy can hope, right??

[TOOLS1]

Speed bleeders SUCK!!!!!! Hope, you have better luck with them then I did.
TOOLS

[cdaiscool]

I hope they don't! I spent a fair bit of money getting 'em for the Integra and bike. I've been told they're a very worthy product.

I'll give 'em a shot and see what happens.

[TOOLS1]

They are OK on cars, but the master cylinders on these bikes do not move enough fluid to compress the air, and open the valve in the bleeder. It's best to back bleed them with a pump type oil can, and a short piece of hose.
TOOLS

[cdaiscool]

I'll have to give it a try. Thanks!

[shinyribs]

LOL. I cringed earlier when I saw your post about the speed bleeders. I knew Tools wasn't gonna let that slide!LOL.   I have used them on autos and they did ok.  Never used one on a bike. I would say Tools knows what hes talking about on this one. And back bleeding is definitely the way to go. I'm glad Tools told me about it. If not I'd probably still be squeezing that lever.

[cdaiscool]

So, I just got the tracking info for my brake rebuild kit and my valve adjustment tool. Apparently vintageCB750 shipped them out today. After I had to inquire about 'em. I ordered 'em Thursday night. :(

It's 'initiated' which means Fedex hasn't gotten 'em yet, but if they do I'll have them Thursday. Same time the rest of my gaskets will (hopefully) be in.

Anyways, I got antsy. I got my master cylinder yesterday, as well as my 2nd mirror so I threw those on. And then I went ahead and reassembled my brake caliper so I could make sure I had everything as well as see how it goes together.

That door on the top of the caliper, is that supposed to be hard to close? I know the metal pieces in there are hitting against the pads, I'm assuming this is by design so the pads go back after I let off the brake? It feels as though they aren't going back much - they spring back, but gently.

Btw, anyone need a new computer? I hope you don't use this one.

http://arstechnica.com/business/news/2012/03/the-pwn-plug-is-a-little-white-box-that-can-hack-your-network.ars

Anyways, pictures! We all know by now I'm good for 'em. :D













And now I'm kind of stuck. Not a whole lot I can do now that those are together. Can't put any gaskets on the bike, can't install the caliper, can't bleed the brakes...

C'mon parts, get here asap!

[cdaiscool]

Haste makes waste.

I may have just messed up guys. I'm not used to working on these bikes like this. I mixed up the Intake and Exhaust when positioning the cams, and accidentally had the intake cams up and exhaust cams down when I put the valve adjustment tool in. I needed to remove an exhaust shim.

Realized my mistake after a couple minutes, pulled out the tool, and realized it'd shed flakes of the chrome. I wiped 'em off, rotated the cam to the correct spot, and did it again. Removed the shim, got the correct shim, put it in, rotated the cam half a dozen times, and measured the valve - .004! WIN.

Now I'm working to put the valve cover back on. My question, then, is are there any tricks I should know before I put it on? Hondabond in the corners or on the cam covers, how to put the cam covers on without ripping the gasket, etc.

I'm going to do some Googling now to find out otherwise, but hope someone can tell me quick so I can go outside and get 'er done before the light's gone.

Also - does anyone know if there's a good chance I messed up anything by putting the valves down while the cam had 'em pushed down? I'm thinking the danger is valve-to-piston contact; how close are they in the head?

-Off to research-

[shinyribs]

Well heck,you gotta take it all apart and check for bent valves now!!!!

Since you were turning the motor by hand,if a valve would have hit the top of the piston I imagine you would have felt it.Thats my opinion at least.

[TOOLS1]

If, you had the valve open, the piston had to be down. I doubt it hit the piston. On the valve cover gasket. I would glue it to the valve cover with weatherstrip adhesive, and a dab of silicone over any joints, or rubber plugs on the head.
TOOLS

[cdaiscool]

Hmm. Need some advice.

Trying to rebuild my caliper now that the bike's running again. The seal, however, looks to be too small:





Is this normal, and if so what's the secret to getting the seal around the piston? Or is the seal too small, and did Vintagecb750k send me the wrong kit?

[cdaiscool]

After looking at it some more, it appears the piston is simply too large. The seal barely fits into that groove; in the picture you can actually see it's fallen out the other side!

So: any tips how to identify what the size of my piston is, so I can get the proper kit? Perhaps by measuring the bore size? This piston is definitely larger than that gasket is good for.

[cdaiscool]

Piston measures out to 1.685 inches, which comes out to 42.8mm per an online calculator.

When measuring height, does one measure from the beveled low, or beveled high side? 1.391 inches on the low side, 1.431 inches on the high side. So 35.3314 mm on low side, 36.34 mm on high side.

Per vintagecb750, the 1981 CB750K piston is: Brake Caliper Piston (1) - 38 x 35mm. So my 43 mm wide piston isn't going to fit into the 38 mm seal.

I'm looking to see what size it happens to actually be. Any way to tell off the caliper? It's got the letter 'c' on it a few times, I wonder if that's supposed to mean it's off a 750c at all?


Arghhh. I guess this means I won't be riding anytime soon. :/

[TOOLS1]

That seal looks way too small. It should fit snug in the caliper. Measure the old piston, you took out of the caliper, and see what size it is. The piston should just barely fit into the caliper.
TOOLS

[cdaiscool]

I measured it and put it into the post above yours. It's 42.8mm across. And yes, it does JUST BARELY fit into the caliper.

I've got a feeling that caliper isn't a stock CB750K caliper. I need help with finding out what it is, otherwise I'll need to get an o-ring and make my own caliper seal or something.

[TOOLS1]

Post a picture of your forks. I might be able to recognize if, your front forks were changed.
TOOLS