Tearing down CB750. What should I upgrade? Also: rust.

[nitsua93]

I recently picked up a 77 cb750k. I'm taking out the engine as it's seized up. I've looked at the 836cc big bore kit. Are they really worth at least $300? It's something I'd like to do since this will be the first engine I'm rebuilding, it would be a good experience.

Also, I've noticed a bit of rust on the outsides of the 3 plugs I successfully removed, and in the part of the alternator that is driven by the motor. Should I try to clean the rust off or is it fine to leave it? It appears to just be rust on top of the surface.

[robs750]

To rebuild a top end,it's gonna cost some money.
There may be some quality issues with the lower cost kits.I'm sure someone can chime in on this.Dont forget about labor costs to bore out the holes.

You may also have to go thru the head and then there is a gasket kit you will need.

It is normal for the plugs to rust on the outside.You should replace them anyway after the build.

[nitsua93]

I'm definitely replacing the plugs. Probably all the gaskets too. One of the plugs actually broke so I'm going to have to tap it out. The thing sat for 30 years

[robs750]

If the plug base is stuck in the thread,soak it with PB Blaster for a day then try to remove it.Some heat (propane torch)around the plug on the head may help also.

[Piute]

   I would go with the bigg bore and plan to myself when the engine needs to go anything over in ring size.
 Sure it's going to cost but not as much as a new bike would,collect and put on all the rest of the goodie's / upgrades you can find that will make it run N "look great for you"   I'd do all electrical (h.i.d. light,coils,a.m.g. batt.)you can if pulling engine may as well get complete new harness,changing all the contacts N plugs will work also IF you have a long winter.Also any N all bearings that are off may as well put in upgrades,just did my rear wheel what a pain in the rear .

                Have ready rust treat and Paint of all parts of course as you put back together.



                                            Rule#1Have fun,

[nitsua93]

I'm not going to do H.I.Ds unless I can get halos. I'd rather not blind people. I'm buying a battery so I can start messing with the electricals and see what condition they're in.

I'm going to try to get the motor to run out of the frame so I don't have to fight it in and out. First I need to know if this rust I've been finding is bad. I've only managed to get the valve cover off after removing the engine. There's rust on top of the lifters/rockers/whatevers, but no pitting. Is this a really bad sign or just normal for a motor that sat for 30 years?

[Piute]

    Trying not to blind others is great ,A proper adjusted light will take care of that,Mine is great you can see the beam pretty clear,The bulb in my faring is just the best Sylvania sells witch goes in a car,bit pricey.I also have some extra light added that i like alot ,also some extra for side view.
  If rust is just surface there nothing to worry about,(i wont judge without seeing)there is no telling how the moisture got in,but happens,how did the oil look ,was there lots of water in there? remember there is no radiator on these so there should never be water in your oil.Now those parts a raw steal so there going to rust if oil not splashed on(witch happens while riding),So treat all raw metal you see through out your bike as soon as you see.
   While messing with my bike i used a car batt. so not to put drain on bike batt.found these great deals on agm batt.
                                                   ABOUT 80 BUCKS on e-bay

[robs750]

nitsua93 wrote

 There's rust on top of the lifters/rockers/whatevers, but no pitting. Is this a really bad sign or just normal for a motor that sat for 30 years?

It's probly normal if the bike sat outside those 30 years!
Before dumping money into the top end,might be a good idea to split the cases and clean inspect everything.
Thirty year old crancase oil with the possibility of water inside is not good IMO.

[nitsua93]

What was left of the oil was pretty brown amd sludgy with a couple white streaks. Felt nasty. All I've seen so far is surface rust. I haven't got down past the cam yet. Still deciding how I want to keep everything organized :p

[shinyribs]

robs750 is right. if an engine has sat for 30 years I'd want to tear it down first. The acids in the engine oil can really eat up bearings when let sit for that kind of time.They sort of dissolve and flake apart.You don't have to do a total rebuild. Just a tear down and clean. Your cam,piston,valves...etc. could very well be reusable.You might really need a new set of bearings on that crank though. JMO of course.


FWIW,I just got a bike that sat indoors for over 20 years.I got it running and all seemed well. Silicone in a cam tower hole burned out the cam and a follower, and now the oil has alot of metal flake in it. Flaking bearings I suppose. I should have tore it down and cleaned it first. Good luck with your project!

[nitsua93]

I can't get the pistons separated from the jugs so I'm buying a "new" motor. $175 for a clean, not seized motor.