1974 CB750k - Engine "rebuild" - first timer over here

6pounder

6pounder

New Member
Hey guys/gals, I have done plenty of lurking and searching and figured I might as well make a post to see if I can get some quick tips and tricks. I have a 1974 CB750k, which I am trying to do a simple cost efficient "rebuild" on. I have stripped everything down to the frame, rebuilt the carbs, working on replacing all wheel bearings and just took the valve cover off.
My questions are:

The bike has 40k on it supposedly, so take that into consideration and it ran well when I bought it. I currently have a complete gasket set, all new bolts, new bearings for the wheels, new battery, new stock coils and other miscellaneous things. I just opened up the motor and would like suggestions on what to tackle next!

- Lap valves?
- Piston Rings?
- Pistons?
- Chains?
- I don't have a clue

Here are some pictures. Any help/info/guidance is appreciated! Just looking for it to run stock/oem. No mods thus far.

Carbs - before and after


New 18" rear wheel - the start anyways


Progress - before bearings


Under the valve cover -


Question is.....where should I go from here. What would you do in your experience??

Here it was before I pulled the motor...
 
how did it run and ride before you started to tear it down ? what did you get on the compresion tests ?
 
xb33bsa said:
how did it run and ride before you started to tear it down ? what did you get on the compresion tests ?
Click to expand...

Ran and idled well before I tore it down, never did a compression test so that I can not answer. Thinking about just doing piston rings, valve seals, lapping and all new gaskets. Depending on what it looks like inside that is.
 
Got the head off and making progress. For your viewing pleasure.
Question; Do most of you replace the cylinder studs? Just trying to get all the info I need to prevent all oil leaks. Appreciate any insight!




 
Purchased some new rings and APE heavy duty studs as well. After that she should be all ready to go back together! Finished the valve cover the other day as well as some valve lapping!
 
You don't need to replace the studs as long as they aren't visibly damaged. You should be able to reuse the pistons as long as they clean up well and the bore is still within the service limits...just give the bore a light hone to provide the proper break in surface for the rings and ensure you clean them well to remove all abrasive particles from the honing process. Check the threads on your cam tower mounts...I have found that people tend to over-torque them and strip the holes in the head. You are going to want to have a look at the valves as well, they may only require a bit of lapping to ensure that the seats and valve faces are in good condition, since you have the top end torn down this far, you might as well go all the way...check the valve guides and change the seals while you are at it.
So far so good :)
 
Thanks goldy!! I am cleaning the pistons up currently and have new rings for them, so hopefully I can get most of the buildup off. Trying to replace all that I can while I am in there of course. Did the valve stem seals, lapped the valves, cleaning pistons, replacing rings, bought a complete gasket set and also ordered new studs just to be safe. I will give the honing a shot and post more pictures as I go! Appreciate the kind words and advice!
 
It isn't a car engine so you don't run out and replace everything ::)
Fit middle pistons into bores first
Motor weighs 212lbs 'dry' (used to work in a Honda dealers ;D )
 
This is all sort of trial on error. I read too many forums/threads/posts about what you should just "do while you're in there" kind of thing. I have most everything I need now to complete the motor rebuild - minus the fact I dropped a piston today while cleaning it and cracked the damn thing :-\ I have another used one to use in its place. Hopefully that works?!

 
don't "clean up" the pistons .it is not an old car motor what needs to have the ring grooves scraped...you can clean the looser carbon off the tops, but that is all (you need and want to have a layer of carbon)anything else is doing more harn than good
 
also dont change the wheel bearings unless they are obviously bad ....too many motherfuckers change wheel bearings when it is extremely rare to EVER have a need to do so.
however DO get new rear brake shooze if they are old originals
 
Grab some new cush rubbers for the rear wheel. They're cheap from vintagecb750.com. Actually, pretty much everything can be got from that site. Personally, I replace all outer bearings on anything I buy used. That's just me. Does it need new bearings? Maybe not, but a C-Note worth of bearings gives me peace of mind.
 
Wheel bearings where more for a piece of mind. I am really just dipping the pistons in the chemical dip, which works wonders! Take a look at the before and after. This is only about an 8 hour timespan. Berrymans chemical dip is what I used. Around $20 for a gallon at WalMart. Ordered 4 used pistons from Ebay for about $12 to replace the one I cracked. Hopefully it will measure out ok. After I get the pistons all clean, I will button up the engine and start putting her back together. Any recommendations on tires? "Old School" look, but i want to be able to take corners.
 
Re: 1974 CB750k - Engine "rebuild" - first timer over here


Jewbacca said:
Grab some new cush rubbers for the rear wheel. They're cheap from vintagecb750.com.
Click to expand...

Call me blind but I can't seem to find them on the site
 
Personally I really like Dunlop K81 (TT100) but they are no longer made in sizes I want. :(
Majority of people find them a bit 'strange' as they have more tread in contact with road when leaned over than when upright.
They were called TT100 as they were on Malcolm Uphill's Triumph 650 when it was first production bike to lap IOM TT course at 100mph (1968 or 69, I forget stuff ;) ) K70 is an older design but seems to work well. Pair of AVON 'Roadrunner' or AVON 'Speedmaster' ribbed with a Dunlop K70 rear were a more contemporary tyre set for CB750 and was a better pairing than AVON ribbed front and AVON 'Safety Mileage' rear
 
Still working on the motor before I make the jump to tires. Today I was replacing the head studs and took it one by one. Got everyone out and in except for the very last one......of course. Couldn't get the last one to budge so I pulled out the torch and heated it. Few more cranks and snap. FML. The stud snapped right off so I have nothing to grab. Any ideas or tips would be GREATLY appreciated. Replacing with APE heavy duty studs....does 17 foot pounds sound correct for these? I read 8ft lbs, 17ft lbs and 22ft lbs, so I am not so sure anymore. Thanks again!
 
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