Topic: CB750 Motor rebuild... for dummies

[mystic_1]

Hiya OTool, welcome to the forum, do introduce yourself in the New Member section, and post some pics of your project if you can.  It's nice to hear about these old girls staying within the family, so to speak.

And yes, you CAN do it

mystic_1

[bunghole]

OTool,

Just go for it!  Take it slow.  Read and reread these forum posts.  Get as much printed and electronic information as you can.  Develop a list of parts and suppliers and take it slow.  I've screwed up enough by being impatient and have finally learned that a methodical and organized approach is the way to go.

Good luck!

[Kevin D]

Well, I got it back together, back in the frame, assembled, fired right up, had my 80 mile shakedown ride and all is well and guess what -                                                                                   NO OIL LEAKS!!!!   

Now that doesn't mean it won't leak later on, but as of now - bone dry top end - I used the Vesrah head gasket, torqued/retorqued the stock head bolts at 18ft/lb, and used viton o-rings on the oil supply and return studs/passages.

I haven't yet added it all up but I think I spent about $400 on parts and tools: primary chains and tensioner, cam chain and tensioner, piston rings, Vesrah kit and lots of other orings and seals. Had to buy a cylinder hone($30 Murrays), valve spring compressor($15 Sears), valve lapping kit ($20 Murrays).

This is my third time tearing this engine down, first and second were over thirty years ago and nothing else since. I was more than a little apprehensive going in to this, I was thinking: future basket case.

You need a fair amount of undisturbed space to spread it out, used lots of ziplocks, boxes, 6oz yogurt cups for the screws/bolts. Took my time, read the manual, took pics when necessary, most of the help came right here >>sohc4<<.

Break-in oil change is next, new rear tire, and I hope it'll be good for a good long time.

[Terry in Australia]

Looks great Kevin, well done!

[andy750]

Well done Kevin and nice looking engine - my K4 engine got the HD studs this year and cometic head gasket so lets see who stays leak-free!

Im hoping we both do 

Good luck with it and ride it and enjoy!

cheers
Andy

[Kevin D]

Well thanks for the compliments guys, and thanks for all the help. I couldna done it without all the help and encouragement a guy can get here.

lets see who stays leak-free!

Im hoping we both do

 +1 on that!!!

I'm no seal designer but I have an idea that extended high speed running contributes to HG leaks. I dont have an oil pressure gauge but I've heard that the oil pump makes 10psi per 1000rpm. So at 75mph you have about 50psi at those two rear center studs. That seems like a big job for a little oring.

[birdcatcher]

As far as stud removal goes, if I'm taking them out I'm not putting the OEM ones back in, so no problem  butchering them. I just bend the suckers at right angles to "crack" them, then bend them up to some semblance of straightness to wind them out. No heat, no penetrating fluid, no double-nutting, no vice-grips. Down and dirty, but effective. Cheers, Terry.      

I followed this advice last monday when I removed my cylinder studs.  Shot a blast of penetrating oil at the bottom of each stud everyday for 3 days prior.  Once I was ready to remove them, I bent them down with a rubber mallet, then twisted them out by hand.  Didn't take me more than an hour of my time, and I didn't have a single stud snap.  Just make sure you hammer them down to at least a 60 degree angle.  If you don't they tend to just bend around the base rather than twist out.

Oh and thanks big time for this piece of advice Terry, I was a bit afraid of removing my studs with all the horror stories, but found this method very easy.

I'm at this stage in my engine rebuild, and am wondering exactly how to bend the cylinder studs.  My concern is that hitting them with a mallet will put strain on the bottom threads and damage them.  Is this a possibility?  Do I need to support the bottom of the studs with anything?  I'm a mechanical newbie, and I don't want to damage the threads in the crankcase while trying to remove these things.

By the way, thanks for this thread, I've been following it closely to help with my K6 rebuild.

Randall

[Jerry Rxman Griffin aka MuthaF'er]

Randall,

I can see this as a valid concern if not done properly. However I have not attempted it this way. Other methods are to use heat &/or a stud removal tool on your socket wrench along with a pair of Vise Grips. I did this on mine but without heat and it worked fine. Just don't get so close to the case mating surface that you bugger it up.

[Terry in Australia]

Yeah, what Jerry said mate, don't go whacking anything with a mallet. I bent my old studs using a length of pipe (length of old handlebar from memory) and used the pipe as a lever to turn them.

Once you've "cracked" each stud and it's free, you can either bend it up straight(ish) again, or chop the bent section off with an angle grinder, bolt cutters etc, and turn out the remainder of the stud by hand. Cheers, Terry.

[roadkill savior]

Ok question........what's the worst that could happen if for some reason the entire motor was buttoned up ready to start and then you realize that the 4 really skinny rubber rings that go around the bottom of the sleeves, between the base gasket and jugs, were accidentally left out?

[Jerry Rxman Griffin aka MuthaF'er]

The way I see it is the o-rings for the cylinders keep oil from exiting between the cylinders and the block. Anyone else??

[birdcatcher]

Thanks Terry and RxmanGriff, that makes a lot of sense.

[Terry in Australia]

Ok question........what's the worst that could happen if for some reason the entire motor was buttoned up ready to start and then you realize that the 4 really skinny rubber rings that go around the bottom of the sleeves, between the base gasket and jugs, were accidentally left out?

The sleeves are "shrunk" into the cylinder block, and the holes the sleeves go into are bored, just like the cylinders, so it'd be a very tight fit. Considering the size of those O rings and the fact that they just sit there, I'd say that there's no real oil pressure to worry about, but maybe some oil mist.

From memory (and I can check an empty cylinder block if you want) the oil can't exit anywhere between the bottom case and the head, so the head gasket should stop any oil mist from exiting the engine, but if it blows smoke, it could mean that oil mist is entering the combustion chambers, and then it'll be up to you whether you want to tear it down again. Look at the bright side mate, at least your pipes won't rust out!

[paulages]

regarding the bent stud removal method... if you bend them in the correct order, you can work them all out without straightening them back out. It's a bit of a puzzle figuring out which ones to turn which way to allow another to spin all the way around, but it's possible.

[Joe Efinger]

This is all great advise.  TORQUE WRENCH! and a good one only! The cylinder studs gave me the most trouble. I had three inch pound wrenches and 3 different torque values when tighening down head.  In 77 they deleted rubber 4 grommets/seals inboard of 2 and 3 cylinders and those studs rusted alot. Honda must have had a reason. Cam caps and other parts require very low torque values as well so that's why I say it's very important to have a good torque wrench. Good Luck

[HondaMan]

Ok question........what's the worst that could happen if for some reason the entire motor was buttoned up ready to start and then you realize that the 4 really skinny rubber rings that go around the bottom of the sleeves, between the base gasket and jugs, were accidentally left out?

Those itty-bitty O-rings are there to prevent oil creep up into the sleeves, between them and the fins. This ensures that in the future, they won't become loose, and also ensures that they won't leak oil at the head gasket, because the outside of the liners is also outside of the sealing area of the head gasket. In short, they prevent head gasket oil leakage, especially on the carb side. (Insider's note: Kaw 900 didn't have this at first, and they leaked a lot!)

In the past, when Honda did not even sell these O-rings with their top-end gasket sets, we would apply some hardening Permatex in a circle around the deck (i.e., UNDER the head gasket) just outside of the liner at the top. This situation would arise when riders would send their cylinders to the local auto machine shops for an overnite soak in their carb-cleaning tanks, which destroyed the O-rings. The hardening Permatex would stop the oil leaks, but upon subsequent teardowns, we often found that you could lift the sleeves right out of the cylinders by smacking them with your fist, because they were then well-lubed.

Which led us to the next revelation: when we had to change out sleeves (because of some catastrophic racing mishap) we would remove the affected one's O-ring, then would heat up the cylinders, lay them upside down on a bench (in a big cooking pan), and pour refrigerated 10w oil around the desired sleeve. It would suck that cold oil up into the sleeve, and as soon as the fins were cool, you could tap that sleeve right out.

It's fun to learn from the mistakes: makes you smarter. 

[SandwichEnthusiast]

I'm just about finished re-assembling my crankcase,

Any advice about applying the liquid gasket (im using threebond 1194)?

Also I have seen recommendations for torquing and then re-torquing the bolts, to prevent seepage, any advice about that?

[birdcatcher]

Thanks for the cylinder stud advice.  It really helped.

I've got my engine completely torn apart, and am having the cases and fins sandblasted and prepped for painting.  After painting everything, I hope to reassemble all of it.

I have run into a snag, though.  The guy who's doing some work on the head cracked one of the intake flanges while removing it.  Does anyone know where I can find a replacement intake flange?  Or is there anyone willing to sell me one off a donor motor?

I don't mean to turn this into a Parts Wanted thread.  However, I'm working on a restricted timeline, and I'm really hoping someone can help me out.

I've checked CMS and Dynoman, but I haven't had any luck.  Any thoughts?

Thanks,
Randall

[MCRider]

Ok question........what's the worst that could happen if for some reason the entire motor was buttoned up ready to start and then you realize that the 4 really skinny rubber rings that go around the bottom of the sleeves, between the base gasket and jugs, were accidentally left out?

Other comments considered. My expereince was with drag motors we routinely left them out. Unnecessary. Of course those motors were torn down regularly, but they were also stressed heavily. In a street bike, I don't think you'll ever know.

The very early motors did not have valve guide seals on the exhaust. They were put on to keep that little bit of misting from occurring, a consumer oriented thing. Again, we left them off drag motors to reduce friction on the valve stems.

Perhaps another reason (among very many) that the early motors had an HP edge.

[Terry in Australia]

You're probably quite right MC, I've only just discovered (when I found them in the gasket kit last week) that I didn't replace the cylinder sealing O rings on my Suzuki GS1000 engine when I swapped cylinder sleeves about 5 years ago, with no obvious leaks or smoke. I've probably done 10,000 miles or more, since I did this work. Cheers, Terry.

[MRieck]

Ok question........what's the worst that could happen if for some reason the entire motor was buttoned up ready to start and then you realize that the 4 really skinny rubber rings that go around the bottom of the sleeves, between the base gasket and jugs, were accidentally left out?

Those itty-bitty O-rings are there to prevent oil creep up into the sleeves, between them and the fins. This ensures that in the future, they won't become loose, and also ensures that they won't leak oil at the head gasket, because the outside of the liners is also outside of the sealing area of the head gasket. In short, they prevent head gasket oil leakage, especially on the carb side. (Insider's note: Kaw 900 didn't have this at first, and they leaked a lot!)

In the past, when Honda did not even sell these O-rings with their top-end gasket sets, we would apply some hardening Permatex in a circle around the deck (i.e., UNDER the head gasket) just outside of the liner at the top. This situation would arise when riders would send their cylinders to the local auto machine shops for an overnite soak in their carb-cleaning tanks, which destroyed the O-rings. The hardening Permatex would stop the oil leaks, but upon subsequent teardowns, we often found that you could lift the sleeves right out of the cylinders by smacking them with your fist, because they were then well-lubed.

Which led us to the next revelation: when we had to change out sleeves (because of some catastrophic racing mishap) we would remove the affected one's O-ring, then would heat up the cylinders, lay them upside down on a bench (in a big cooking pan), and pour refrigerated 10w oil around the desired sleeve. It would suck that cold oil up into the sleeve, and as soon as the fins were cool, you could tap that sleeve right out.

It's fun to learn from the mistakes: makes you smarter. 

That's interesting. I have never had a problem with "oil creep" using overbore kits with oversize sleeves. You can't use the o rings with the larger sleeves.

[Terry in Australia]

I wonder if that's because we use HD cylinder studs on our bikes Mike? I guess it's possible that with standard studs only 15 or so ft-lbs of torque, the head gasket isn't "pulled down" as much as with 25, so there might be some seepage? The big Suzy's torque values are bigger again, so that's probably why mine is ok? Cheers, Terry.

[IBOMILES]

What happened?  All of the good wisdom stopped . . . and here I am holding my crankshaft in my hands not knowing what the heck to do next . . . Help!

[Magpie]

Your first post and you've got your crankshaft in your hands? You're a fast learner. What are you doing to what bike?
Cliff.

[CBcafeJunkie]

So I have ripped into my 1975 cb750f and found some rust on the valves. I read earlier about porting the valves what is that, can I do it, and if I can't do it how much will it cost? Also I want to paint the crankcase when is the best time to do that , and if I sand blast it what should I def cover thanks a ton!!!!

CBcafeJunkie