Topic: CB750 Motor rebuild... for dummies

[ealanm]

Question of the day:  Every visible fastener is out of the cylinder head. What is a good method for breaking it free of the gasket material without doing any damage?

I have a medium-sized rubber hammer that works well.  A couple of taps on either side and the head pops right off.  I strike at an angle on a corner of the head, so that it imparts a bit of rotation.  A plastic hammer would probably also work, but I've always used the rubber one, on the theory that it's much softer than the fins and therefore less likely to do any damage.

[Terry in Australia]

Yeah, be vewy vewy careful whacking your head or cylinders with anything, a rubber mallet will smash brittle fins off just as easy as a sledge hammer. (well, maybe not, but I still wouldn't do it) Sometimes it's necessary to "shock" the head off the cylinder block, but often, you can grab the head and "rock" it off the cylinder block.

When it comes time to clean off old gasket residue, try paint stripper, it works a treat! Cheers, Terry.

[ocarr18]

First time to the forum and looking for some answers/suggestions. Bought a 1976 cb750 from a friend who is a serious traveller and bike has been stored for about a year. Last time he messed with the bike he noticed a little gas in oil pan? Whats best case/worse case?

[borzwazie]

leaky petcock seals. get that fixed, don't want your oil all thinned out.

[cafesomeday]

Okay, so I just read this whole thread in one sitting and am pretty overwhelmed. I got a '77 cb750 in the beginning of this season for cheap money which I thought was in great condition. I was planning on rebuilding the engine and going through the entire bike nice and slow, but I'm gushing oil out of the engine (neighbors have started leaving me notes) and I'm afraid that I've got to tackel the engine sooner than I had hoped. I'm working a 9-5 and full-time in school, so I was just hoping to get this thing through until may when I'm done with school and had some time to tinker.

My question is, realistically, can I complete a top end engine rebuild in a weekend (I'm a complete nube and only have the internet, a shop manual, and a clymer's manual). Or, is it more likely than not that I'll come up with many of the issues that have been documented and I'll need to go back to ebay or another seller? Thank you for taking a minute to reply to my silly inquiry, I'm sure I'll be back with pix and all kinds of frustrations.

[ev0lve]

Okay, so I just read this whole thread in one sitting and am pretty overwhelmed. I got a '77 cb750 in the beginning of this season for cheap money which I thought was in great condition. I was planning on rebuilding the engine and going through the entire bike nice and slow, but I'm gushing oil out of the engine (neighbors have started leaving me notes) and I'm afraid that I've got to tackel the engine sooner than I had hoped. I'm working a 9-5 and full-time in school, so I was just hoping to get this thing through until may when I'm done with school and had some time to tinker.

My question is, realistically, can I complete a top end engine rebuild in a weekend (I'm a complete nube and only have the internet, a shop manual, and a clymer's manual). Or, is it more likely than not that I'll come up with many of the issues that have been documented and I'll need to go back to ebay or another seller? Thank you for taking a minute to reply to my silly inquiry, I'm sure I'll be back with pix and all kinds of frustrations.

Being still closer to the complete noob side than seasoned veteran here's my two cents.

Can it be done in a weekend? Yes. Can you, right now, do it in a weekend? Probably not.

From having all the right tools to actually ordering what you need to finding out you need an extra person to wondering how to get this or that in or off or spun around properly or you'll break something you had no idea could be broken that way. Oh, and don't forget the dropping of things through the motor. Always something.

First time in is a learning thing. You might be a fast learner though and lucky   Worse thing that would happen is you'll have it apart and ready for the next weekend  

[eastoak]

Iggy is right on the money. It's always something with these bikes. If you have a nice set of tools, shop and all of the parts you need but no knowledge of what you're doing it's going to take more than a weekend.

[cafesomeday]

That's what I thought - okay, last stupid question of the day. with oil leaking out, not gushing yet, is it incredibly stupid of me to keep riding it until I can get to it during the winter sometime, ie: dangerous, harmful to the bike despite checking the oil regularly? Thanks guys.

[ev0lve]

Where's the oil coming from? If it's leaking while it's parked I think you might have some other problem. Otherwise, wipe it down when you get home and check your oil level.

BTW, mine was throwing a fair amount of oil out the top end in the last 6 months - turned out replacing the tappet covers o-ring gaskets dried it right up. Find your leak first  

[cafesomeday]

Thanks for the tips. I'm pretty sure that its coming from the head. I have oil all over the top left (if sitting on the bike) of the head trickling down over the rest. Is it possible to just replace that gasket for a quick job or not worth it if I've already got the engine opened up. This of course is assuming (crossed fingers) that i don't come into other problems.

The only reason I'm trying to take a short cut here is because its my transportation three days a week for a total weekly mileage of about 50 miles. I was just hoping to be able to wait for May to rip it apart, but.... damn. I'll post pictures once I get a chance to take them.

[ev0lve]

Might want to open up a new thread with pics and whatnot. You'll get more and better advice.

My vibe is if you have to remove the engine you might as well buy the cow. Or somesuch mixed metaphor. 

[cafesomeday]

Doh@#@! Knew you were going to say something along those lines. I'll let you know where it goes.

[mystic_1]

Thanks for the tips. I'm pretty sure that its coming from the head. I have oil all over the top left (if sitting on the bike) of the head trickling down over the rest. Is it possible to just replace that gasket for a quick job or not worth it if I've already got the engine opened up. This of course is assuming (crossed fingers) that i don't come into other problems.

The only reason I'm trying to take a short cut here is because its my transportation three days a week for a total weekly mileage of about 50 miles. I was just hoping to be able to wait for May to rip it apart, but.... damn. I'll post pictures once I get a chance to take them.

The rubber pucks under the cam towers are a common source of leaks like this, you can do them without pulling the head but you do have to pull the motor and tear down the valvetrain.

mystic_1

[Riley001]

Hey everyone, my wife bought me a CB750K4, 1974. I've tornit apart and have to motor out looking to rebuild it.  This is my 1st bike and 1st bike project.  I'm looking to restore it as close to factory as I can, cosmetically anyway.  Ive read thru the entire thread which has some great info.  I'll be starting work on the engine next week/end and will post picks and questions as I'm sure I'll have a few.

Thanks in advance.

[mystic_1]

Welcome Riley,

That one hell of a wife you have! 

I'd suggest starting a new thread of your own rather than extending this one, you'll probably get more traffic that way.

mystic_1

[Riley001]

Welcome Riley,

That one hell of a wife you have! 

I'd suggest starting a new thread of your own rather than extending this one, you'll probably get more traffic that way.

mystic_1

That she is!  Thanks for the tip, ill start a new post next week.

Cheers.

[brah pitt]

excellent thread! I know what I will be doing this winter.

[rubicon151]

HOwdy -

So I am wrist deep in the engine of my 1976 cb750k.  some history - several PO's and I am not sure if it had ever been well cared for.  it has 23k on the clock and when i rescued her she was leaning up against the back of a barn with a birds nest (literally) under the seat and a rats nest in the tailpipe.  changed the oil and plugs - went through the carbs quick just to see if it fired.... it did and i was happy that there werent any terrible noises...

so that brings me to today.  engine is apart and I have some decisions to make...

look at my picks - valves and heads need some serious cleaning, piston skirts are scuffed.. and although i dont have a pic the no 4 outboard cam bearing has signs of wear...

I have rings, circlips, seals and springs coming from cycle ex - i figure a quick hone and rings for a cheepie refresh and I am on the road again in the spring wo any trouble...

any problems with my plan? suggestions?

 - pistol

[MCRider]

You didn't mention lappin the valves in. A hand lapping, is fine. Search on "lapping valves". Or I'll repeat the procedure. From their condition (bad) and the seats in the head, its highly recommended. That with new rings, light hone, and you'll have a sweet running stocker.

[rubicon151]

Good stuff - Thanks MCRider 

I forgot to mention that lapping the valves in was in the plan as well...

What is the best wat to get this boat anchor clean... anything easier than the wire brush

What solvent should I be using?

[MCRider]

Good stuff - Thanks MCRider  

I forgot to mention that lapping the valves in was in the plan as well...

What is the best wat to get this boat anchor clean... anything easier than the wire brush

What solvent should I be using?

If you mean by boat anchor, just the head, glass beading is the ticket. But it needs a tourough disassembly, and a more thorough cleaning afterwards. Soda blasting is more benign, but not quite as effective. Neither would require solvents, which is the direction I always go if I can. Glass beading should be finished off with high pressure spray, and or wash tub workout. Soda is water cleanup as well, but not so necesary you get every last molecule out, though you should still try.

I'll soda blast your pistons and valves for a small price, but the head is too much to ship unless your close?

[MCRider]

Getting the base gasket off the crankcase is the worst. Razor blade and acetone and patiencs. I think there are some commercially available gasket removers at the auto stores. All depends what its stuck down with.

[westfieldandy]

Hi everyone,

just read all of this topic, and need some answers to the following:-

1. when you split crankcases, should you pull the top off from the bottom, or turn crancase over and remove bottom from the top,  also what is correct proceedure when re assembling.

2. I have a mixture of engine parts, heads and cases can be identified from stampings but how do you know what barrels to use, ie if i use head stamped 410, with 410 pistons what barrels work with this, this would be an f2.

[brandEn]

Hi everyone,

just read all of this topic, and need some answers to the following:-

1. when you split crankcases, should you pull the top off from the bottom, or turn crancase over and remove bottom from the top,  also what is correct proceedure when re assembling.

2. I have a mixture of engine parts, heads and cases can be identified from stampings but how do you know what barrels to use, ie if i use head stamped 410, with 410 pistons what barrels work with this, this would be an f2.

Welcome to the forum.

To seperate the engine cases turn the engine upside down and remove the bottom from the top. All the gears and the crank will be laying in their spots. Reassemble the same way.

Check ot the engine FAQ section. Post #6 may be of some help.
http://forums.sohc4.net/index.php?topic=369.0

[lucky]

So since this is an engine rebuild thread lets get a noob question of mine out of the way.

Why do you have to hone a cylinder and replace piston rings? What happens if you dont?

Good source to buy rings? and maybe a DIY honing page? (I'm sorta at this point in my engine rebuild.)
And while I'm here.

That piston is fine.
You have to hone and replace the rings or it will smoke a lot, and it will never break in.