Topic: CB750 Motor rebuild... for dummies

[CrisPDuk]

I'm liking this thread, inspiration abounds

Although I have to confess that I've already bottled it with my engine, the decision has already been made to send it down to BryanJ for it's rebuild

The warmth might be from the few bloody marries I've had this afternoon 

Doesn't more than one marry at a time make you a bigamist though

[Terry in Australia]

Doesn't more than one marry at a time make you a bigamist though

Not if you live in Utah................ 

[CrisPDuk]

Apparently they even frown upon it there these days Terry


I've never understood why any guy would want to marry more than one at a time anyway   Surely just the one gets in the way of bike time too much already

[legendary]

This thread is really taking off...thanks to everyone who is adding in.

dustyc- nice tip on the cardboard with the 1" grid I'll be doing that when I catch up.

paulages- my humble opinion your CB750 motor pics would add to this thread! (As will anyone elses.)  I think that
              including  350/500/550/650 would muddy the water. Start a thread for each.

smack doogle Jack- You are way ahead of me. But then again ..I already have a valve spring compressor.
                      I'd loan you mine but they are pretty reasonable. Round trip shipping would be close to the cost of your
                      own.

                 -switching to the frame? Motor thread man! Focus Jack, focus!

Honda? -   You are way ahead! Your pics and questions will really help me so keep it coming.
               "Good source to buy rings?" You asked.
                On page one... 9th post is by Terry in Australia gives a great rundown on the parts we will all need.

On the same topic...Terry in Australia's recommended source for Primary Chains is Terry Quail or Quail "owner of the comfortable K8"

When I asked about getting a Primary Chain- Terry replied:

I buy oem new/old stock through a buyer in Japan.  I order once a week. It takes 10 days to arrive.  If I special order anything I want the buyer to provide the part number and it has to be piad in advance since I can not make returns.
(Terrys e-mail address removed)
I can sell it straight up or through e-bay. pay by pay-pal.  Let me know.  E-bay store is Iron-Horse-Honda
Terry Quail

I just saved you and Terry some time- take advantage! :-)

[Jerry Rxman Griffin aka MuthaF'er]

Doesn't more than one marry at a time make you a bigamist though

Not if you live in Utah................ 

MAKES YOU A BIGA-FOOL IN ANY STATE (well, in Terry's DRUNKEN state that's a different story)   

[legendary]

Took four days "off" from the engine to do my "day job".  Very much appreciate the help so far!

Here are the latest pic's

Last time I posted I had the valve cover off. This afternoon I took apart the top end and followed the advice of keeping my parts organized. Fasteners are arranged on card board at the moment. Tomorrow I am going to print an enlarged digital pic of the head and attach it to the back of a scrap of Lexan. Then I am going to drill holes where each fastener goes and arrange them on that. My attempt at making it idiot proof.

The interior of my motor looks good with the exception of the crud on top of the pistons. When I get it torn down some more I will take close ups of individual components. Hopefully someone can tell me which components look like they need to be replaced.

No real problems with this.....

Open the advice gates guys..... whats next??

Steve

[RRRToolSolutions]

Stave, are you taking the bottom-end apart? If not, I would certainly encourage you to pull the oil pan and clean the screen to the oil filter and to clean the inside of the pan. It goes on with a gasket, so it's an easy clean and re-install.

Is this a top-end build only?

One more piece of advice - that engine is a whole lot easier to lift/handle with all of the side covers, clutch, stator and rotor removed.

Regards,
Gordon
 

[bikehenge]

I appreciate this timely thread, I'm in the process of reassembling a bike (minimal engine work), but I have big plans for a second bike.

I am considering a higher performance cam, and from what I have gathered so far I should also replace the cylinder studs (great how-to info BTW).  I would like to experiment with this bike and have pipe dreams of adding boost of some sort (please don't laugh...).  Because of the additional stress I may put on the engine I was considering having my con rods shot peened (~$85 from APE) , but I see forged rods on ebay periodically for ~$200.  Are either of these options worthwhile?  The Carillo rod option is out of my league and I would prefer to use the stock crank.

Thanks for the input, I think this is within the scope of this thread.
Rob

[legendary]

Hello Gordon,
                    I am going to take the bottom end apart as well.  I am doing this to learn for the most part. The toughest part of this so far has been getting over the fear of ripping into it. 

Then again...I haven't put anything back together yet!

After today I need the mental reset of getting organized and cleaning up a bit.

I think your advice on removing the side covers etc. to reduce the weight is a great idea. Especially before I get into the bottom end.

I really need to get a shopping list going as well. Have to get the brain caught up to the hands.

Thanks Again!   Steve

[paulages]

well i've forgotten my camera through most of my build so far, but i have another 750 coming up right after this one. i'll try and document that one well. i'm almost ready to start putting together my 718, which will have its own pictorial thread as well.

[Terry in Australia]

well i've forgotten my camera through most of my build so far, but i have another 750 coming up right after this one. i'll try and document that one well. i'm almost ready to start putting together my 718, which will have its own pictorial thread as well.

Woohoo!

[Honda?]

  Well I measured my piston to bore gap. It was supposed to be at .004 and under. Well I was able to (with ease) fit a .05 feeler guage into there and the two pistons on the left side of my engine (#1 and 2) I could shift from side to side in the cylinder. It's time to shop around for a machine shop to bore them and find pistons. Any advice on pistons/rings and a Wisconsin machine shop that can bore my cylinders would be much appreciated!

-Tyson

[Jerry Rxman Griffin aka MuthaF'er]

Tyson,

That's really a shame    More expensive for you but you'll likely end up with better low end power and do less downshifting around town. May as well throw in a cam while you're in there and increase the top end power too. If you do the cam be sure to go with some heavy duty springs. About another $250 for both. You don't need to go nuts at this stage and use Titanium retainers or port & polish the head unless you have another $1000 to spare. 

Kit depends on what size you want. New kits are available in 836 (Wiseco) & 850 (CycleX) which allow you to reuse your cylinders after a bore. I don't know anything about the CycleX but a few members have used them. They can be had for about $100 less than the Wiseco at $250. If it were me I'd go with either kit as long as it has 3 piece oil rings for better oil control. Of course there's always used stuff floating around too but try to find 3 piece oil rings to fit! Now, if you have another $2000 you can get everything you need to go to 1000cc but thats another story that I probably don't need to go into at this time.

[smack doogle]

Legendary - Nice job on taking it apart.  I too am feeling a whole lot better now that I've got to this point.  Should start getting interesting in the weeks to come.

Honda?  - I'm sorry to hear about that.  Sounds like a big pain if you asked me.  Good luck!

I'm moving slowly but surely.  Keep the good posts coming folks. All help is greatly appreciated.

[Terry in Australia]

Here you go mate, don't spend a squillion bucks, (Jerry, stay off the hootch for 5 minutes, can't ya?) this 3rd over rebuild kit is 51 bucks with 54 minutes to go, so get bidding!

http://cgi.ebay.com/ebaymotors/Honda-CB750-SOHC-PISTON-KIT-75mm-AHRMA_W0QQitemZ300186621787QQihZ020QQcategoryZ35581QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

[Patrick]

Good catch, Terry.

[Terry in Australia]

I gotta tell you Pat, I'm tempted to bid on this myself, just in case the other boys are asleep!

[chippyfive50]

GREAT TIMING ON THIS POST FOR ME,  LAST NIGHT I DID THIS.
IT HAS 10K ON THE CLOCK, SHOULD I DO THE RINGS??  (GOOD COMPRESSION THOUGH)
IT HAS A BAD VALVE SEAL AND O RINGS.

[RRRToolSolutions]

You've got to pull the head to service the valves. Look at each cylinder wall - top to bottom and look for scoring. If it's not burning oil, then no - not mandatory.

Me - I'd hone the cylinders, replace the rings, and clean the piston groves to make sure there is no carbon build-up. That way you've got 40,000 or more trouble free miles ahead.

Regards,
Gordon

[legendary]

Hey Guys,
               no pics at the moment...but I took Gordon's advice this afternoon. Took off the side covers, drained the oil, and while I was at it, removed the starter motor.

What remains is light enough that I can move it myself which is nice.

Bought a 1000 lb engine stand from Harbor Freight today.($55)

A traditional v8 engine stand will need to be modified to support a CB750 motor...... and this is worth doing. Working on the floor is not the ideal situation. I need to either make or buy a fixture to adapt the engine stand to the CB750. If anyone knows of one (or has drawings) let me know.

Thanks All. Enjoying this a lot and so far nothing in my motor to make me cry! 

12 hr days Mon & Tues. So see ya'll Wednesday.

Steve

[RRRToolSolutions]

I'll take a photo of the 90 degree elbow I had made that allows the horizontal position as shown in the photo of my 836 build. The 750 Kaw engine photo shows the stand-offs I had welded onto sturdy bolts. These allow any engine to be mounted and turned 360. I've had 750 Kaws, 500 Kaws, a CB750, and my Black Bomber engines on there just in the last 2 years. The elbow simply fits into the orange stand's round neck and has a pipe welded into a 90 degree so I can insert the plate you see down into it like a turntable.

My son is away on a deer hunting trip in Alabama and he has my digital camera with him. I'll take some upon his return.

Regards,
Gordon

[SandwichEnthusiast]

Hi guys,

I just stumbled along this forum thread, and I'm in the same boat as legendary and smack doogle I've asked a few questions on the UK SOHC board, but this thread seems like a better place to get some informed answers.

So I guess I'll lay the basics down:
I'm 25 year old Canadian, and working on a Blue Metalic 1975 SOHC CB750K that my dad bought for $500+/-. mileage unknown as there were no gauges. He bought it off a buddy who got it in non-running condition who rebuilt the carbs, and wasn't that.

My father, who owned a different CB back in his salad days (put it through a pop up camper that cut in front of him) and who has some mechanic background turned it over a few times, only got compression on the left two cylinders: uh oh.

Honda? posted a picture that looked a little like this:


So now we're talking some parts (that I've already got, more on that in the future)

I've gotten the engine down to about where the other guys are: cylinder head off, looking at the carbon on the cylinder heads.

I can feel the steel grit from the camshaft in the grease and grime in the engine, so the old man and I are in agreement on the full tear down, but not on the ridiculous windjammer fairing he wants to bolt on to it when we finally piece it together.

I'll put up another post about the things I'm using and what I've learned so far.

Thanks everybody, and good luck to Leg. and Doodle

[SandwichEnthusiast]

So now the details:

I'm working from two main sources: the PDF of the shop manual and the Clymer Honda manual (eighth edition). I like the shop manual better. Also I have the parts fiche jpegs and parts lists off http://www.crotchrocket.com which I numbered (1 to 52 alpha order) and am using the part numbers on each diagram to bag and tag parts along with a bit of a description i.e.: #15 - 22, 21, 15, 16 Cylinder head bolts and nuts + washers.

heres some pics of the cylinder head


and the top of the pistons


So we're assuming that the camshaft broke (see my user photo) because something clogged up the oil baffles (baffles right word there?) and ground the sucker to dust.

Any experiences with broken camshafts? what should i be looking for? hows those pistons look?

Also the new camshaft holders i bought (which include rocker arms) and such only have two screws per holder where the old and ground up ones (1975) have three. are they not compatible? whats the story?

oh and for a laugh: I spent about thirty minutes trying to get the cam sprocket bolts off (try to turn the bolt, camshaft rotates) before i realized there's a bolt on the other end to hold the thing in place/ rotate it.

So I guess its a foot race, who wants to pull apart the next part of the motor and risk breaking something the next guy will learn from the first?

Thanks Guys.

[Honda?]

Ok. I need some good news guys.

I've decided to go for the 836cc kit for my bike since it costs about the same(in my area) for an 836 as it does for a first overbore. I will not be racing my bike. My bike will be used for commuting and some fun stuff so I don't need any insane performance parts.

My first question is can I(safely/reliably/effectively) run stock valves/springs/camshaft on an 836cc engine? Reccomendations?

And here are some "can I reuse it" questions with pics!
There are no pits on the mating surfaces at all....only discoloration. If that matters

Can I reuse my intake valves? They all look this way.

Are my intake valve seats good? (all look this way)

Can I use my exhaust valves?

Are my exhaust valve seats good?


One other business detail. I wire brushed all of the crap off of my valves and valve seats. Once I did that I put the valves back into the head and jammed a light into each port to check how well they seat with the head. I was unable to see any light escape from between the valve seat and valve. So i'm thinking they mate up to eachother very well.

Thank you guys for all of your help! Haha I never would have dreamed I would be spending this kind of money on my bike. When I bought it for $700 I thought to myself "this should last at least another 20k miles so all I have to replace is chain/sprockets" I sure opened a BIG can of worms!

[Terry in Australia]

You should "lap" your valves and seats in mate, but they look like they'll lap in ok. Stock cam and valve train works fine with the 836cc kit, you'll just have a bit more torque and overall power, and you can always enhance it with a nice cam later. Cheers, Terry.