Topic: '73 CB750 redo for a clean "machine" look

[dumbquestions]

Hi all,

My name is Arthur, I'm a longtime lurker here with a '73 750 in pieces. I want to put together a semi-cafe with a clean "machine" look and decided to get off my duff and do it  once and for all. As my username indicates, I will have plenty of dumb questions, but right now I'm struggling with photobucket and trying to set my album to public and copying over the links here. Hopefully I've got it right.

Here is the bike I'm seeking to imitate:



Here is how my bike currently exists:





Have the bible by hondaman to steer by. Onwards and forward with the dismantle. This is a top to bottom dismantle- there's a slight crack in the case from a loose chain whiplash. It also had trouble shifting in and out of first. Oil weeps out between the jugs and head, so I suspect the gasket is shot.

[dumbquestions]

Was it a dumb move to loosen the cam bearing caps before extracting the rocker shafts? Because as soon as I did so, the left cam tower just started to rise off its base 



FWIW, the cam chain is absolutely stuck and refuses to move even though I removed the cam chain tensioner. I'm thinking to break the cam chain, I plan to replace it anyway because I  have absolutely no idea how many miles is on the engine, bike came to me minus speedo and tach, from an abusive PO.

[mystic_1]

Hrm, that frame is a 78 F model.

Yes you should have pulled the rockers before uncapping the cam shaft, because now spring pressure from the valves is pushing things up as you found.  You risk bending or breaking things that way imho.  Put the caps back on, and then disassemble the rockers before unbolting them.  It's important to do things in the order presented in the manuals for reasons like this.

As far as the cam chain is concerned, what do you mean by "it won't move"?  Can you rotate the crankshaft, and therefore the camshaft?  Or is the motor locked up?

If the motor isn't stuck, then removing the cam chain tensioner (which may be stuck) from the motor entirely should give you enough slack to unbolt the cam sprocket and slide it to the side when the time comes, which then gives you enough slack to wiggle the cam shaft out to the side.

Take your time and don't rush things, ask plenty of questions, and plenty of people here will be happy to prattle on about whatever topic you care to choose (and maybe a few you didn't lol).

mystic_1

[dumbquestions]

Thanks for the welcome, Mystic. You are right, haste makes waste. I did read Hondaman's recommendation to slide out the rocker arms before loosening the cam bearing caps but got excited and carried away with loosening all the nuts.

Fortunately got around it by improvising a makeshift valve spring compressor and pushing the arms out one at a time, that did the trick and the cam tower came back down and, what do you know, the cam chain loosened up.

On with the tear down:

Noticed some wear on the cam tower bearing (bottom):





View of the base of the cam towers, L and R:





Organizing stuff into zip bags:



Off came the head and jugs, in two pieces, even after removing all the screws.. is that normal?



Pistons:



Carbon fouled..



Here's where things get weird. I can't believe that this is the correct gasket, can it? Doesn't look right to me:



This took me by surprise- dirt encrusted inside the jugs, they didn't end up there by accident, was pretty encrusted:

[dumbquestions]

Separated the head from the jugs by inserting wood shims between the fins (carefully) then once a sliver of a crack opened, inserted the shims into the crack and separated the heads. Those fins need a serious cleaning:



Looks like the bike was running too rich?

[mystic_1]

I've seen worse 

Should clean up nice.

mystic_1

[mystic_1]

View of the base of the cam towers, L and R:

One comment I'll make here, is that in these two pictures, you can see that on either side of the cam chain tunnel, on the intake side, next to the head nuts, you can see two circular aluminum pieces which are pressed into the head.  On one of your pictures there's still an o-ring around it, on the other pics the o-ring is missing.

These are the oil control orifices which control oil flow to the valve train.

Two important items here:

• Don't loose them!
• Make sure they aren't clogged.
• Make sure to reinstall the o-rings around these at re-assembly.  There are matching o-rings that go under the outside corners of the cam towers, to keep things even.

Hmm, that's three things.

• Clean the oil passages in the cam towers too.

OK, four.

mystic_1

[Tews19]

What does the head have engraved ? Possible decking done prior or head work? You can see some writing in the one pic.

[dumbquestions]

One comment I'll make here, is that in these two pictures, you can see that on either side of the cam chain tunnel, on the intake side, next to the head nuts, you can see two circular aluminum pieces which are pressed into the head.  On one of your pictures there's still an o-ring around it, on the other pics the o-ring is missing.

Do you mean the orifice that the arrow is pointed to?

[dumbquestions]

What does the head have engraved ? Possible decking done prior or head work? You can see some writing in the one pic.

Your guess is as good as mine.

[dumbquestions]

Also, mystic, to answer your question, yes that is a 78f frame. That is what the bike came with. I am debating whether to keep it or trade it (or buy) a '73 frame.  I love the '73 oil tank with the metal cap, but don't know if it'll fit on a '78 frame.

Not digging how far out the hoop goes on the '78 frame anyway. I hate the custom seat that comes with the bike- it could sit three comfortably.

[mystic_1]

Do you mean the orifice that the arrow is pointed to

Yup!

mystic_1

[dumbquestions]

Yup!

mystic_1

Ok, yes I have both top end oil jets and all O-rings in a zip-loc for safekeeping.

Also was very particular about labeling rockers/arms, etc. so they will go back in the exact same position they came out of.

Any thoughts on the gasket? It seems to be a very poor fit.

[70CB750]

Welcome, DQ

[dumbquestions]

Thanks 750.

Would you or anyone else have a thought re: the gasket on the upper case? It's hard for me to believe that it's the correct one for a K3 engine.

[calj737]

Would you or anyone else have a thought re: the gasket on the upper case? It's hard for me to believe that it's the correct one for a K3 engine.

It is not, but doesn't really matter as you will be replacing it anyway.

[dumbquestions]

Thanks Calj. You're right, pointless to fret over it.

Well today I got the cases split, and I was sweating bullets loosening those bolts. Read too many horror threads on snapped bolts on here, I guess. But all of them came out and after quite a  bit of time carefully working wood splits in every where and tapping it apart, the cases came apart:





However, my joy at finally splitting the case was tempered by a horrific discovery:









And the upper case in the same place:





Thorough eye check of the gears amazingly show no chipped tooth or disfigurement, 3rd and 4th gear seems to have gotten the brunt of the damage.

My guess is that at this point, the cases are shot and I'll have to find a replacement case not only because the space to weld those crack is nonexistent, but also the bolt holes are tore apart as well.

But I will defer to the expert opinions here.

[calj737]

There's a member here, Bill Benton (http://forums.sohc4.net/index.php?action=profile;u=11731) that is likely to have cases available to ship you. I do know at the moment he is traveling on the West Coast, but he is due back (ATL) by end of the week. I'd suggest you PM him and get new cases.

If its any consolation, that crack is not an uncommon occurrence due to a thrown chain off the front sprocket. But your cases are pretty much worthless unless you are a very competent welder.

[SOHC4 Cafe Racer Fan]

Yes, those cases are toast.  Remember to get a matching set of cases.  Also not a bad idea to stick with the same bearing pattern stamped on your existing cases.

[dumbquestions]

Thanks Cal and CRF, I suspected as much. Not too upset about it- old bikes are old bikes, labor-intensive and a money pit, but I made my peace with it a long time ago.

I will PM Bill.

CRF, can you please elaborate on the bearing pattern #'s I'm looking for? Where would those numbers be?

[mystic_1]

The size codes for the crankshaft main Journal bearing sizes are stamped into the  crankcase, on the bottom side of the mounting boss for the front engine hanger bolts.  It will be five separate letters, for instance ABBAA

You don't technically need to keep these identical to your Originals, so long as you replace the upper and lower crankcase halves as a set, and get all new bearings after cross-referencing the charts in the factory service manual.

mystic_1

[SOHC4 Cafe Racer Fan]

The size codes for the crankshaft main Journal bearing sizes are stamped into the  crankcase, on the bottom side of the mounting boss for the front engine hanger bolts.  It will be five separate letters, for instance ABBA

You don't technically need to keep these identical to your Originals, so long as you replace the upper and lower crankcase halves as a set, and get all new bearings after cross-referencing the charts in the factory service manual.

mystic_1

True, but you better get matching case halves!

[dumbquestions]

Would these be the numbers/letters I'm looking for? I didn't find anything else on the inside of the bottom case.



Also I removed everything except for this bearing inside the case. I guess I can easily rig some sort of bearing puller with a long bolt and a couple thick washers and nuts, or is that bearing easily replaced?

[riffman12]

what on earth happened to that case? looks like a grenade went off in there.

[SOHC4 Cafe Racer Fan]

Nope.  The bearing sequence is actually stamped on the outside rear of the lower case.