Topic: Rebuilding Top End CB750 (Updated 2/27/13)

[NewOldSchool]

I have finally gotten the head off and disassembled to the point where only the valves remain and have two questions:

I did a leak test by flipping the head upside down and filling the combustion chambers with fuel. Sure enough fuel slowly dripped out of the number 2 cylinder exhaust valve over a 30 min period. The rest sealed flawlessly.

First:

My plan is to take each valve one by one and lap them at home. I know the cheap and easy way is to slip fuel line over the end of the valve and put the other end of the line over an extension in the chuck of a drill and slowly turn them in the seat with lapping compound. I have also heard to never do this because it could wear the guides so which method do you recommend?

Second:

When I go to reassemble the top end I have heard two ways to install the head and base gaskets. Either dry or sprayed with copper compound. Which do you all recommend? ( I have the standard cycle x gasket kit)

[Old Scrambler]

I would skip the cheap and easy approach for more of your time invested in searching the CORRECT & BEST way to grind valve-seats and lap valves to the seats...........

[NewOldSchool]

Maybe I shouldn't have said cheap and easy haha... I am going to carefully take my time and do it right, with all other valves but one sealing flawlessly and compression testing good before I disassembled everything I was going for more of a clean up and fresh gasket job than a total head rebuild.

I'm on a tight budget and just wanted to stop the leaky head issue. You should see how CLEAN this engine is inside, the cam and bearing surfaces look almost new and its the first time I believe this motor has ever been apart! (31,000 miles)

[madScientist]

If I remember right the manual says to install the gasket's dry.

As for lapping valves with a power drill...i'm not sure i'd try it.

[lucky]

Before you work on the valves you need to make sure it does not need new quides.

You cannot just take old valves out and lap them. The seats need to be recut AFTER new guides are installed!!!

So measure the amount of valve guide wobble.
Then recut seats.
Put in new valves and lap.

UNLIKE a car you cannot grind valves.
On a motorcycle you just replace them.

WATCH THIS:
Honda CB750 cafe SOHC4 Neway Valve Seat cutting.AVI

[NewOldSchool]

Thanks, I was not aware that I couldn't do that to CB750 heads. Goes to show you can't trust all you read and see on the interwebs. (Not to mention they don't seem like the most trustworthy bunch?)

For reference:

cb750 sohc top end rebuild on a budjet

[MCRider]

No need to be paranoid about the cam chain. It ain't goin anywhere and will easily mesh back into the crank sprocket. Just let it drop down and out of the way. Pick it up with a magnet when its needed.

RTV/SILICONE!!!  NOOOOOOO. Suicide. May not hurt if only used on pucks, but its killed many a motor.

[lucky]

Yes... you have to be careful what you see on the internet.

Separate the wheat from the chaff.

There is no such thing as a budget rebuild.
It is either rebuilt correctly, or it is not rebuilt correctly. Price may vary.

[NewOldSchool]

No need to be paranoid about the cam chain. It ain't goin anywhere and will easily mesh back into the crank sprocket. Just let it drop down and out of the way.

Yea I thought the same thing, no big deal. I just tied a clean red shop towel through it and stuffed it into the case a bit to keep stuff out and keep the chain easily accessible.

[NewOldSchool]

What about sealing it all back up?

I've heard coat the cylinder base gasket and do the head gasket dry?

[MCRider]

What about sealing it all back up?

I've heard coat the cylinder base gasket and do the head gasket dry?

Everybody and every manual has every method and they all work better than the other, Just ask someone. Very nearly an oil thread clone.

I've heard those who swear do them all dry, some are all treated. For convenience sake I treat one side with a non-hardening sealer like Gasgacinch. But that's to hold them in place as much as anything. Sealer on the pucks is a good idea.

[MCRider]

I think they probably end up with a decent result for bottom dollar. We can nit pick it as long as we want.

[NewOldSchool]

What about sealing it all back up?

I've heard coat the cylinder base gasket and do the head gasket dry?

Everybody and every manual has every method and they all work better than the other, Just ask someone. Very nearly an oil thread clone.

I've heard those who swear do them all dry, some are all treated. For convenience sake I treat one side with a non-hardening sealer like Gasgacinch. But that's to hold them in place as much as anything. Sealer on the pucks is a good idea.

HaHa definitely don't want this thread degrading into one of those!

I keep thinking of new questions while I have the attention of you more experienced guys.


Can I leave out the two extra bolts that hold the rocker shafts solid (so they can't rotate freely) and only install the center retaining bolt on the rocker blocks? I understand this leads to more valve noise which is why Honda added the bolts in the first place, but also a tad more HP.

[MCRider]

Yes you can leave them out. If they are the notched in the middle sort of pin, yes they will click if you're good enough to hear it. HondaMan talks about this.

If you have the wasp waist pins, they will rotate 360deg and won't click or wear unevenly. These are more rare. HM was threatening to make some but i don't think he has yet.

[NewOldSchool]

Yes you can leave them out. If they are the notched in the middle sort of pin, yes they will click if you're good enough to hear it. HondaMan talks about this.

If you have the wasp waist pins, they will rotate 360deg and won't click or wear unevenly. These are more rare. HM was threatening to make some but i don't think he has yet.

It is a 1974 K4 and yes they do have the notch in the middle for the center bolt (three bolts were there stock, total for each shaft) I am not sure I understand what you mean about wasp waist pins?

[MCRider]

Yes you can leave them out. If they are the notched in the middle sort of pin, yes they will click if you're good enough to hear it. HondaMan talks about this.

If you have the wasp waist pins, they will rotate 360deg and won't click or wear unevenly. These are more rare. HM was threatening to make some but i don't think he has yet.

It is a 1974 K4 and yes they do have the notch in the middle for the center bolt (three bolts were there stock, total for each shaft) I am not sure I understand what you mean about wasp waist pins?

The notch goes all the way around the pin, giving the appearance of a wasp's waist. That allows the pin to rotate as well as go side to side a little hitting the bolt that holds it in place. least resistance, most hp, most noise.

[NewOldSchool]

Yes you can leave them out. If they are the notched in the middle sort of pin, yes they will click if you're good enough to hear it. HondaMan talks about this.

If you have the wasp waist pins, they will rotate 360deg and won't click or wear unevenly. These are more rare. HM was threatening to make some but i don't think he has yet.

It is a 1974 K4 and yes they do have the notch in the middle for the center bolt (three bolts were there stock, total for each shaft) I am not sure I understand what you mean about wasp waist pins?

The notch goes all the way around the pin, giving the appearance of a wasp's waist. That allows the pin to rotate as well as go side to side a little hitting the bolt that holds it in place. least resistance, most hp, most noise.

I can shoot a picture when I get home, but yes my bike does have the groove all 360 degrees around the pins in the center of them. It is not just a notch on one side.

Anyone know of a reputable shop or forum meber and the average cost to do a fresh cut on the valve seats?

[MCRider]

APE is in Rosamond, is that near you?
http://cbrzone.com/sohc.html
Course people ship stuff all over the world these days.
They would do the whole 9 yards, but you could ask them for something less ala carte.

[NewOldSchool]

APE is in Rosamond, is that near you?
http://cbrzone.com/sohc.html
Course people ship stuff all over the world these days.
They would do the whole 9 yards, but you could ask them for something less ala carte.

Thanks you've been a big help. I'll post here if something goes awry with reassembly.

[NewOldSchool]

Is this still serviceable?

The teeth are long gone, but they roll freely and aren't falling apart.

[MCRider]

Sorry that tensioner is shot. See all the roughness? That's bits of rubber that has gone into the oil. Get a good look at a new one and you'll see.

Those raised center humps should be square shouldered and smooth.

[NewOldSchool]

Since I've owned it I have religiously changed the oil and and checked chain tension, etc.

Was this caused by a P/O not tensioning the cam chain over an extended period, or is this how they look at 30,000?

[MRieck]

Since I've owned it I have religiously changed the oil and and checked chain tension, etc.

Was this caused by a P/O not tensioning the cam chain over an extended period, or is this how they look at 30,000?

....and almost 40 years. Think about it.

[MCRider]

Since I've owned it I have religiously changed the oil and and checked chain tension, etc.

Was this caused by a P/O not tensioning the cam chain over an extended period, or is this how they look at 30,000?

....and almost 40 years. Think about it.

like Mike says...

If it were 15 yrs, religious oil changes and tensioning, I'd be surprised to see it that way.

But time wreaks as much havoc as use does. Outgassing of the rubber softening chemicals can cause even a lightly used part to become hard and come apart.

[NewOldSchool]

More great news, It seams 1 or 2 of my valve guides are worn, I checked them with a micrometer and they are outside of Honda's spec...

I see cycle X has a "Rev Kit" that comes with guides, valves, beehive springs, retainers, and a CX1 cam.

Any experience with this kit or know anything about that cam?