Topic: CB750- What's Needed for Head Gasket Change?

[Mikey G.]

So, I'm pulling the motor of a barn-find CB750 K2, and because there's a huge oil stain on the front of the case, I figure I'll replace head gasket and o-rings while I'm at it.  Before I get too deep though, what's a list of all the gaskets I'm going to need when I do this?  Here's what I got-

-Valve cover/rocker arm cover gasket
-Head Gasket
-Oil passage o-rings (2)
-Oil line o-rings (maybe?)
-Tach cable seal (maybe?)

Anything else I should grab before I go any deeper?

[ekpent]

 Have you heard the engine run yet or know for sure if the tranny is good ? I like to try and hear something run first to get a good assessment unless its a bonafide rebuild situation from the start. Be a shame to do the top end and find a problem in the lower.

[Mikey G.]

Heard it run?  No.  It DOES turn over, and seems to have compression.  Shifting seems alright, best I can't really tell with a frozen clutch.

Really the only reason I'm replacing the head gasket is because of a frozen spark plug (which I need to remove- I may need to pull the head to do so) and the giant oil stain on the front of the block.  If there's problems lower in the engine, I'll just sell the entire project- that's where I'm at with this bike.  I can't tell if it runs until I pull the spark plug, and if the engine is out it's worth fixing what I know is wrong.  If bigger issues show up later, I'll tackle them then.

I'm really only fixing the oil leak up front- it's hard to tell with a 30 year old barn find what's wrong until I at least get the motor running.

[Jerry Rxman Griffin aka MuthaF'er]

6 sealing pucks under the cam towers are a definite to replace.

[754]

You do know that removing topend,  means pulling the motor out of the frame..
Unless your frame is already cut under the tank, or has a frame kit,  then you need a frame kit to keep the frame from flexing..

[mrbreeze]

6 sealing pucks under the cam towers are a definite to replace.

And 4 small o rings. 2 go over the oil nozzles and the other 2 on the ends. And some sealer, high temp thread lock, and...............there could be more.
I just did mine the other night. My bike still ran great but what was once a trace of oil became a weep. After that it became wet but not really hitting the ground. I decided that I would not even pull the head but just change the pucks. I had the luxury of not pulling the engine.....I'm putting in the frame kit at the same time. Also have the luxury of knowing how good it ran before teardown which you don't know about your's.

[mrbreeze]

 Another couple of things since you are going deeper than I did.........tensioner gasket, tensioner wheel, guides?.

[mrbreeze]

One more other thing.....find and read Brandon's leak free top end thread..........good info.

[PeWe]

If using MLS head gasket... It has rivets that will probably will make problems when they are thick and make it impossible to tighten the head  completely sealed.
- Hammer all rivets flat
- Drill dimples where the rivets hit the cyl block and head. 10mm drill will make a wider dimple that will fit even if the dimple is not perfectly matched with the gasket rivet. Not needed to drill a hole thru the flange.

[Ericcb750]

On my 78K which was my first re-build I tore into it thinking it was the head gasket but it was those darn pucks under the cam towers like Jerry has mentioned....Now knowing what I know it makes it appear to be a head gasket leak when one or more of those are leaking....IF you have the funds too , I would take a look at Frank's (754) weld less frame kit...it's a work or art and EXTREMELY well made. I haven't installed mine just yet but there are others who have and are happy with them.

I hope this helps a little. For my top end I ended up using a Vesrah top end kit (but keep in mind it doesn't come with those sealing Rubber pucks for the cam towers)....why I have no darn clue....

[Eydugstr]

So, I'm pulling the motor of a barn-find CB750 K2, and because there's a huge oil stain on the front of the case, I figure I'll replace head gasket and o-rings while I'm at it.  Before I get too deep though, what's a list of all the gaskets I'm going to need when I do this?  Here's what I got-

-Valve cover/rocker arm cover gasket
-Head Gasket
-Oil passage o-rings (2)
-Oil line o-rings (maybe?)
-Tach cable seal (maybe?)

Anything else I should grab before I go any deeper?

Like everyone said, the rubber pucks that go underneath the cam holders (x6).  One thing to note:  Measure the pucks with a caliper, and make sure they're all consistent.  One time I ordered a set from both Honda & Cycle X, and the Honda OEM's wound up being smaller diameter and thinner! http://forums.sohc4.net/index.php/topic,115057.msg1374645.html#msg1374645

Also, if you decided to have a machine shop deck the head flat, cut the cylinder head dowels down as well.

Also there's a honda service bulletin concerning early K cylinder heads, calling for extra o-rings to be used around the cylinder head studs.  Here's a link that has a picture of that service bulletin (Look for Harsh's post) http://forums.sohc4.net/index.php/topic,155043.msg1795388.html#msg1795388

[Mikey G.]

Thanks for all the advice, guys.

This thing with the pucks under the rocker towers has me intrigued.  I just checked the oil leak on the engine best I could (again- it's nothing but a stain on an engine that hasn't run in 30 years), and it may be coming from above the head gasket line.

If it's just the pucks that are the problem, that would be fantastic.  I don't really want to crack the head if I don't have to- the engine seems to have pretty good compression already, and the less I have to mess with it the better.

I'll keep everyone informed once I start tearing into this beast.

[evanphi]

FYI, if you need to cut shims for the pucks, they are 1" diameter. I did the same for mine last winter and no leaks!

[newday777]

Thanks for all the advice, guys.

This thing with the pucks under the rocker towers has me intrigued.  I just checked the oil leak on the engine best I could (again- it's nothing but a stain on an engine that hasn't run in 30 years), and it may be coming from above the head gasket line.

If it's just the pucks that are the problem, that would be fantastic.  I don't really want to crack the head if I don't have to- the engine seems to have pretty good compression already, and the less I have to mess with it the better.

I'll keep everyone informed once I start tearing into this beast.

I know this is an old thread.....
Lucien
You never really updated this thread as to what you ended up doing on this motor leakage.

[Alan F.]

He hasn't been here since last November 25th, hopefully all is well.

You might try reaching him with the email icon in his profile, tiny envelope icon opens a forum form to write your message.

[newday777]

He hasn't been here since last November 25th, hopefully all is well.
I know about the pm
You might try reaching him with the email icon in his profile, tiny envelope icon opens a forum form to write your message.

[Alan F.]

I don't mean the pm link but the email link that'll send messages directly to his registered email address.

[Mikey G.]

Oh hey!  Yeah, I may have.... gotten distracted by something shiny for a bit.  When you've got three other bikes that tends to happen.

In any case, replacing the pucks was about as deep as I went.  I got everything buttoned up and back together, and everything seems to be fine.  If I'm 100% honest, though, I genuinely haven't put enough miles on it to be sure.  I want to say that at some point I did get another small weep around the head gasket area, but it was minor enough for me to chalk it up to a combination of "old bike" and "it's sorta something these engines just DO".

I also noticed what looked like a casting void in the valve cover by the tachometer drive cable, which allowed a LOT of oil past the seal.  I'm curious as to how much this influenced my problem too. 

Once the weather clears up I plan on riding this one a bit more consistently, so I'll have a better idea of what's going on, but for right now it's satisfactorily "good enough" for me.