Topic: CB750K4 what did I get myself into?

[PeWe]

Much better!
The o-ring should raise a little over the surface so it will compress when tighten the cam holders.

Do not forget the other o-rings. There are 4. The other seal the end of oil feed to the cam and rockers.

[BrockCB750]

Are all 4 supposed to be the same size? Looks like they are 4x required in the parts diagram.

The two I put on the oil jets had to be stretched around the oil jet. As in the inner diameter was too small to just lay flat around it. They would look too small in the two other non-oil jet locations.

Sounds like I just need I order exactly the correct size. The parts blowout calls for 5.8x1.9 91305-323-000 like you said. I am going to break out my meter and measure all the rings in the kit and if none match order new ones from Honda.

[PeWe]

The earlier heads have different oil orifices and different o-rings.
Here a K2 head to zoom in with a bag of o-rings.

[BrockCB750]

Dropped the frame, swing arm, oil tank and engine mounts, etc off at powder coat. Powder coat won out.

I got the head on the other day, looked at my bag of gaskets/o-rings from the kit and realized I had 8 cylinder stud gaskets I never put in. I got to reading and watching a couple youtube videos and am I crazy or do people not put these gaskets in sometimes? These are called "GASKET, CYLINDER STUD BOLT" in the parts diagrams.

Here is what I am talking about (pictures after I put all 8 in):




Since I had the head off another questions that feels REALLY dumb. In the case of the head gasket I am using which end goes up? The writing side up, or non-writing side up? haha. I feel like a dumbass asking that.

Also, is this a decent quality gasket?

[PeWe]

Head gasket only fit one way.
Usually wrong first time independent of how you do!

[BrockCB750]

PeWe really appreciate you taking the time to reply.

Yeah woke up this morning and remembered its all offset a little bit so it can only fit one way. Probably why I didn't ask before, haha. I am getting REALLY paranoid now that I am probably three weeks away from starting everything up. Really don't want to find out I did something dumb like missing an o-ring or gasket and trash the engine.

I watched another video of someone reassembling the head and they didn't use the head bolt gaskets mentioned.

I also did some reading on CycleX and other places and this it what it sounds like to me:
1) There were head bolt gaskets pretty early on but not in the first couple engine interations.
2) There were oil leak issues.
3) Dowels and head bolt gaskets were added to help resolve that issue.
4) Later on larger sized head bolt gaskets were added to further resolve that issue.
5) They are not totally necessary today because head gaskets are better? Not just the MLS ones like what CycleX sells.
6) The larger head bolt gaskets that come in kits are fine to use and cause no issues but may not be necessary anymore.

Does that all line up with reality?

If so I am just going to use them since I have the head off the engine again.

I don't know what the hell I would do without this forum!

[newday777]

"Also, is this a decent quality gasket?"
Who is the gasket from?
Hondaman says he uses the Veserah gaskets on his engine builds.

"2) There were oil leak issues."

I stopped by a friend's shop yesterday that is a longtime race engine builder. Kurt Jordan. He worked for Action Fours back in the 70s and would occasionally get to take some of the customer bikes to the dragstrip(per the request of the customer) to get the ET times of the build to see if they would produce the claimed increase the Action Fours said would happen in their ads. He also track raced his own bikes. And he has engineering degrees. He also at 70 yrs still remembers most details of his engine builds and the associated customer.(he still has all his notebooks on each build to refer back to when needed too)
In our conversation he mentioned that the leaks (and iterations of the 750 motors) were from the lack of heat cycling of the motors to retorque the head bolts because of the lack of room in the frame to pull the head cover off to retorque the following day after cooldown.(at the factory and when dealers warranty replacement of head gaskets because of the leaking head gaskets)
So what he does is to do a heat cycle with a propane torch to mimic a run engine for heat cycling. Then retorque it the following morning after it cools down. He also says he likes using the MLS head gaskets as the take less amount to retorque after cool down. The important thing is to heat Cycle it to retorque before putting the motor back in the bike.

(Hondaman or maybe Mike Rieke use a salamander construction heater to get the motors up to temp for heat cycling, CycleX has a mule test stand to run the motors on to heat cycle.)

[calj737]

I'm dubious a propane torch would properly heat cycle an aluminum motor where the torque would be changed. First, the size of the engine is an extremely large heat sink and a torch is too concentrated to effect the motor much farther away than the point of flame contact.

Second, even distribution of the heat to uniformally raise the temperature sufficiently to cause the torque to adjust is not probable with propane, again due to concentration of the flame.

Convection heat would be far better is performing an out-of-running condition.

I completely agree that cycling the motor and re-torque is advantageous, but with the composition of the head bolts, they are more apt to properly stretch after sitting and not truly require a heat cycle to confirm the torque. Plus modern gaskets will be compressed after the first torque, so after 24 hours a simplle release-torque again is totally adequate.

I'm sure your friend has lots of experience, but it is all too likely that his methods were developed "then" with "early tech" and not necessary "now". But old habits die hard...

[BrockCB750]

Getting very close to buttoning the engine up.

Also, probably a risk putting all this effort into the bike and not having it tagged and titled in my name first but took care of that this morning. Got through the abandoned vehicle process. Also, have my motorcycle endorcement finally. Definitely two things that could have been done before but thankfully went well. haha

[HondaMan]

I'm a little late to ask this, but...did you have the top of the cylinders milled down 0.010" when they were bored?
If not, and if you didn't get some thicker O-rings for the 2 oil ports thru the head gasket, it will leak oil. I can send a pair for free: I try to 'catch' builders here first so they don't go thru all the work, only to get leaking between the head and cylinders.
I used both" mill the cylinder deck 0.010" to match up to the modern head gaskets that are all thicker than Honda's OEM versions, by 0.2mm. Some of the MLS gaskets are thicker than that, even. Then, the O-ring (1.9x5. isn't thick enough to seal those oil ports.

[BrockCB750]

I didn't have the cylinders bored and no milling.

Very much appreciate you trying to catch a mistake!

I do have the engine all back together. Frame gets back from powder coat Friday.

Is there anything I can do now to make 100% certain I won't do anything to trash the engine? I triple checked the gaskets as I put it together.

I am getting a little nervous after all this work.

[BenelliSEI]

Did you manually prime the oil pump?

[BrockCB750]

Frame, swingarm, other stuff back from powder coat!

With a little help from the wife engine is in the frame

This is really going to pick up steam it feels like.

[BenelliSEI]

Looks very good!

[BrockCB750]

Thanks! It's coming along fast. I am so stretched for time between work/life responsibilities it's hard to keep up with posts here.



There is one snag so far. The Swingarm pivot bolt with the rearsets... The nut doesn't thread on nearly enough for me to be comfortable. Is it common to need to buy a new longer one? I am scratching my head. Sent an email to CognitoMoto to see what they say...



Seems not ideal.

[willbird]

I did see somebody selling custom made engine bolts who said to tell them if you have rear sets due to this issue.

https://www.zed-parts.com/product-page/honda-cb750-sohc-engine-mount-kit

The overall price IMHO for the product was awesome but international shipping usually scares me off ;-). Hmm $118 shipped...not terrible overall really. Buying just one longer stud should not be too bad.

Bill

[BrockCB750]

Thanks for the suggestion! Just sent them an email with some pictures to confirm.

[BrockCB750]

Looks like the just make the engine mount bolts and not swingarm.

Also, went to get the rear brake all connected up to the lever and realized I installed the actuator part the brake lever connects to upside down?!? Didn't even know I could do that by accident. Rear wheel coming back out tonight...

[BenelliSEI]

Might be easier to open up the ID of the rear set bracket and have a machine shop make you a “tube nut” that reaches down to contact the frame?

[willbird]

I did do a little googling on this and I am finding info that a DOHC 750 swing arm bolt was born longer. Looks like they had gone to needle bearings by then so in ebay pictures anyway they are not drilled for a grease zerk. That is a 1979, they say an 80+ is a bigger bolt.

here is where I found the info

https://www.cb750.com/threads/will-a-79-swingarm-fit-my-70-frame.4389/

Bill

[HondaMan]

You might want to use the swingarm bolt from the 1977/78 750F2/3 bikes. It is the same diameter, but a little longer.
Honda's partnumber for it is: 90121-393-770.

[BrockCB750]

Thanks for the suggestion HondaMan! I snagged one from eBay for cheap.

I also emailed that company linked before and they offered to make one to whatever length I need once they get the correct metal/bar/whatever in stock.

Hopefully will make good progress tonight. I want to attempt to start the bike this weekend.

[HondaMan]

The only 'down side' to the longer swingarm bolt is their lack of greasing passages. I drill them in: it uses up 4 drill bits. Then the collar in the arm either needs to be the early style with grease passages and grooves, or make them yourself: just be sure their feed holes align with the ones you put into the long bolt!

[BrockCB750]

Thanks for the heads up HondaMan.

Got the rearsets on. It took literally all day yesterday. When you're first folding with rear brakes and trying to figure that all out... Let's just say it would take anyone with even a little experience much less time and I struggled.

Today is going to be hooking up the controls, getting oil tank in place and priming oil through the engine.

[BenelliSEI]

Looking very nice!