Cylinder Head Replacement

[xlr8]

Greetings,

I adopted a '77 750K last year.  A mechanic buddy who is doing a top end job for me says the exhaust valve guides need replacing.  The engine has 33,000 miles and I don't have any mtce info on it.  Not burning oil (no smoke on a cold start).  

After discussing the options with my friend, getting a used head in good condition seems to be the way to go. (I know a new set of 8 intake and exhaust guides can be ordered from Honda.) Here's my question:

Will a '76 750F head work?  I was told the valves are bit larger on the "F" versus the "K" otherwise the head is the same.  I have found one that seems to be in very good condition for about $150.

Thanks in advance for your help.

[Re-run]

The valve thing is only the later F bikes, not the 76. The 77/78 K bikes use the 76 engine anyways with a few changes. What was your compression before the work was done? I mean if you had ok compression and did not burn oil, then there is no reason to replace the seals unless you freshening things up but then that would be something to do to a head from a different engine so you would not be ahead anything. Just do the work, if you need to, on the 77 head.

[xlr8]

Thanks for the advice re-run, the work was done (top end) on the original head and she is running much smoother.  This bike is starting to consume more of my spare time, both riding and tweaking. One thing I have noticed at higher speeds (above 60 mph) is moderate to prohibitive vibration felt in the handlebars/grips.  

Now, I'm not trying to set any land speed records with this old girl but I would like to be able to cruise at 75+/- (rpm's around 5K) with less vibration if possible.  Is this realistic? I know these bikes were spec'd originally to break 100 mph, but given the vibration I'm encountering at 65 mph I can only imagine how severe it would be at 100.

One thought I had was to install some rubber dampers at the engine mounts, however I noticed that the front ones are lateral so not sure if that is even possible and/or would make any difference. Is there more likely some other contributing factor that makes more sense to chase and solve? I am planning to do some 100 mile rides come spring so hoping for a feasible solution.

Thanks in advance to all who have ideas and experience on this matter.

[Re-run]

Well it is an air cooler 4 so vibration will happen. However, the F bikes in 77/78 were geared a bit more for acceleration. The front sprocket is a 14 tooth I think. The 77/78K used a 15 tooth sprocket. now this is assuming you still use the 630 chain size.

[buttcan]

OUCH ! $150 for a head ? I would keep looking personally. If it is rebuilt professionally then MAYBE. If I had another one we coulda made a deal but I recently gave my extras away to a couple friends.

[Re-run]

Buttcan, if you have friends with cb 750s, maybe send them this way?

I kinda overrun here but we have so few frequenting members and I don't want to see the site die.
If we can get more people on here to contribute, we can grow more and I can stop posting everywhere!
I would like people to get better info, especially on the dohc bikes than what I can give sometimes.
It also would free me up to add additional content to make the site a better resource.

[buttcan]

I agree. I am on several other 750 forums as well and I will pass the info on.

[PeWe]

The "F" head with larger inlet valves? This is the F2 head that has larger combustion chambers. You will get lower compression when used with the older model K pistons.

The pistons for a F2 have a different dome. There is a good description at Dynoman. http://www.dynoman.net/bikepages/F2/pistons.html
As the 1977-78 Honda CB750 F2 has an 18% larger combustion chamber to compensate for the larger valves, through the years many people have installed the standard 836 (ex: Wiseco K836) kits for the 69-76 engines only to discover that the compression was too low to give them the extra power that they were after.

[Re-run]

The OP was talking about a 67 F head which does not have the larger valves.

[77honda]

My cam chain just broke on my 77 k im in the middle of replacing it i discovered that two of my valves hit  pisons one valve is definitly bent.

    i have a 78 k parts bike i was wondering if i could swap out the two best looking valves  from the 78 with the bent and hit ones from the 77????    


thanks in advance for all feedback

[Re-run]

Should be a-ok. But it might not seal properly. You might have to freshen the seat for the valve and make sure the guides are ok.

[77honda]

if the other head from the 78 is ok would it fit on the 77 i was told it would be a pain to swap them out. they say i would have to get some special something to make sure they seat properly

and another ? would it hurt to put the head in a parts washing tank??   do u think it would mess anything up    

i run pods on my 77 and there is a rubber hose ran up beside my battery box is that the crank case breather?? it smokes like a chimney do they all do that my brother has a 78 and runs pods and his does the same thing.   I have an original air box im going to put on when i get it cleaned up and i think that hose plugs into it.    I just wonder why it smokes so bad out of that hose


hey thanks for your time man i appriciate it

[Re-run]

The 78 head will fit just fine on the 77. nothing any different then pulling the 77 head off and putting it back on. There is a difference from earlier years though but 77/78 is all good. As long as it is staying K or staying F.
 I doubt it would mess anything up, you would just need to clean it well. Before parts washing it though, you will want to remove everything from the head.

That hose is the crankcase breather. So it does smoke, just the design. It would normally plug into the airbox.

[77honda]

remove everything as in valevs and all or what. how would i go about getting them cleaned in one piece. im not very handy when it comes to valves and springs and all

[Re-run]

Yup, remove it all if you use a parts washer. Probably should anyways and check the parts over to make sure they are ok.