Topic: Cylinder head swap from other cb750 to cb750a

[Newcb750aguy]

Hey y'all, I'm new here,

I realize this isn't the hondamatic section,
I'm posting here since the used head I buy will most likely be from
a manual cb750 and people in this section will have a larger
Knowledge base.
If this post has to be moved into the hondamatic section
Then so be it, I understand.

I bought my 1977 cb750a mostly complete but
neglected a couple months ago.

My problem,
I tried everything under the sun to get the plugs out in
one piece, I ended up breaking one off flush.
I used pb blaster, diesel fuel, molly spray, heat, square and
spiral easy outs, and various verbal enticements.
Nothing happening, stuck like Chuck.

I have drilled out and tapped plug threads in old mopar car heads
In the past, I restore old muscle cars when I find a decent one.
No way to attempt it on this head at that angle even if I wanted to.

My option now is to go through the process of off frame repair if
I'm going to keep the bike, which I am.
Or frame modification for a frame on swap, that may happen,
But not why I'm posting now.

The head I have has the number 392 in a small rectangle indention and stamped
5 X to the right of it "picture attached"

Any long timers here know how I should go about locating a head to fit it without
Trying to find the same numbered head, or modifying a different head ?
Is there a manual or cross reference book I haven't found ?
P there is just let me know, I'll read it.

I've got many years exp. with old automotive applications
I'm a fair hand at figuring things out if it's got grease and oil smeared on it.
I'm only a few months into learning the old school Japanese bikes, and
It seems to be a vast amount of "it's the same except for"

I'm sure I'll have other things to work through before it's a daily driver.
I apologize for the long post, I wanted to give an accurate description
of my situation and what I need.

Thank you for any knowledge you can pass on

Cheers

[05c50]

The 750A has a frame that does not require the engine to be removed to remove the head. Pull the head and take it to a good machine shop. Most likely easier and cheaper than replacing the head.

........Paul

[Newcb750aguy]

Thank you Paul,
I've gotten conflicting information about the head removal on the
Cb750a.
A few people I spoke with said the head is removable with engine in place
 
A few people also said the engine has to be removed to do the work required.

I'm actually going out to attempt removing the head after I post this.

That is an excellent suggestion and info.

[grcamna2]

Does the CB750A have different ports or valve sizes than a standard CB750K ?

[david 750f]

The “392” head is the same as the 75/76F head and 77/78 K head.

[05c50]

Valves and head are interchangeable with K model of same years. Ports and valves are the same.

....Paul

[Don R]

 I can confirm my 76A had a 392 head the same as my 76F. The carbs are smaller though.

[Newcb750aguy]

That's great info guys, thanks.
I got the head off and there was a pile of bug bodies, leaf debris and crud
in the cylinder.
Looks like there was oil blow by in them from the old rings as well.
I took it by a machine shop and asked about getting it out, guy said guesstimate
between $50 and $100
Depending on how difficult, if it unscrewed or had to be drill and tapped.
Thats not checking for cracks, etc.

It's a toss up as to what I will do.
I'm looking at finding a used head with good spark hole threads with the same specs
Cheaper, then just have it checked for cracks and warp, possibly
A basic port.
I'm thinking with the condition of the rings, now may be the time to do a piston, cam, and carb.
change with new rings.
More than I intended investment wise with my current limited knowledge to get it running,
but probably the best thing to avoid tearing it down a second time later.
This is going to take me a little while to read over the manual sections a few times.
 
I appreciate everyone's input,
Thank you

[grcamna2]

I think it would be great if you could purchase another head w/ rubber intake manifolds/carbs + an airbox set of correct size rubber boots and a set of cylinders & matching pistons,all 'in spec' from a known good running 750F.A complete top-end from someone here,if possible..

[Newcb750aguy]

I'm looking at getting the  CB750K carbs and stock K cam or the DP295 cam
Or something comparable to it.
New rings, and the 836 pistons as suggested by several knowledgeable people
On this forum as it appears to be a proven combination.
I'm not trying to win any races, just a little more pep will be fine
The air box fits for the k carbs,  so just needng new holders.

I've already installed a new complete wiring harness, handswitches, cables, etc.
I'm sure there will be other small stuff along the way.

Thanks again for all your help

[Don R]

 It sounds like you are on track to have a sweet running machine. Keep us updated.

[PeWe]

DP295 cam need pistons with deeper valve pockets, like JE 65mm 836cc.

Not Cruzinimage cheap 836 kit that has no pockets for higher lifting cams as DP295.
Webcam 41 aka CycleX CX-1 used. See the long thread about those pistons.

I'm sure you need to replace ex guides (and valves) on your existing head or old head found on eBay.

About so called drop in cams in stock engine.
I tried a Kenny Harmon D cam with stock K7 pistons, K6 head, valve/piston contact.
That cam's lobes look rather fat so its strange it is written to be a drop in cam on stock engines. Adjustabe sprocket, open/close as cam card say.
Engine got a stock K7 cam.

Deeper pockets needed for more fun.

[Newcb750aguy]

Don, I'm hoping it does turn out to be a really nice ride, I'd like to have it as a daily ride in decent weather.

PeWe, You blew me away with all the brands and numbers comparisons LOL, it was great to read that info though.
That kind of knowledge will be very useful when I understand and know what I'm doing

I'm not really looking at just drop in options since I'm just now accumulating the needed knowledge to be successful
at rebuilding a bike.
I'm an old Mopar muscle car guy, so I'm kinda going off that knowledge of building old challenger, charger motors.

I'm reading a ton of the threads here, searching the internet for the limited actual specs of the aftermarket parts for sale, rather than just going by them advertising fits this, this, and this.
Really not wanting to put this bike together and first crank bend the valves because I lacked diligence in my research and learning and went off a sales brochure

I'm not looking at the DP295 now that I've read up a little on things, sounds like more than I need

At the moment I am learning the basic specs of factory design, eg cylinder wall thickness, piston height, diameter, pin diameter, heartline to top, etc.
It's probably more than I need to do to figure out a piston combo with all the information being openly exchanged.
I feel like knowing some of the basic manufacturing details may help me and possibly others somewhere down the line.
Things that were overlooked on the car motors that could cause problems apply somewhat to these as well

Ive noticed just in the short time Ive been working on this bike that most of the parts come from over seas and even though
these bikes were made there.
It looks like there is a wide variation of measurements between manufacturers/sellers online and the information detail they are willing to supply vs OE specs.

When I was searching for the piston kit, I ran across a so-called Big Bore 65MM 836CC 4 piston set with rings, pins, and clips included, with no instructions included, fits CB750Kmodels 1969-1978 for about $80 shipped from Cali.
That's the insane price of $20 bucks a piston set.
I personally don't see a snowballs chance on hot pavement those things will work any length of time if at all.

No details on material construction, no measurements other than diameter, listed as manufactured to Honda OE reference number 1301-300-030 which is non existent since it's missing 1 and I couldn't find one that ended in 30
So I'm taking it slow and learning a bit.

Point well taken on the head, any head I use will be checked for cracks and warp-age as well as get new valves, guides, seals, springs, etc. all new and matched to the new cam.
I'm going to look at the connecting rods to see their condition as well.

I intended to get it running enough to see if it knocked, smoked, caught fire, was cracked, etc. then make the major decisions to fix or part.
After the broken plug and seeing the bugs, etc. in the cylinder, things went sideways.
At this point, for what I paid for the complete bike, with keys and title, though neglected outdoors, If I have to completely redo the engine, it won't be a loss.
I really would have like to checked the shifting a little, but that's gonna have to wait I guess.

I'm going to learn what I can to make some proper, slightly over OE, mildly fun modifications without
tearing it up inside LOL

Thanks again to everyone that has had input on this, all of your knowledge helps a beginner like me to move forward,
I really appreciate it.

I hope everyone has a safe and awesome holiday
 

[PeWe]

DP295 is a very good profile.
You need higher CR pistons like 10 something to 1.
Ported head with race springs and a good flowing exhaust.

My K2 runs cheap Cruzinimage K7 61.5mm pistons +0.5 mm, restored head with KPMI race springs.

56-57 whp on dyno. Yamiya no numbers 4-4 is a part of it. A good 4-1 will work too. A fun well running ride.

If going for the cheap 836 stock style, perfect with a good std cam or the drop in cams CycleX sells. Head with good guides, valves and seats.

This forum has tons of info about restoring and modifying the entire bike.

An old engine also need to be restored with chains, tensioners, bearings....

[Newcb750aguy]

I found a good used head on ebay and it will be shipped out on Monday

I found a nearly complete 77 cb750a like mine a couple hours away that
I'm going to buy at a reasonable price, it has un-weathered parts and minimal
Chrome surface rust, so that will be a large help to speed things along

I will be actually ordering the piston kit this weekend, the pistons I'm looking at
are forged 10.25-1 comp. 65mm as many on this forum have posted as having
good success with.

PeWe, it sounds like you have experience with the different aftermarket and stock parts,
maybe you or someone else here has insight into my questions.

The new pistons will be 4mm larger in diameter than stock, presuming
This motor has never been into before as the bore diameter is stock.
Are the factory cylinder walls thick enough to remove that amount of material
And still handle regular daily use ?
Car blocks are steel and much thicker, I plan to add an oil cooler, my concern in the thickness, I haven't
Found any information on the factory numbers on wall thickness, maximum bore, etc. ?

I'm most likely going to buy a 750k stock grind cam or something close.
I'm going to put my carb set back on to start, since it's now cleaned and rebuilt.
If I can find a decent set at a decent price, I will be trying the K carbs as well
since it has proven success and it's a bolt on adaption with OE parts.
It looks like those years used keihin 657A, ,657B, and 086A

My questions about the valves and springs,
The head and cylinders and piston will be going to the shop to get the checking,
Boring, port, etc. I'll need to send the valve stuff as well.

There will be new guides, seals, chain, adjusters, etc..
Should I be looking at anything particular concerning the new valves for the 750a head
With the K type cam, and should I have a different valve angle, spring, keepers, etc.
from the stock parts ?
Is there a better type of, or longer life guide or seals for the valves or clearance ?
Not looking for an actual brand, but a material or the way it should be installed

My main goal, besides bringing this bike back into operating condition, is that it
has a long life mechanically, I don't actually have to get 65mm pistons if it will
Cause stress failure next year.
I can go smaller, but it's got to have something after looking in those cylinders.
I'm still new to bike engine modification, If I don't have something figured correctly
Or not looking in the right direction, just let me know

Thanks for everyone's input

[PeWe]

If you plan to port the head, bigger inlet valves (33.5mm), hotter cam as DP295, those pistons are the correct path if chamber volumes are around 22.5cc. If over 23cc, you need to deck it. Compression is the key for power.
Better carbs. I guess stock K2-K6 carbs will work.

If you do not plan to port the head. Cheaper oversize pistons will work fine. Like K7 +0.5mm from Cruzinimage.
Head needs most likely new ex guides and valves, maybe in too.

My latest 392 head purchase had bad Ex guides as another K6 head where Ex side was really worn while In looked like a low miler.

With earlier carbs to open up a bit. Plus CX-1 cam or similar

There are others that have modified CB750A.

I recommend forum member MRieck for headwork and all parts you need. He has lots of experiences on these bikes.

Modifying the engine will cost a lot when you need to refurbish everything else at the same time  if not recently done or a low miler.

[robvangulik]

My main goal, besides bringing this bike back into operating condition, is that it
has a long life mechanically, I don't actually have to get 65mm pistons if it will
Cause stress failure next year.
I can go smaller, but it's got to have something after looking in those cylinders.
I'm still new to bike engine modification, If I don't have something figured correctly
Or not looking in the right direction, just let me know

Thanks for everyone's input

When you take care of all the points mentioned in your list, it will probably do another 40+ years, how old are you in 40 years?

[Newcb750aguy]

Thanks again for your knowledge and insight PeWe, I'll see if I
Can find some info from MRieck.

This morning I was trying to compare the 64.5mm OE cyl. sleeves
Of the early 80's cb900 to see if they could be fitted into this 750 block.
They would be plenty of metal in the walls after fitting the 65mm in them.
Didn't find any OD. or sleeve length info though.
I'm sure people have done or tried it.
I just have so much other 750 stuff to read and learn, that's a whole
nother journey.

robvangulik, I'm double nickels this year, so in 40 years I'll either be a memory,
Or piloting a really cool space craft from my hover chair, lol
My old cars, trucks, bikes, odd, cool junk, anything I have left will go to youngish family members
In hopes that they can appreciate the history, skill, and art of the way old machines were made

Big dreams, they'll probably sell it, but someone will enjoy it 

Thanks again to everyone
Without forums like this exchanging in-depth hands on information
It would be a far more difficult task