Author Topic: CB750 Cylinder Heads (Read 7293 times)

spiritof67 · « **on:** June 25, 2012, 02:57:12 PM »

I have available to me a 1977-78 CB750K cylinder head and carburetors. I recall that Honda's Last Gasp Hop-Up for the SOHC included bigger valves and ports and accelerator pumps in the carburetors, and that sounds interesting. But the question is, do these things actually have a positive effect in everyday running, enough to swap them onto a pre-77 SOHC? By the way, I chose the K head because of the legendary problems with over-strong valve springs and the attendant problems thereof with the F2 head.

Is this a worhwhile swap, conidering the difference in exhaust pipe head mountings? Or has some beknighted individual made adaptors so you can use pre-77 exhausts with the '77-'78 head?

Spirit

Jerry Rxman Griffin aka MuthaF'er · « **Reply #1 on:** June 25, 2012, 03:18:48 PM »

77-78 K head is 75 - 76 F head. You should find a 392 on the top fin intake side by #3. It has a smoother combustion chamber shape that would remind one of a hemi shape. The head and/or pistons account for a 0.2:1 CR bump to 9.2:1.

78 K 392 head has stronger springs from the F2/3 (good) and the keepers are from the F2/3 also (different grooves in valves).

Only difference in exhaust mounting is screw/bolts vs studs.

spiritof67 · « **Reply #2 on:** June 25, 2012, 03:39:21 PM »

I thought the spigots on the late head fitted the exhaust differently. Does that mean my Bassani 4 into 1 will fit that head without changes? Good news!

Spirit

Sam Green Racing · « **Reply #3 on:** June 25, 2012, 04:03:16 PM »

The mounting systems are different on the K and F but they are interchangable, one uses counter sunk screws, the other studs, not a problem.

Sam.

spiritof67 · « **Reply #4 on:** June 26, 2012, 07:43:27 AM »

Excellent!

spiritof67 · « **Reply #5 on:** June 30, 2012, 04:05:03 PM »

Wait: Let me go through that again. I have a '77"K" head; will any header or set of pipes that fits a KO-K4 fit the exhaust spigots fit that head as well? Just to be clear..

Spirit

Jerry Rxman Griffin aka MuthaF'er · « **Reply #6 on:** June 30, 2012, 04:50:54 PM »

The exhaust holes are all identical. They use studs in the later heads and bolts in the earlier heads.

Don R · « **Reply #7 on:** June 30, 2012, 05:01:57 PM »

Use the hardware that works with the header you have.

Sprocketwerx · « **Reply #8 on:** July 06, 2012, 06:47:38 PM »

He's asking if anyone makes early style spigots for the later K model heads, so he can slip his KO pipes on and clamp them in place.
I have a fresh stock '78K and it doesn't have spigots (the spigots are part of the pipe).

Cycle-X sells spigots and clamps, to answer OP question. You can use your pipes.
I'm thinking of producing stainless spigots for the slip-on style pipes myself.

I only say that because some people are just used to slipping the pipes on and off existing spigots, as if the spigots were an integral part of the head.

Not saying anyone's a dummy, but they're out there...lmao

scottly · « **Reply #9 on:** July 06, 2012, 07:47:01 PM »

Quote from: Sprocketwerx on July 06, 2012, 06:47:38 PM

He's asking if anyone makes early style spigots for the later K model heads, so he can slip his KO pipes on and clamp them in place.

Yeah, and the answer was to swap the early spigots onto the late head....

Sprocketwerx · « **Reply #10 on:** July 07, 2012, 04:36:58 AM »

Yep, and that's different from "use the hardware from the old pipes"
Hardware refers to nuts and bolts to most people, spigots are more than that.

AND this IS the internet, the place where some people don't use search features and noobs ask random questions. Some need a clear answer. I just clarified.
He also asked if anyone produces spigots and nobody else offered a source for them. I did.

Old Scrambler · « **Reply #11 on:** July 09, 2012, 12:40:48 PM »

Just a thought for Sprocketwerx.................If you produce an aftermarket spigot, think about the length and ID for performance with headers

Sprocketwerx · « **Reply #12 on:** July 10, 2012, 05:21:39 AM »

Quote from: Old Scrambler on July 09, 2012, 12:40:48 PM

Just a thought for Sprocketwerx.................If you produce an aftermarket spigot, think about the length and ID for performance with headers

Thanks Scrambler, I always keep performance first and looks (style) secondary when building pipes or intakes. My background with headers includes some high horsepower drag race motors. Pro Mod 700-800 inch nitrous motors.

Sure some of the stuff I do for choppers has to look good and might bias a little more toward style, but if the pipes are not effective performance-wise... why bother?
I always try to keep pipe lengths balanced with as few bends as possible. Rarely will there be more than an inch or so difference in length across the bank of pipes.

The crossover pipes in my other thread are 1 5/8 16ga, matches the stock spigot on OD/ID, so I swedged the pipes for a smooth transition from spigot to header.
I'm not big on huge fat pipes unless there's some sort of boos involved that requires that kind of volume. Velocity is our friend in exhaust!

I'll stop now before this turns into a commercial for my pipes...lol
I just like to make these old Hondas fast and help others do the same!

754 · « **Reply #13 on:** July 11, 2012, 07:08:48 AM »

The stock spigot uses an inner pipe that is almost never there, on headers.. meaning there is a wide spot inside the spigot.. be nice to eliminate that or get better port to pipe matching.
You are thinking Aluminum. I hope?

Sprocketwerx · « **Reply #14 on:** July 12, 2012, 04:58:46 AM »

Port to pipe matching would be my goal here as a performance objective. Wide spots in spigot don't remove restriction, just creates another farther down pipe.

As far as aluminum, I don't think the weldable alloys will withstand the heat without deforming or melting, given the material thickness. Stainless will allow a thinner material and retain shape and strength in that harsh environment of heat and pressure (clamping pressure as well as exhaust pulses).

The main benefit of them though would be ease of maintenance and no need to replace crush gaskets every time you remove pipes, as well as swapping pipes from earlier models or building your own pipes.

Most of my customer base are not racers so economy and ease of installation are of more interest to most of them. And most don't really understand the performance aspects.

754 · « **Reply #15 on:** July 12, 2012, 06:36:21 AM »

Welding? Aluminum is fast to machine.. could be one piece. 2024 handles heat wsell, and 7075 is quite tough..
Aluminum wopuld be about 35 to 50ish% of the weight of steel..

voxonda · « **Reply #16 on:** July 12, 2012, 07:01:55 AM »

Never had any prob's with the aluminium spigots I make/use.

Turned from 6082T6, no welding.

Pipe fits flush in the spigot and they match internal, no wide spots.

Rob.

Sprocketwerx · « **Reply #17 on:** July 13, 2012, 05:59:24 PM »

Voxanda, how are the pipes retained with your slip-in spigots?
I hadn't thought of internal spigots as most I see on the street are inverse of what you have.
I like it! and looks fairly simple to produce.

voxonda · « **Reply #18 on:** July 14, 2012, 03:01:31 AM »

Hey sprocket,

The pieces you see here are not the finished products, what's missing is the tiny holes were the springs that retain the pipes are located. Second option is to use the higher nuts as in pic#2, and use these to retain the springs by driling a small hole in them.

Rob

Sprocketwerx · « **Reply #19 on:** July 14, 2012, 08:52:52 AM »

Thanks, I wasn't sure if it was a setscrew or something like that hidden from view.
Springs make sense, been using them on my collectors for years.

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Author Topic: CB750 Cylinder Heads (Read 7293 times)