Topic: Black Engines

[Vintageguy]

Been told that the black 750 engines had a tad more horsepower than the shiny ones. True of false? Thanks you.

[MikeSimon]

Black engines have a better heat dissipation. It is possible that this would translate into a tad more efficiency, thus more hp. Maybe ¼ hp.
Considering that the gasoline internal combustion engine has an efficiency of only about 30% and a tolerance of +/- 5% from the nominal output, it is probably negligible. Then again, there are guys who chase any fraction of a hp.

[Vintageguy]

Me.

[hsas.69]

The original black engines were from the 77&78 "F" models. They did make a little more power due to internal engine changes.

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[TwoTired]

The black 77 and 78 engines had more cam and valve size increase.  Indeed they made more power and had a higher red line, too.

In order to get the bigger valves fitted, they had to change the valve geometry, which placed more pressure on valve guides.  Many of the guides wore quickly, possibly associated with the different material chosen for the guides themselves.  Honda replace a lot of these guides (and valves) under warranty, with I believe a different hardness guide material.  An Un-revamped black engine head should be looked upon with a jaundiced eye.  I don't think many survived much beyond 20K miles, with out really good oil change and selection regimen.  It also helped if the engine didn't spend a lot of time at or near red line, which is where it made more power.

Cheers,

[seanbarney41]

being the owner of 78f and also every other generation of cb750 except k0, yeah the black(from the factory) engines got some significantly bigger balls.  My valve guides lasted till 30,000 miles, still runs great, but huffs and puffs oil and smoke like a chimney.  This was not a well cared for bike.  Previous owner beat the #$%* out of it, let it sit for years, I unstuck it, immediately burned up a camshaft, replaced cam, towers, rockers, lovingly beat on it some more for 12,000 more miles.  It WILL ride again!

[lowriderbud]

Is there a visual identifier for the 'F' heads?  I have an extra head I acquired through some trading; the guy I got it from thought it was a head from a '77 F model.  The valves have been removed and it's just a bare head.

Later, Bud...

[seanbarney41]

look for "410" cast in above carb spigots...not sure they all have that

[kmb69]

.....
In order to get the bigger valves fitted, they had to change the valve geometry, which placed more pressure on valve guides.  Many of the guides wore quickly, possibly associated with the different material chosen for the guides themselves.
.....

Not to be confused with changing the valve angle. The valve angles remained the same but they were "sunk" deeper in the head and created a different rocker arm contact geometry even though the valve stem length was modified. Combustion chamber became larger requiring the domed pistons to get the compression ratio. Pretty sure the guide material was changed for the worse.

look for "410" cast in above carb spigots...not sure they all have that

Yep, the 77-78 F heads had the 410 product casting code and the intake ports are approximately 2mm larger.

[IVPeters]

Out of curiosity, were the head changes exclusive to the 77-78 F? The F0 I'm rebuilding had a K7 engine swapped in by the previous owner.
I ported/polished the head, and got new valves but never thought about or had reference to the original F0 engine.

[kmb69]

Out of curiosity, were the head changes exclusive to the 77-78 F? The F0 I'm rebuilding had a K7 engine swapped in by the previous owner.
I ported/polished the head, and got new valves but never thought about or had reference to the original F0 engine.

The 410 product code was unique to 77-78 F's.

There was a 392 product code cylinder head used on: 76-78 A's, 75-76 F's, and 77-78 K's. Valve sizes were identical to the 300 product code equivalents but the combustion chamber was reshaped and somewhat enlarged. The 78 A's and 78 K's used different valves with 77-78 F style spring retainers and cotters. I personally prefer the original style retainers and cotters.

[IVPeters]

Interesting! Thanks for the info kmb69.

[Jerry Rxman Griffin aka MuthaF'er]

The 77/78 F and the 78 K had stronger valve springs also.

IVP, The 77 K engine is basically a 75/76 F engine and it's head would be an identical -392 head. Piston tops may look a little difference with a corresponding slight drop in compression that the "full top" dome F engine allowed. The top fin rear side probably has a 392 cast into it by #3 intake.

[IVPeters]

Cool, good to know Jerry thanks.

[lowriderbud]

My head does have the '410' molded in between intake ports 2 and 3.  It's a bare head - who would be a good source to rebuild it or supply the parts for me to rebuild it. It will need spring, guides, valves, retainers, keeper, and seals.

Thanks for the info, Bud...

[slikwilli420]

My head does have the '410' molded in between intake ports 2 and 3.  It's a bare head - who would be a good source to rebuild it or supply the parts for me to rebuild it. It will need spring, guides, valves, retainers, keeper, and seals.

Thanks for the info, Bud...

Mike Rieck or Ken at Cycle X.

[kmb69]

My head does have the '410' molded in between intake ports 2 and 3.  It's a bare head - who would be a good source to rebuild it or supply the parts for me to rebuild it. It will need spring, guides, valves, retainers, keeper, and seals.

Thanks for the info, Bud...

MRieck on this forum is probably the best SOHC head porting/rebuild guru you can find. Send him a PM.

[70CB750]

I rebuilt my 78 supersport head with valves and retainers from CycleX.  Not sure where I got -bronze - guides from.

Also new tappet adjusters.