Author Topic: K7 combustion chamber Q?  (Read 895 times)

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Offline Don R

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K7 combustion chamber Q?
« on: February 01, 2023, 08:38:10 AM »
 I recently read the 77/78K has a bigger combustion chamber than early K's, how does that compare to the early K?
  A similar question was recently asked about the F2 head, if I used 10.25/1 Wiseco's in 65mm, how much compression reduction would there be? A wild ask guess is close enough.
 
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Offline PeWe

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Re: K7 combustion chamber Q?
« Reply #1 on: February 01, 2023, 09:17:12 AM »
I bought an ugly 392 head with good fins. It should be ported anyway. Most guides really bad, all ex and 2 in. All replaced. Bigger valves.

The chambers were between 22.5-22.9cc

Someone had tampered with it so I do not know if chambers had been smaller. Head height as stock, not milled.

392 mark is upside down.



Look like residues of a Honda sticky head gasket.



Chambers 23.7cc after restore and porting as ordered. Here with 72mm gasket 33.5/28.5mm valves.


Drilled the 2 stud holes for oil with 9.5mm drill. HD studs are 8mm so not much left for flow.
Maybe more 392 heads are like that.
« Last Edit: February 01, 2023, 09:40:15 AM by PeWe »
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline scottly

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Re: K7 combustion chamber Q?
« Reply #2 on: February 01, 2023, 06:14:57 PM »
IIRC, the K7 has an approximately 24cc chamber according to my rough measurement, the early chamber was reported to be 22cc, and the F2/3 was reported to be 28cc.
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Offline Don R

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Re: K7 combustion chamber Q?
« Reply #3 on: February 01, 2023, 06:26:15 PM »
 Thanks, I had forgotten the last two K models may have 392 heads.
No matter how many times you paint over a shadow, it's still there.
 CEO at the no kill motorcycle shop.
 You don't need a weatherman to know which way the wind blows.

Offline PeWe

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Re: K7 combustion chamber Q?
« Reply #4 on: February 01, 2023, 09:18:15 PM »
A burette and a piece of clear plexi with 2x  5-6 mm holes will help you to find out.

Very useful to measure chambers, verify volumes and if equal.

I use a 25ml version
Head is resting on a valve cover with cam holder bolt mod that levels it.



« Last Edit: February 01, 2023, 09:34:59 PM by PeWe »
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline Don R

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Re: K7 combustion chamber Q?
« Reply #5 on: February 02, 2023, 02:19:30 AM »
 Once upon a time I had a graduated syringe, I do have lexan left from our dragster windshield too.
No matter how many times you paint over a shadow, it's still there.
 CEO at the no kill motorcycle shop.
 You don't need a weatherman to know which way the wind blows.

Offline Don R

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Re: K7 combustion chamber Q?
« Reply #6 on: February 11, 2023, 02:36:44 PM »
 I noticed that CycleX made mention of using these 392 heads with 12-1 pistons to make a little more sane compression ratio. My thought exactly.
No matter how many times you paint over a shadow, it's still there.
 CEO at the no kill motorcycle shop.
 You don't need a weatherman to know which way the wind blows.