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Author Topic: cb350F Head Gasket Seep  (Read 492 times)

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Offline 574hondarider

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cb350F Head Gasket Seep
« on: July 25, 2024, 10:13:46 AM »
Hello All,

It's been awhile since I've been on.  Fully restored 73 cb350F back about 3 years ago.  I used oem honda gaskets with the copper spray.  No leaks or seeps for 2000 miles.  I haven't ridden the bike much at all this year and took it out for a spin and noticed the infamous weeping on left side of the bike around the head gasket.  I'm thinking I may try to loosen, then tighten all the engine bolts in the correct pattern, but was wondering if anyone had any thoughts on this, esp. given the fact I used the copper spray.  Thanks so much!

« Last Edit: August 23, 2024, 01:02:19 PM by 574hondarider »
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Offline BenelliSEI

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Re: cb350F Head Gasket Seep
« Reply #1 on: July 25, 2024, 11:04:44 AM »
Could be just the copper spray. Clean it up and run it some more…..
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Offline Mark1976

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Re: cb350F Head Gasket Seep
« Reply #2 on: July 25, 2024, 11:56:16 AM »
   I'd go John's route, then if it continues you can decide what to do, you could re torque and then if that fails its time to replace the HG and accompanying seals. It's a common issue (the leak), but it could be the copper coat weeping thru. One thing at a time.
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Offline 574hondarider

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Re: cb350F Head Gasket Seep
« Reply #3 on: July 25, 2024, 12:59:27 PM »
Thanks guys.  It's definitely oil but just a few drops.  Maybe I'll try retorquing in a few weeks or let it roll until it gets worse.

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Offline BenelliSEI

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Re: cb350F Head Gasket Seep
« Reply #4 on: July 25, 2024, 02:48:15 PM »
Quote from: 574hondarider on July 25, 2024, 12:59:27 PM
Thanks guys.  It's definitely oil but just a few drops.  Maybe I'll try retorquing in a few weeks or let it roll until it gets worse.

Good plan. I had a K8 that did that for +10 years before I pulled the motor. Wiped it down regularly and really the oil loss was virtually nothing…..just annoying.
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Offline newday777

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Re: cb350F Head Gasket Seep
« Reply #5 on: July 25, 2024, 07:53:09 PM »
Maybe it's not the headgasket but the oil sealing pucks have shrunk from lack of use. Wipe it down as you said and ride it more to get them to seal up as you ride.
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Stu
Honda Parts manager in the mid 1970s Nashua Honda
My current rides
1975 K5 Planet Blue my summer ride, it was a friend's bike I worked with at the Honda shop in 76, lots of fun to be on it again
1976 K6 Anteres Red rebuilding project, was originally my brother's that I set up from the crate, it'll breath again soon!
Project 750s, 2 K4, 2 K6, 1 K8
2008 GL1800 my daily ride and cross country runner

Prior bikes....
1972 Suzuki GT380 I had charge of it for a year in 1973 while my friend was deployed and learned to love street riding....
New CB450 K7 after my friend returned...
New CB750 K5 Planet Blue, demise by ex cousin in law at 9,000 miles...
New CB750 K6 Anteres Red, to replace the totaled K5, I sold this K6 at 45k in 1983, I had heavily modified it, many great memories on it and have missed it greatly.....
1983 GL1100A, 1999 GL1500 SE, 1999 GL1500A

Offline HondaMan

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Re: cb350F Head Gasket Seep
« Reply #6 on: July 25, 2024, 08:40:08 PM »
The O-ring that passes the oil pressure thru the head gasket is VERY LIKELY too thin by 0.2mm or so. This is because all these head gaskets are now thicker by that much since asbestos was outlawed: the gaskets that replaced those are 0.2mm thicker.

BUT...here's the kicker...there are not seals available for the oil jets that are 0.2mm taller. So what must happen to restore the seal integrity of that spot is:
mill the cylinders down 0.02mm (0.010" works fine, too) on their top.

Sorry, but that's what the cause is, and how to fix it for real... :(
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Offline PeWe

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Re: cb350F Head Gasket Seep
« Reply #7 on: July 26, 2024, 12:57:33 AM »
I used copper spray on a graphite head gasket on a CB750. A mistake.
It leaked oil rather soon. Higher compression with 836 helped the leak to happen early.

My local shop's mechanic that has tuned/modified all kinds of bikes since mid 70's told me to mount graphite gasket dry, no copper spray.
« Last Edit: July 26, 2024, 05:52:47 AM by PeWe »
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CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
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Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
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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
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Offline bryanj

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Re: cb350F Head Gasket Seep
« Reply #8 on: July 26, 2024, 03:05:57 AM »
Never use anything on a standard gasket
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Offline Mark1976

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Re: cb350F Head Gasket Seep
« Reply #9 on: July 26, 2024, 06:43:38 AM »
Quote from: bryanj on July 26, 2024, 03:05:57 AM
Never use anything on a standard gasket
+1 to that.
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Offline 574hondarider

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Re: cb350F Head Gasket Seep
« Reply #10 on: August 23, 2024, 01:01:33 PM »
It does not appear to be leaking anymore, so I'm thinking riding it a bit fixed the issue!
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Offline newday777

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Re: cb350F Head Gasket Seep
« Reply #11 on: August 23, 2024, 02:31:15 PM »
Quote from: 574hondarider on August 23, 2024, 01:01:33 PM
It does not appear to be leaking anymore, so I'm thinking riding it a bit fixed the issue!
The rubber pucks swelled with the heat and oil hitting them to seal it.
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Stu
Honda Parts manager in the mid 1970s Nashua Honda
My current rides
1975 K5 Planet Blue my summer ride, it was a friend's bike I worked with at the Honda shop in 76, lots of fun to be on it again
1976 K6 Anteres Red rebuilding project, was originally my brother's that I set up from the crate, it'll breath again soon!
Project 750s, 2 K4, 2 K6, 1 K8
2008 GL1800 my daily ride and cross country runner

Prior bikes....
1972 Suzuki GT380 I had charge of it for a year in 1973 while my friend was deployed and learned to love street riding....
New CB450 K7 after my friend returned...
New CB750 K5 Planet Blue, demise by ex cousin in law at 9,000 miles...
New CB750 K6 Anteres Red, to replace the totaled K5, I sold this K6 at 45k in 1983, I had heavily modified it, many great memories on it and have missed it greatly.....
1983 GL1100A, 1999 GL1500 SE, 1999 GL1500A
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