Topic: Fixing the leaky CB400f oil passages

[vames]

Hi all. Years ago, I tried all the "easy" stuff to try and fix the oil seepage around the front left head/jugs mating area on my 400F.  I retorqued the head, changed all the copper crush gaskets, changed the valve cover gasket, and none of that solved it. So I decided that I'd tolerate the oil seepage and mop it up with a rag after each ride.

Now, I'm starting to use the bike to commute and I don't want oil spritz on my shoes, so I'm finally going to take this on. In previous posts, Hondaman has suggested that most of the head gasket kits for the 400f include undersized seals that leak. I think someone even said that OEM Honda replacements can be leaky. So my question: based on what is available in today's market (even if I have to buy separate sets or components) what is my best bet for head gasket and seals? Can OEM Honda still be obtained, and would those be the best to go with. Also, if anybody has any preferred alchemy/tips/tricks/products for buttoning up a dry CB400F head, I'm eager to hear it. The motor runs well with good compression and a fairly quiet cam chain setup, so I'd like to do as little as possible to address the leak and not open any cans of worms trying to make things faster or better.
 

[bryanj]

Check both block and head for flat then use slightly thicker cross section O rings. Aparently newer gaskets, that now have to be asbestos free, are thicker than original but the O rings are still the same

[vames]

Is there a known difference between the old (asbestos) head gasket thickness and newer ones? If so, could I just have a machine shop take that much meat off the head to ensure that the orings seal?

[Tim2005]

I'm not sure this change in gasket thickness thing applies to 400Fs. The "o-rings" on the head joint are not normal o-rings at all, but are shaped seals, much taller than o-rings. Some of the ones in pattern gasket kits used to be nothing more that short slices of pieces of tubing, without shaped ends. And were crap. If you get a genuine Honda kit then you will see the shape of them.  I have assembled them & remained long-term dry successfully by using the Honda seals and the tiniest smear of either Wellseal or perhaps Wurth Ultra Silicone.

[bryanj]

ANY SILICONE IS A BAD IDEA

[Tim2005]

Well I used it on my daily rider when I last rebuilt it in 2007. Those seals seemed ok on today's ride.  Though I do have a weird unrelated leak down one of the centre studs! It is important it's the tiniest smear- certainly must avoid all risks of blocking the smaller one which is the feed oilway. 

[robvangulik]

Just today Hondaman wrote this in another thread
http://forums.sohc4.net/index.php/topic,185384.msg2146988.html#msg2146988

Well, I'll put it out there for 'the record' for the CB350F/400F....

To prevent oil leaks from the oil jet orifices with the modern, thicker, head gaskets, the only ture fix I have found is to deck (flat-grind) the top of the cylinder deck by 0.010". There is no 'thicker' or 'taller' oil seal that I have found that will work to prevent the leaks, Milling the deck by this much is the difference between the OEM and modern head gaskets (about 0.008"-0.010" typically).

[vames]

Amazing that Hondaman answered my very specific question without even seeing it!

But what if I don't plan on taking the cylinders off? Can I have 0.01" taken off of the head instead of the cylinder?

[Bodi]

No. Decking the barrels makes the hole the orifice thing site in shallower, so the seal has less space.
Strangely, I was thinking about this earlier today. Maybe a 0.010 shim washer under the orifice would do the same thing as decking 0.010? Or a thin section o-ring around the drilled tube?
I think an o-ring would be better as it would seal between the bottom of the orifice and the bottom of the hole. As is, the original seal has to seal between the top of the orifice part and the head, and around the hole's bore as oil will seep through the clearance. If the bottom is sealed, the upper seal just has to seal between the brass part and the head surface.

[robvangulik]

Amazing that Hondaman answered my very specific question without even seeing it!

But what if I don't plan on taking the cylinders off? Can I have 0.01" taken off of the head instead of the cylinder?

When you take the head off you'll have to take the cylinders off as well, or the bottom gasket WILL fail as you'll disturb it for certain!

[jonda500]

Amazing that Hondaman answered my very specific question without even seeing it!

But what if I don't plan on taking the cylinders off? Can I have 0.01" taken off of the head instead of the cylinder?

When you take the head off you'll have to take the cylinders off as well, or the bottom gasket WILL fail as you'll disturb it for certain!

That's not true - I have had my head off three times yet still have no leaking around the cylinder base gasket.
John

[robvangulik]

Than you're a really lucky man and should buy a lottery ticket to see if that luck holds on

[vames]

OK OK i’ll pull off the cylinders too. I’d feel stupid if i fixed one leak and created another. Also I have to get that bit removed from the cylinder mating surface.