Author Topic: Dreaded CB350F Head Gasket Oil Leak (Read 17773 times)

donpark1086 · « **on:** August 16, 2012, 06:55:03 pm »

Gents:

I've got about 1600 miles on my 1972 CB350F rebuild. The outboard oil galleries are leaking quite badly and can overwhelm towels jammed in the fins in a 100 mile run. At the cylinder and head joint.

I've pretty well exhausted this forum looking for a proper engineering fix. My head and deck were flat and I used a good gasket kit. Seems to stop leaking once the oil is warm and thin. Those little orifice valve gaskets are very soft rubber, and I guess the crush upon assembly can hold back the oil.

No help from head re-torque.

I really like this little screamer. She winds out nicely on song and will idle gently. I'll try to make it through the riding season, which ends about mid-November here,

Thanks for any advice guys.

donpark1086 · « **Reply #1 on:** September 17, 2012, 05:41:51 pm »

Gents:

I'm getting ready to de-head my CB350F to address the copious oil leaks on each side of the head cylinder gasket.

I have a plan to use Viton O-rings in a special way, and I'll take some photos when I do it to report

But here is a question.

There is a multi-hole orifice valve in each of these galleries. First, I believe these small holed restrictors have full oil pressure below them in the vertical gallery, and the small holes prevent over-oiling the valves, cams, and rockers. But what about oil pressure above the restrictors? Is the oil pressure quite low above these restrictors?

If oil pressure is low above the restrictors, then I want my design to limit the head gasket seal to being exposed to this low pressure, and not the full oil pressure.

Thanks.

donpark1086 · « **Reply #2 on:** September 19, 2012, 05:29:03 pm »

Hi:

I got the CB350F disassembled to the point where the cam is out. The head is still on.

As far as I can tell the oil to the valve gear is delivered by two little pipes, a pipe for cyls 1and 2, and a pipe for cyls 3 and 4. These pipes have pin holes aimed at each of the cam lobes, so 8 pin holes. My guess is that these 8 pin holes are so small that there is substantial pressure in these pipes, so there is, by hydraulic logic, substantial pressure above the oil valve orifice at the cylinder block head joint.

The cam bearings have no drillings to deliver oil. Just by splash.

So, I'll need good sealing below and above this oil valve orifice that does not rely at all on the head gasket doing any of the sealing at all.

More later......as this progresses.

Mister Gumby · « **Reply #3 on:** September 19, 2012, 09:16:44 pm »

Interesting, I'll be keeping an eye on this thread. I'll have my 350 on the road daily in a couple of weeks and I'm curious as to the quirks I'll be up against

donpark1086 · « **Reply #4 on:** September 21, 2012, 05:43:49 pm »

Hi:

I rebuilt this CB350F engine with an NE gasket set as in the image.

After 5000 miles, the sealing sleeves around the oil valve orifice have gone rock hard and cannot seal. Also, these sealing sleeves are so short that they are very little compressed by the cylinder head and the cylinder deck.

So, the NE gasket kit has inferior materials and poor dimensional design. These sleeves should have been made of Viton and they should have been about 30% longer for more rubber compression to improve sealing. Since the sleeves failed to seal oil pressure, the secondary containment was the head gasket, which it not up to this containment task.

In my next note, I'll try to explain my improvised seal approach with Viton O-rings.

donpark1086 · « **Reply #5 on:** September 25, 2012, 06:21:05 pm »

Hi:

Here are some updates on trying to keep oil inside the CB350F.

The two images show the factory approach to sealing the oil going to the cams at the cylinder head joint.

The rubber seal on the valve orifice is very little compressed between the head underside and the cylinder deck topside because the head gasket is more than 0.040" thick. And the hole in the head gasket is no where near filled with the crush of the orifice seal.

In the next post, I'll show the improvised approached to sealing this area.

HondaMan · « **Reply #6 on:** September 25, 2012, 10:15:48 pm »

It appears you've been stung by the recent crop of cheesy gasket kits we are seeing: even the once-great Vesrah kits are coming with undersized (or even missing...) O-rings in the kits I often buy. Their O-rings are now consistently undersized, so I have begun creating proper 'kits' for the 750K and 750F bikes. Next is for the 350F/400F (of which I have just one, but relish it).

I just had the factory (Parker) make me a batch of 50 of the O-rings on the oil pickup funnel (16.5x2.00mm), a critical one that no one offers today. I'll be posting those in the For Sale section and on my website in the upcoming "Baby Fours" section.

After that will come the full O-ring kits for the 350F/400F, because almost everyone here who has recently rebuilt these Fours has ended up with that same oil leak, due to the seal you are now working on.

BTW: Viton isn't necessary. The Buna will seal better. Viton is good for contact with ethanol fuels, but they don't seal as well and tend to be stiff enough to cause other problems unless you get the exact size, which must be 3% to 5% SMALLER in cross-section than the equivalent Buna ring. (Just a hint...)

The oil PSI inside the cams "squirt tubes" is similar to the top end of the 500/550 and 750: there is about 15 PSI inside the tubes when the engine is cold, 5-8 PSI when hot.

hoodellyhoo · « **Reply #7 on:** September 26, 2012, 04:54:27 am »

Just wanted to say thank you for documenting all of this. I'm hoping to reassemble my 350f motor in the spring and will deffinitely be checking my o-rings and seals now.

kyre · « **Reply #8 on:** September 26, 2012, 06:03:52 am »

You can still buy that orifice valve gasket from service Honda (OEM). I'm rebuilding my 400f now and bought 2 because I knew the vesrah kit didn't supply the correct gasket. I'm also finding out like Hondaman said, the vesrah kit also forgets a bunch of other gaskets too.

camelman · « **Reply #9 on:** September 26, 2012, 01:16:01 pm »

Good to know about service Honda. I've been struggling with a few leaks too and can't figure them out. I wonder if the vesrah kit just let me down.

donpark1086 · « **Reply #10 on:** September 26, 2012, 06:40:09 pm »

Gentlemen:

Thanks for all your productive input. I'd like to get this CB350F reasonably leak free. With its Marving racing exhaust, this Green Hornet is a double howl; a howl from the engine, and a howl from my helmet. One of the most fun bikes I've had. And if you like attention.....it attracts.

I've rebuilt all manner of Triumph, BSA, Norton, Kawasaki, Yamaha, Briggs, Ducati, Guzzi, and OMC engines, and they all been reasonably oil tight, at least for 10 to 20 kmiles. So my spewing CB350F was a real exception.

I do believe that the NE Brand gasket kit from Japan was worse than 3rd rate in many regards.

I'm really glad the the SOHC site will have a Baby Fours section. I've got a lot of rebuild images to share when it is ready.

On O-rings, they have a Duro hardness rating, and my Viton ones were the same 70 Duro as the Nitrile. Also, I was doing a bunch of micrometer work on the O-ring dimensions to get their toroidial volumes so the compressed O-ring would fill about 92% of the space and get a 40% maximum compression. My Viton O-rings were slightly bigger than my nitrile. I was surprise at this manufacturing variability, but I really don't know who made them. Maybe they were "off-specs" sold in various kits.

I did find that the orifice valve gasket is still available from Honda, but I wanted Viton for those cylinder head temperatures, and Honda did not specify their material. Also, if my NE Brand headgasket is a different thickness than Honda OEM then I could have incorrect crush on the Honda orifice valve gasket.

I'm just doing some rides now on the refitted O-rings, and she seems pretty dry. There is still some oil, but this might be earlier splash that I didn't clean up.

More later............

danfr · « **Reply #11 on:** December 04, 2012, 08:19:43 pm »

Any updates on the leak?? I rebuilt my 350F and it leaks out the left side. I'm thinking its the Vesrah orings??

donpark1086 · « **Reply #12 on:** December 05, 2012, 05:33:07 am »

danfr:

My CB350F still leaks on both sides. Frankly, this disgusts me. There is little doubt that Honda made a lousy engineering design for oiling the top end of this engine.

Like so many others, Ill ride this Honda Valdez and just mop up after every jaunt. Until someone can offer a hopeful fix I'm not going to do the top end again just to have it leak again in a week or two.

The CB350F was short lived, but the popular CB400 lasted awhile, and I'm surprised that no one has yet found a way to stop this spewing.

By comparison, my 1964 BSA 650 Lightening is dry and clean!!

camelman · « **Reply #13 on:** December 05, 2012, 10:37:03 am »

Donpark,

I'm sorry to hear you're still having problems. I've been leak free on my head gaskets for my last four builds, but i typically use coppercoat on the head gasket. Did you try that? Also, a leaking valve cover gasket may emulate a leaking head gasket, which turned out to be the issue i referenced in my prior post on this thread. Have you installed a new valve cover gasket yet?

Camelman

donpark1086 · « **Reply #14 on:** December 05, 2012, 04:19:10 pm »

Thanks camelman.

I have not coppercoated the head gasket. Perhaps that the secret.

By valve cover gasket, the serpentine O-ring, is very oil tight. Not a wisp.

I will admit that I use this bike, I put on about 7000 miles from August to December.

I like many aspects of this bike, but this engineering deficiency does have me annoyed. I was glad to put a cover on it for the Winter to get it out of my sight!

At least I'm emotionally involved with my bike!

hondamatteo · « **Reply #15 on:** December 05, 2012, 09:20:53 pm »

I also used the coppercoat on my head gasket when I replaced it for this very reason. I have been leak free for a few thousand miles since. Dont remember the name of the gasket kit I used though.

donpark1086 · « **Reply #16 on:** December 06, 2012, 06:02:54 am »

Thanks guys.

No matter how achieved, no leaks are a good thing and the right goal. So I may do the copper coat approach next Spring.

From an engineering standpoint, the head gasket is not really the sealing surface for these oilways, the rubber bits are the seals, but the head gasket can act as a back up. I'd like to devise something that actually seals the oilways. But why be a purist. A clean engine is the desired outcome.

camelman · « **Reply #17 on:** December 06, 2012, 08:04:31 am »

Donpark, just to be sure, do you have dowels for the holes directly forward of those oil restrictors? Those forward holes should have a metal dowel and a rubber ring similar in design to the oil restrictors. Those forward holes are well known to leak.

Prospect · « **Reply #18 on:** December 06, 2012, 01:08:49 pm »

I'm planning to put back the top end of my 350f this winter after a light hone and a valve job. I'll try the copper coat for the head gasket. I've actually found an old gasket set from the late 80's which I think might be from Honda but I'm not sure. Hopefully it's better than the stuff coming out these days. My Athena set for the cb750 has been holding up OK but I haven't heard great reviews as of late.

donpark1086 · « **Reply #19 on:** December 06, 2012, 05:05:35 pm »

Guys:

Thanks for your continued input about keeping oil in the CB350F.

Camelman, I think that these dowel pins and seals are well located in my engine, but I did notice that the rubber sleeves were quite hard.

Am I correct that these are oil drain holes to duct oil back into the crankcase, so that they don't hold back pressurized oil. But oil will find it way out if there is any clearance, especially hot oil.

Come Spring I'll try to do a bit more "source diagnosis".

camelman · « **Reply #20 on:** December 06, 2012, 08:55:53 pm »

Donpark,

That's your leak source. Those things leak like sieves when they're old. Those little seals on the oil restrictors hardly ever leak.

Camelman

HondaMan · « **Reply #21 on:** December 06, 2012, 10:34:37 pm »

I think you both have hit on something important here: the return oil paths do have quite some pressure because of the turbulence that comes from the high revs of the Baby Fours. The 350F/400F were always dry as a bone, until they sat too long.

One of the big culprits I am finding with all of the head gaskets today: the spec is off in 2 ways.
1. The new ones don't compress as far as the old ones. Probable reason: different crush materials, as asbestos is now outlawed. In some cases, higher torque helps, but usually only on the OEM gaskets we have last seen.
2. The head gaskets today come with a slick, sealed, hard surface. All of the old Honda gaskets had a coating that resembled paraffin mixed with rubber cement, and it was quite soaked into the [porous] asbestos surface. This had a LOT to do with their sealing, and with the difficulty of removing the head gaskets. Today's head gaskets, even after 10k miles, come apart like they were never sealed when disassembled.

So, the search is on: even if I have to INVENT something, there must be a sealant that resembles the old stuff that can be applied with a roller (which was how Honda did it) or a brush, that is thin, rubber-y, and sets with heat, that can also get a grip on these head gaskets (and others, for that matter).

The Cometic head gaskets are the hardest of all to seal. Their surface is irregular like cloth, yet completely impervious to anything sticky. These come with Wiseco piston kits, and are real hard to seal around the oil ports. They also come in the red-colored "Japanese made" gasket kits, I've discovered.

Maybe something like Walthers GOO and high-temp paraffin, heated to about 200 degrees, stirred slowly in the pot (with a chant and a lizard's liver?) might work...

donpark1086 · « **Reply #22 on:** December 07, 2012, 05:27:18 am »

Thanks guys.

After my first engine build the leaking was certainly from the oil valve orifices. I wired some rolled up paper towel bits to stay put in the fin area between the cylinder and its head, and the oil always originated from the region of the orifice.

And when I got the bike there was much external oil baked onto the sides of the engine.

After the sealing attempt around these orifices described earlier in this thread, she was dry for about 2000 miles, then started to leak again.

I have not done the paper towel trick again as the riding season ended.

Hondaman makes a good point about crankcase pressure and those drain seals. Although this engine is vented to atmosphere via the breather tube, there is great windage in that engine from pumping pistons and a whirling crank. So it might be at atmospheric pressure but it is windy in there!

HondaMan · « **Reply #23 on:** December 09, 2012, 07:06:17 pm »

Wow, that's a beautiful engine!

donpark1086 · « **Reply #24 on:** December 09, 2012, 08:21:02 pm »

Hondaman.

Old Soichiro had an eye for industrial art before Steve Jobs claimed it for himself.

Through its Marving exhaust the sonic signature dancing up through the gears is a thing of beauty as well.

But those oil leaks!

News:

Author Topic: Dreaded CB350F Head Gasket Oil Leak (Read 17773 times)