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I have always thought that if the crankcase was vented, since the space under the valve covers is connected to the crankcase through the pushrod tubes, the valve covers didn't need to be vented.

I have some new valve covers that are already drilled and tapped for vents.  So, I am reconsidering.

I have read that some people vent both sides and some vent only the 1-2 side saying you shouldn't vent the 3-4 side.

Some say that venting the valve covers facilitates the oil draining back through the pushrod tubes.

What do you all think?  Should I plug the holes, vent one side or vent both sides?

I have a CB breather/oil fill setup venting the crankcase and have opened up the outlets.  I also have a breather box above the transmission that two transmission vents are connected to.  If the consensus is that I should vent them, how should I route the hoses?  Should I tee them together into a single hose I connect somewhere?  Some suggest using an air filter type thing right at the valve cover.  Others say that something like that would get clogged up. 

Again, what do you all think?

1957 CMC (Speedster) in Ann Arbor, MI

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Michael........I would drill another hole in that breather box and connect both hoses from 1-2 & 3-4.  I like that you put it in that "void area", out of site. I too, had trouble with those "C" channel CB covers plus the bolts that hold them on leaked too so I went back to stock steel/snap bail covers and never had another leak. Also I welded steel tube connections into these covers (again, no leaks now) to connect the hoses to. I have found that using the valve cover gaskets that are made of black rubber impregnated cork work the best.  lastly, I spot welded a small flat piece of 22ga sheet metal tothe the bottom/inner cover wall to keep the gasket from drifting up and out of it's sealing area. Similar to what's on the top.

I have to mention too, that I used "Total Seal" piston rings during assembly. Measured with my trusty Manometer, I had no crankcase pressure what-so-ever ! When you order a set of Total-Seal rings for the 90mm pistons, only the second oil ring is used and it has the trade-mark piston ring end gap seal. When testing piston ring seal using standard leak-down procedure I was surprised how high it was compared to stock ring set readings. 

The only reason I mention this is if any of you are building and engine right now, I recommend installing a set of Tota-Seal rings. You won't have any blow-by to worry about, plus the compression stays up there where its supposed to and no increased friction from these rings as well.................Bruce

Michael McKelvey posted:

Even with my CB breather/filler with enlarged hoses and a sand seal, I am still getting oil behind the crank pulley.

@Michael McKelvey I have this issue on my engine.  It's not much oil, just enough to make an annoying little oil stain on the engine tin.  I cannot detect blow by from the oil filler.  However, I am running an aftermarket aluminum pulley and feel that it may be the problem.  My plan is to replace the pulley with a stock one.  The oil return grooves on the hub of a stock pulley are much deeper and sharper than the aftermarket pulley.  I am hoping that this will help return the oil to the crankcase.

DCD7024E-AB5D-43AE-8C4C-91F71C3443175708D76D-1C11-44CC-935A-02B3123AEC02Pat Downs uses stock VW valve covers on his engines. 

My engine that Pat built has a breather system that I designed. I have an electric fuel pump so used an aluminum block-off plate where the mechanical pump would've been. I drilled and tapped it for a pipe thread and used -8 stainless hose to connect it to a Porsche oil filter canister which hangs on my fan shroud like the P-cars did. This can is filled with scotchbrite to stop any oil from making it on through the exit hose which goes to the #1&2 side air cleaner. 

Haven't had a drop of oil leak from this engine in 5,000 km.  


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Last edited by Will Hesch

What most venting systems need is volume for the air to slow down enough for the oil to drop out of suspension. If you're using the little flat rectangular breather box use bigger hose- it will add volume, air in the hose won't be moving as fast and most of the oil will fall out of suspension and run down the inside of the hose before it ever hits the breather box. Gordon had success when he upped the hose on his from 3/8- 5/8".

As Anthony noted in the CarlosG Spyder breather thread, the CB oil filler/breather tower does not seem to have enough volume(and they leak).

My friend Brian got the CB filler/breather on his 2332 and it does not work much at all. Puking oil from his crank pulley like crazy. He has a CB thinline 1.5 quart sump extension, so I told him to lower the oil level. But he has no VC vents, and only the filler/breather with the fuel pump blockoff plumbed to the CB unit.

In short, he(and you) need to ditch that unit for one with some volume. I think it is important to have a good deal of volume in the bottom and the scrubber pads in a much smaller area at the top. My homemade tube is about a quart in volume, has two SS Chore boy pads in the top end(one tube to one carb), size 1/2" ID. Each VC has a 1/2" vent tube. I also ran a huge 1" ID hose from the distributor hole to the breather. It works pretty darn well with the dry sump pump. I also have a 1/2" hose from the dry sump tank to the carb top. I used some nylon elbows in the system and they were small ID, so they all got drilled out to a 1/2" ID.

I don't seem to get any residue at all in the right carb where both vents go.

I know they are expensive, but both CSP and RLR make some really nice breathers that can be mounted on the firewall.

Last edited by DannyP

I found that the CB Oil filler/breather box leaks oil.  (Part#1771)  The leak occurs thru the two bottom machine screws holding the cover on. The threaded holes in the main body go all the way thru thus allowing oil to seep past the threads. Additionally, when the cover is removed, there is oil trapped at the bottom which spills out. This is where the lower screw is and is probably why it leaks. Of course a little silicone gasket sealer will fix this but at $70+ for this, I don't think it should leak at all. I removed it and went for a different one and a std. breather box with only valve cover tubing connected. The new filler neck had an opening for a vent hose but all I put on it was a small paper breather filter. Still, the Total Seal rings are holding back the blow-by for the most part.



I knew a guy once that welded a small tube angled front to backish on an exhaust pipe to create a Venturi effect and he connected the other end to a crankcase vent. No more venting concerns.

( sent from my vintage iPad from where excellent blackened grouper can be had while witnessing girls beach volleyball without backs in bikinis )  Someone's got to keep score, eh ?

Last edited by David Stroud IM Roadster D

I got the CB 1771 to finally stop leaking but it took a while.  

My cover screws thread into bosses cast into the main body, but the threaded holes do not go all the way through the body so they don’t leak out the back.  The original CB design uses threaded rod and wingnuts for the cover.    Those got tossed for flanged, hex-head 10-32 screws with soft-ish nylon sealing washers to seal the screws against the cover.

The cover came with a formed seal that feels like neoprene and that got wrecked the first time I pulled the cover.  In its place I just shoot a bead of that black, fuel resistant silicon caulk around the cover sealing slot and that has been sealing for several years, now.

IIRC, CB uses some sort of flat rubber washer between the breather and the filler pedestal and I couldn’t find a direct replacement locally so I went with a suitable, slightly stiff O-Ring squashed between the pedestal and the breather.  I also bought one of those cool round, toothed, gland nut adapters for my ratchet drive so I can crank the breather up nice and tight with no leaks.

I only drain into the filler port so the drain port on the bottom has teflon tape on a pipe plug.  The breather outlet ports on top have press-fit 90° plastic fittings (they are modified PCV fittings) going into 5/8” ID heater hose to each carb filter cover.  I needed the bigger 5/8” hose to keep the case pressure down and it works.  I do not vent my valve covers.

After all that........   The breather doesn’t leak and the only time that I get weeping from behind the crankshaft pulley is when I’ve been hammering the interstates at 80+ mph for over an hour or so and then I get just a little wetness at the drain lip in the tin under the pulley.

If I had it to do over again, though, I would probably just go with a Berg breather and declare success.

Last edited by Gordon Nichols

My two sense............get a Ron Lummus or CSP breather box. Then hose and fittings in either AN 8 or An10 depending on the breather box fittings. Don’t use AN6 fuel line, it’s to small! Mount it as high on the firewall as possible without blocking the fan inlet on the shroud

use a stock VW oil filler breather on the generator/alternator stand.

run each valve cover to the breather. Route the hoses so there’s no low spots. 
on the breather box the lower outlet will be used as a breather line and drain back  therefore you route it to the oil  filler breather tube.  With a stock shroud we use the 2 qt unit, if you have the porsche shroud set up you can use the 3 qt unit.y


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