If I understand correctly...

To follow up on a thread originally posted by Stephen Hill.
Will having a PCV valve in place, alleviate enough pressure build up on a wet sump start up to avoid blowing the crank seal?

Lineslinger said:

To follow up on a thread originally posted by Stephen Hill.
Will having a PCV valve in place, alleviate enough pressure build up on a wet sump start up to avoid blowing the crank seal?

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Yep

baz said:

Yep

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If it's a reed type valve attached to the crankcase (i.e. so it would be submerged in the excess oil if the bike had 'wet-sumped.')

oldschool said:

If it's a reed type valve attached to the crankcase (i.e. so it would be submerged in the excess oil if the bike had 'wet-sumped.')

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Yes it would
Until you kick the bike over

Not it its just on the breather line to oil tank. The crank seal is sitting below pistons and if oil is high enough in sump, there will be a pretty high pressure build up as pistons drop down. That will continue until sump oil level drops to near its usual level, where now pistons stay clear of oil level and air is what gets displaced every revolution.

I had a seal spilt in two parts from starting with a significant wet sump oil level. Did not notice until finding rear wheel getting oil soaked while out riding.

There was a video posted here once of a '72 750 with a reed valve placed in the line off the breather (back of the case) which was wet sumped and started and the oil pumped into the oil tank via the reed valve. Now, with that said, which engine model are we talking about, where is the valve placed? If you added the Comnoz reed valve to the back of your 850 cases, you are probably fine. If it is off the timing chest, maybe not so good.

I had discussed this briefly with Matt R. during rebuild and he noted that with my setup drain the CC before cranking. I assumed I had this pressure thing analyzed but wanted to ask others. I have PCV outlet line off the timing chest leading to a Mikes XS reed valve between the outlet and below the oil tank on a 74' 850. Never an issue on my automobile engine builds but different configuration methinks.

I have never tried wet sump start on Norton engine. My bike sits for long periods when hot or cold weather prevails without marking the shop floor. I spent a lot of time on rebuild making it that way. I shouldn't even be talking about it, I'll jinx it.

So, unless the PCV is set up lower crank case like a Commonz/cNw/NYC offering the pressure would and maybe could bite me with my current configuration.
I'll just keep dropping down to drain it, no big deal. Sometimes I can grab a quick nap while doing so.

YES, if you are referring to an aftermarket reed valve type breather, and NOT simply an automotive one-way ball & spring type PCV valve in-line on the OEM oil breather line.

BE SPECIFIC, because more than one aspect of the above reply can get misinterpreted in other people's comments relating to "PCV" valves!

Lineslinger said:

I had discussed this briefly with Matt R. during rebuild and he noted that with my setup drain the CC before cranking. I assumed I had this pressure thing analyzed but wanted to ask others. I have PCV outlet line off the timing chest leading to a Mikes XS reed valve between the outlet and below the oil tank on a 74' 850. Never an issue on my automobile engine builds but different configuration methinks.

I have never tried wet sump start on Norton engine. My bike sits for long periods when hot or cold weather prevails without marking the shop floor. I spent a lot of time on rebuild making it that way. I shouldn't even be talking about it, I'll jinx it.

So, unless the PCV is set up lower crank case like a Commonz/cNw/NYC offering the pressure would and maybe could bite me with my current configuration.
I'll just keep dropping down to drain it, no big deal. Sometimes I can grab a quick nap while doing so.

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Does your bike habe a wet sump issue? Check oil level after a ride, then again following days/weeks layup to see it significant drop in level. Act accordingly.
My 74 850 will lose nearly all standing oil above the tank strainer gauze in about 2 weeks. Unless of course I flip my feed line tap with ign interrupt to off.

Smartypants.

I have 2 reed valve breathers that are both well above the oil height of a sumped motor and I never drain the oil to start the bike. I have JSmotorsport's camshaft port breather and I have a second timing chest mounted breather with 2 extra 3/8" holes drilled between the timing chest and the crankcase to facilitate less restrictive air movement. I have no leaks really except the tiny oil mist dribble out of the oil tank breather which is normal since oil is atomized inside the oil tank and exits with the air.

The falling pistons compress the air inside the crankcase to induce the crankcase pressure. IF your breather is underwater (under oil actually), then the air has to push a volume of oil to aleviate the pressure so it would be higher resistance and higher pressure created than just pushing air, so in theory your underwater reed valve should be MORE prone to blow out a crankcase seal....

Not only is the air lighter per measured volume of fluid, the air can also be compressed where the fluid can not and must be moved to reduce the pressure.... so the crankcase reed valve may be better in setting up a sympathetic pulse to the engine rpms because it's closer to the source of compression, but I would bet that it's not in any way superior in reducing crank seal blow out....

Tornado said:

Does your bike habe a wet sump issue? Check oil level after a ride, then again following days/weeks layup to see it significant drop in level. Act accordingly.
My 74 850 will lose nearly all standing oil above the tank strainer gauze in about 2 weeks. Unless of course I flip my feed line tap with ign interrupt to off.

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Two weeks standing to see the gauze filter is pretty quick
I'd have thought it'd be more like double that time at least?

9 times out of 10 you'd get away with starting a sumped engine without dislodging the main crank seal
If you can kick it over fast enough that is,and don't mind a bit of smoke as it's scavenging
That's what we did back in the day
But any reed valve fitted to the crankcase will relieve some of the pressure
Whether it is air or to a lesser extent oil
Therefore helping to not build enough pressure to blow the oil seal out of position, which is what the question was about

Tornado said:

Not it its just on the breather line to oil tank. The crank seal is sitting below pistons and if oil is high enough in sump, there will be a pretty high pressure build up as pistons drop down. That will continue until sump oil level drops to near its usual level, where now pistons stay clear of oil level and air is what gets displaced every revolution.

I had a seal spilt in two parts from starting with a significant wet sump oil level. Did not notice until finding rear wheel getting oil soaked while out riding.

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When your seal split did you have a reed valve fitted?

So we can all agree that air ( a gas ) is compressible and oil ( a liquid) is not . Two pistons descending together ( as they do in a 360 degree crankshaft) compress the air in in the crankcase which in turn tries to compress the oil which is not compressible and therefore has to escape somehow . If the air can escape ( through a breather of some type located above the level of the oil in the crankcase) before exerting too much pressure then all may be well .
If the air cannot escape ( as when the breather is below the level of the oil ) then it becomes a matter of which gives way first - valve of the breather or the seal . This is pure physics and the laws of physics are very democratic. It doesn’t matter who you are , rich or poor , who designed your breather, new to Nortons or lifetime owner . It really is that simple . Those that have been starting their bikes for years with no regard to a crankcase full of an incompressible liquid best count their blessings.

Lineslinger said:

I had discussed this briefly with Matt R. during rebuild and he noted that with my setup drain the CC before cranking. I assumed I had this pressure thing analyzed but wanted to ask others. I have PCV outlet line off the timing chest leading to a Mikes XS reed valve between the outlet and below the oil tank on a 74' 850. Never an issue on my automobile engine builds but different configuration methinks.

I have never tried wet sump start on Norton engine. My bike sits for long periods when hot or cold weather prevails without marking the shop floor. I spent a lot of time on rebuild making it that way. I shouldn't even be talking about it, I'll jinx it.

So, unless the PCV is set up lower crank case like a Commonz/cNw/NYC offering the pressure would and maybe could bite me with my current configuration.
I'll just keep dropping down to drain it, no big deal. Sometimes I can grab a quick nap while doing so.

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The 72 case breather position is below the oil level in a badly wet sumped engine. Same with the NYC sump plug type.

With these types, the crank case pressure will force the oil through the breather and to the oil tank quickly. It basically means that high excess pressure cannot build up as any pressure acts on the oil, pushing it to the tank.

The one way valve aids this by allowing oil out, but not back in. Without a one way valve oil is pumped out but a large portion is immediately sucked back in again with each rise and fall of the pistons.

With a timing cover breather none of the above happens, only the oil pump can return oil to the tank, that means the engine will have to run for some time before the oil level in the cases is corrected. In this case, with reduced space for air, and being run for a longer time in this way, excess pressure will act upon the oil in the cases, but oil cannot egress via the breather, or be compressed, so it’ll find the weakest link, ie the PTO seal.

If your PTO oil seal is sound, and if you’re lucky, it’ll hold and the excess pressure will be handled by the breather. But the oil seal is the weakest link, so there is a high risk of it being blown out in these circumstances.

On a ‘74 your best option is the NYC sump breather. I have one one both my road bike and track bike. I accidentally started the road bike with a sump full of oil (it has a cNw e start so it’s BOOM when it starts) and the oil seal was fine. I’m not saying I’ll do this every time, but it’s nice to know the system can handle the oversight when needed !

baz said:

When your seal split did you have a reed valve fitted?

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Yes, on the timing cover hose to tank. Oil seal likely 50 yrs old....it was split in two around the periphery, not like a semi-circle.

Fast Eddie said:

The 72 case breather position is below the oil level in a badly wet sumped engine. Same with the NYC sump plug type.

With these types, the crank case pressure will force the oil through the breather and to the oil tank quickly. It basically means that high excess pressure cannot build up as any pressure acts on the oil, pushing it to the tank.

The one way valve aids this by allowing oil out, but not back in. Without a one way valve oil is pumped out but a large portion is immediately sucked back in again with each rise and fall of the pistons.

With a timing cover breather none of the above happens, only the oil pump can return oil to the tank, that means the engine will have to run for some time before the oil level in the cases is corrected. In this case, with reduced space for air, and being run for a longer time in this way, excess pressure will act upon the oil in the cases, but oil cannot be compressed, so it’ll find the weakest link.

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But,... The low placed breather has to chose which is the least path of resistance to reduce the pressure.... It comes down to whether the force needed to move the mass of the oil blocking the reed valve is greater than the resistance of the oil seal to that amount of force... The path of least resistance is the path that will give way.

The compression ratio is the given displacement volume...... divided by the total crankcase volume... to give you the crankcase pressure ratio. Did you plug the extra holes into your timing chest?? well if you did then you have a smaller case volume than I do so your pressure is even higher yet. I drilled 2 3/8" extra holes between my crankcase and timing chest to allow the crankcase pressure to be distributed over a larger total volume, so lower crankcase pressure with same piston displacement..... BTW, I do have a reed valve on both my breather exit ports so I get the one way flow thing...

I may not get my oil back to the tank as fast as a low positioned crankcase breather, but that just means I get more splash of oil on my cam lobes at start up... probably not a bad thing...

As I said, the best thing about the Comstock breather is that it is so close to the source of displacement that it can harmonize with the pressure pulses better at high RPM's...

I would bet you that my dual breathers, not obstructed by any sumping fluids with a larger crankcase/timing chest volume, exerts lower case pressure than the oil sumped, plugged crankcast, comstock style breather.

This happened on my 920 engine, so even more swept volume!

Yes, the timing case holes are plugged (actually they’re Maney cases that I bought from Steve sans holes).

So I kinda had a max pressure thing going on !

Of course, the oil seal is not very old, which helped.

But as I said, with the cNw or NYC type breather, pressure acts on the oil which has an open passage to the tank, so it can't really 'build'.

At least I know I can get away with it if / when needed. But it obviously must put much greater pressure on the seal than it was designed for, hence I wouldn’t do this as normal practice.

Well, I'm glad that you are getting away with it when it happens occasionally.

You probably should think about doing the O-ring trick that Jim Schmitt suggests to prevent any chance of crankshaft seal displacement.... If you buy the correct O-ring, next time you take the front pulley off, you add the O-ring and it's a little insurance against the seal blowing out...