Crank case breather

Sorry to dig up this old thread... but it got me thinking... I have an old Bonneville Air injection valve setup in a box somewhere in my garage.

Well, after much head scratching with my friend and Norton expert, he came up with this. It is mounted on the back of my 72 Combat case at the existing breather location. We haven't tried it out yet, but I think that it will do the job nicely.

Looks perfect. More details on how you did that?

I've got those Triumph ones too. Still on the bike although the rest of the crap is disconnected.

Indeed looks like factory issue and wonder why Norton didn't think of it. Hope it just sucks too.

hobot said:

Indeed looks like factory issue and wonder why Norton didn't think of it. Hope it just sucks too.

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Reed valves usually do.

It should do well, as long as no oil accumulates in that area inside the crankcase.

That's just my opinion and includes ANY Reed valve mounted to the case down there.

Those breathers do better in the old magneto location on the backside of the timing case.

oil is not a problem on the reeds. what do you think a ding pop puts through them.

pete.v said:

It should do well, as long as no oil accumulates in that area inside the crankcase.

That's just my opinion and includes ANY Reed valve mounted to the case down there.

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absolutely 100% wrong. as comstock has PROVEN with testing the best location is in the crankcase where the pressure is generated.

grandpaul said:

Those breathers do better in the old magneto location on the backside of the timing case.

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bill said:

absolutely 100% wrong. as comstock has PROVEN with testing the best location is in the crankcase where the pressure is generated.

grandpaul said:

Those breathers do better in the old magneto location on the backside of the timing case.

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I said "better", for those type breathers. In the lower crankcase is NOT the better or best place for them, due to high likelihood of "slugging".

I have had nothing but positive results with them on the backside of the timing chest (8 bikes so far); results are zero oil leaks from previously leaky spots that are typical of commandos.

Best? I didn't say that.

(also, the backside of the timing chest is very near the relief channel cast into the cases, so the internal pressure has a reasonably clear path out the reed valve)

Here we go again! Not sure if I should get popcorn or a riot shield.

What I have found as far as oil going through my crankcase reed breather:

If the engine is setting at idle for a minute or more oil tends to puddle at the front of the engine case. The Commando engine is tilted forward so the scavenge pump pickup is not at the lowest point in the engine as it was in the Atlas.

The first time you rev the engine after it has idled long enough to create a puddle the oil will be pushed to the rear of the engine by crank windage. This does flood the reed valve with oil and about 2 to 4 oz. of oil will pass through the breather in about one second. During this time the reed stops resonating and the crankcase pressure will begin to rise from around 12 inches of depression [h2o] to 4 or 5 inches of depression. It only lasts a very short time and then the reeds start resonating again and the pressure drops. This is clearly visible in the video I did some time back.

Beyond the first time you rev the engine there is only small amounts of oil splash that pass through the reed breather. This small amount of oil does not affect the reeds ability to resonate.

Part of the reason I wanted the breather down low was to help get rid of the extra oil that accumulates in the crankcase at idle or when starting an engine that has wet sumped.

Without the reed breather the scavenge pump will take several minutes to clear the extra oil from a wet sump condition.

It will take around a minute to catch up with the oil that has accumulated from a long idle period. This extra oil that has accumulated at idle is why a lot of engines will smoke through the exhaust for a minute or two after they have been setting at idle for several minutes.

The oil that has accumulated at the lowest point -toward the front of the case -gets thrown around when you rev the engine and floods the cylinder walls until the scavenge pump has time to catch up with the oil that has suddenly been pushed to the rear with the increase in RPM. With the reed mounted low and to the rear that extra oil gets returned to the tank quickly. Jim

once again it is the better option over the timing chest!! as jim has stated "slugging" is NOT a problem

grandpaul said:

I said "better", for those type breathers. In the lower crankcase is NOT the better or best place for them, due to high likelihood of "slugging".

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any place you put the reed valve is better than nothing but is is NOT the optimal location.

grandpaul said:

I have had nothing but positive results with them on the backside of the timing chest (8 bikes so far); results are zero oil leaks from previously leaky spots that are typical of commandos.

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just how do you expect 2 or 3 3/8 holes from the crankcase to the timing chest to allow the pulses from 750-850 CC's to keep up and allow that much air to pass to the timing chest much less show negative pressure IN the crankcase???

grandpaul said:

also, the backside of the timing chest is very near the relief channel cast into the cases, so the internal pressure has a reasonably clear path out the reed valve

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as stated jim has done the testing with real data to prove this point.
as to johnney you have come up with a nice way of doing a proper breather mod on the 72-73 750 crankcase.

grandpaul said:

Those breathers do better in the old magneto location on the backside of the timing case.

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I think what you meant to say is the breather you sell do better on the timing case. :|

In Combats the drain is at the lowest point of cannted engine so nil puddling at low rpm-acceleration when most blow by pressure occurs to assist pump down. A sat up parked wet sumped Combat's crank spin and blow by at low rpm throw oil back and push it out and up the big ole breather tube, in mine for 30ish seconds before only oil pump spittle seen returning and exhaust smoke stops. At high rpm-acceleration the oil mass is moved to the rear so can and does puddle in Combats which are famous for high rpm wet sump mess but only a fairly small amount as the big ole breather tube is well place to let most excess get shoved back to tank, but does not lower crank pressure below ambient so with elastic level engine operation seams weep. Good enough is good enough, even a reed in the lowest sump plug site, may not be the best shot to miss a deer heart or head but a lung shot bleed out still does the job. Better reed location just means can go longer on ring wear before oil weeps show up. If ya want the best vacuum ya need to go with active evacuation like modern racers do, so I think ya'll missing the boat there.

bill said:

(1)once again it is the better option over the timing chest!! as jim has stated "slugging" is NOT a problem

(2)any place you put the reed valve is better than nothing but is is NOT the optimal location.

(3)just how do you expect 2 or 3 3/8 holes from the crankcase to the timing chest to allow the pulses from 750-850 CC's to keep up and allow that much air to pass to the timing chest much less show negative pressure IN the crankcase???

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(1) Better, yes; I never stated or argued otherwise

(2) I also never stated or argued that it was the optimal location

(3) once the initial pulses are expelled, the relative pressure stays low enough to inhibit oil weepage, the sole purpose of the breather valve. I never stated or argued that the timing chest reed valve created a negative pressure, only that multiple real-world installations have been extremely successful for a very minimal modification

swooshdave said:

I think what you meant to say is the breather you sell do better on the timing case.

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No, that's not what I meant. I was replying to the photo in the post which is a similar reed valve. Subsequent replies related to actual successful installations of the same type of valve.

I really don't see the point of arguing with success. If a bee flies, why argue that it isn't the best way to fly, or that it shouldn't fly?

which one is it?? looks like some one is doing a little back stroking.

grandpaul said:

Those breathers do better in the old magneto location on the backside of the timing case.

Click to expand...

grandpaul said:

bill said:

(1)once again it is the better option over the timing chest!! as jim has stated "slugging" is NOT a problem

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(1) Better, yes; I never stated or argued otherwise

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bill said:

which one is it?? looks like some one is doing a little back stroking.

grandpaul said:

Those breathers do better in the old magneto location on the backside of the timing case.

Click to expand...

grandpaul said:

bill said:

(1)once again it is the better option over the timing chest!! as jim has stated "slugging" is NOT a problem

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(1) Better, yes; I never stated or argued otherwise

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better = better = better.

No other argument was presented by me.

hobot said:

. If ya want the best vacuum ya need to go with active evacuation like modern racers do, so I think ya'll missing the boat there.

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If you look at this picture you see a belt guard from the cam going back to a vane type vacuum pump. It would draw the cases down to 15+ inches HG. It was good for power but there were a couple problems caused by it.

First the wrist pin failed almost immediately. It obviously cut down on the amount of oil mist present to lube the wrist pin. I cured that by going to a DLC coated pin.

The second problem was the valve guides ran dry and would seize. No oil could go down the guide when air was being pulled up through the guide. I ended up with beryllium copper guides, coated valves and extra clearance to get by that problem. Even then the guides didn't last long. Jim

Ya think we kicked that beehive hard enough. Hey, where's jimc?