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Four stroke crankcase venting

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GrahamWh05/11/2014 18:22:23
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Hello All,

I have an old OS 70 Surpass which has a venting point at the bottom of the crankcase, and a new-ish ASP FS70 that has one by the camshaft which is plumbed to the inlet manifold. Both run okay, but when I replaced the bearings of the ASP recently I found some evidence of rust starting to form.

On the compression stroke and the exhaust-expelling stroke the crankcase of the OS Surpass sucks in air, but the ASP sucks in fuel mixture from the carburettor. Could the influx of fuel be speeding the rusting process - and is it a sensible thing to disconnect the breather from the intake manifold and plug the manifold, making it run like the OS?

ken anderson.05/11/2014 19:19:03
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hello graham.........the one on the OS was/is for getting rid of the oil after it has come down from the combustion bit(piston) and lubricated the bottom of the engine.......and is continuous as long as the engine is going--later engine design's put it in to the inlet manifold instead of straight out into the atmosphere.....why- I'm not sure as I don't have any of the recent one's ....

ken Anderson...ne....1 ...... 4st dept.

J D 805/11/2014 19:24:35
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The modern way with vents is to fit to inlet manifold and recycle oil through motor and save some mess by sending it out the exhaust,although you may see some mixture going down vent tube when turning by hand when motor is running the suction in the inlet takes mixture into engine.The best thing to preserve motor is to run it dry at the end of flying and store in warm and dry place.

J D 805/11/2014 19:31:20
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Also it is important that vent pipes are not to long or oil may not drain from crankcase resulting in a build up of oil that can result in damage to engine.

Andrew Price 205/11/2014 19:37:22
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Stevo hasn't seen this yet, but, he's on the way for sure - although he is on the sick list at the mo. smiley

Phil Winks05/11/2014 20:28:29
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somewhere up the page Ken said the crankcase breather is to vent excess oil coming down from above to prevent a build up, However my instruction for my SC30fs say that there is no active lubrication of the crank and that a couple of ml of after run oil should be fed into this breather before each engine run, and my experience is this also vastly improves the pre-run compression making starting a breeze, Just the info I have on the sc engines

Shaunie05/11/2014 22:10:24
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I am sure there will be many opinions on this matter, however I am running an RCV91 with the breather running the full length of the fuselage and contrary to the Wizard of Oz's opinion it runs fine and has not yet exploded!

As far as I am concerned net flow is from the crankcase down the pipe due to blow by from the piston rings and any pressure rise would eject the oil residue far before it became high enough to do damage.

When the breather returns to the inlet manifold the net pressure in the manifold will be below atmospheric and that in the crankcase above it, as a result there will be no flow back into the crankcase when the engine is running. The diameter of the pipe is simply not large enough. As far as I am concerned the fresh fuel is not the problem and it is the burnt fuel residue that is corrosive. By returning the breather to the inlet manifold it promotes a positive ventilation through the engine encouraging a circulation of lubricant through it with the added advantage of the lubricant passing through the engine more than once. These techniques have been used in full size (automotive) engine practice for many years.

Shaunie.

Jon - Laser Engines06/11/2014 08:51:44
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Graham you are spot on. The methanol drawn in when the piston rises will be contributory to the rust you are seeing. It might not be the final reason, but it will be a factor. I have an OS91 surpass with the same link pipe and (over the 12 years I have had it) the bearings have rusted up a few times. My Laser, saito and enya 4 strokes have not and I treat them all the same

J D 806/11/2014 09:35:45
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Shaunie I have also run an RCV 60 with long vent and no problems,but an Enya 90 would have a lot of oil draining from it after a run.However this may be to it being a well used unit.

ken anderson.06/11/2014 09:55:16
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one time we were told(in a far off galaxy) that the breather bit of tubing had to be kept to a certain length or else when it was exhausting-if it was too long it would suck back in the rubbish it had attempted to discharge.....

ken Anderson...ne...1 tech/not dept.

Cuban806/11/2014 10:56:02
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Posted by john davies 8 on 05/11/2014 19:31:20:

Also it is important that vent pipes are not to long or oil may not drain from crankcase resulting in a build up of oil that can result in damage to engine.

Bit of an myth I find myself. With older engines that don't recycle the crankcase waste, I've always run long tubes to the outside and have never seen any evidence of the crankcase filling with oil. I remember Brian Wynch warning about this years ago, and at the time I tried to recreate the problem by fitting a long tube filled with oil and running the engine to see what happened. The oil gradually found it's way out!

Braddock, VC06/11/2014 11:52:30
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Bit of a long story but I'll get there in the end, I have an OS 52 surpass which I crashed. Some of the damage was the spigot for the exhaust manifold broke off partially. I actually bought a new head and fitted this.

In the interim I got hold of a ys 110 fz and whilst it has many nipples on the engine none (as in 0) are a vent from the crankcase. I had a peak in the cam cover and, lo and behold, there are several holes into the inlet manifold in the head.

Aha I thought and after some mithering drilled a 1.5 mm hole in the old os 52 head, down into the inlet passage. The engine is, by now, inverted in an 84" span Southerner vintage job so after some clarting the old head went back on and a small socket head bolt plugged the original vent hole.

I must have run the engine for several hours that year and had no problem with it, it idled brilliantly, so I took the plunge and did the same with the new cylinder head which is now installed back in the plane and it had a season of running with no problems.

The inverted rocker cover is filled with oil which is understandable but by and large there are no oily streaks down the side of the plane from the cowling, still get some from the exhaust though.

The engine has the original bearings in, circa 1995 ditto the ys 110 fz, circa 2004.

It's what I've done and whilst I am happy with it it's an individual's choice what they do to their own engine. If you do undertake a similar project, on your own head be it!!!! wink

Braddock, VC06/11/2014 11:52:33
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Bit of a long story but I'll get there in the end, I have an OS 52 surpass which I crashed. Some of the damage was the spigot for the exhaust manifold broke off partially. I actually bought a new head and fitted this.

In the interim I got hold of a ys 110 fz and whilst it has many nipples on the engine none (as in 0) are a vent from the crankcase. I had a peek in the cam cover and, lo and behold, there are several holes into the inlet manifold in the head.

Aha I thought and after some mithering drilled a 1.5 mm hole in the old os 52 head, down into the inlet passage. The engine is, by now, inverted in an 84" span Southerner vintage job so after some clarting the old head went back on and a small socket head bolt plugged the original vent hole.

I must have run the engine for several hours that year and had no problem with it, it idled brilliantly, so I took the plunge and did the same with the new cylinder head which is now installed back in the plane and it had a season of running with no problems.

The inverted rocker cover is filled with oil which is understandable but by and large there are no oily streaks down the side of the plane from the cowling, still get some from the exhaust though.

The engine has the original bearings in, circa 1995 ditto the ys 110 fz, circa 2004.

It's what I've done and whilst I am happy with it it's an individual's choice what they do to their own engine. If you do undertake a similar project, on your own head be it!!!! wink

Edited By Braddock, VC on 06/11/2014 12:00:05

Braddock, VC06/11/2014 12:07:19
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Sorry about the double post, I edited to spell peek correctly and got more than I bargained for.

Braddock, VC06/11/2014 12:12:21
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If you look in my gallery, there's a piccy of the old head with a modelling pin sticking out of the hole I drilled.

PatMc06/11/2014 12:32:40
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Posted by ken anderson. on 06/11/2014 09:55:16:

one time we were told(in a far off galaxy) that the breather bit of tubing had to be kept to a certain length or else when it was exhausting-if it was too long it would suck back in the rubbish it had attempted to discharge.....

ken Anderson...ne...1 tech/not dept.

Using 3mm id bore breather tube the critical length would be 14cm per 1cc engine capacity or using 4mm id tube 8cm per cc. So for a 70 as in the OP it would need to be 160cm or 92cm.
Even if the tube was longer the oil content of the "breath" would cool, stick to the tube & only a fraction of it would be drawn back with the gas in each "up stroke". So for every 4 stroke cycle the contents of the crankcase would be diluted twice.

I agree with Cuban8, the warning on breather tube length, like a lot of stuff that emanates from WOO, is a myth.

GrahamWh06/11/2014 13:19:10
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Thank you for the various replies chaps. I can imagine a very long breather pipe causing some reduction as power as the piston tries to go up and down for a normally vented engine, but making a short tube and having it go into a larger diameter one with an unsealed union would overcome this.

For me the issue is the fuel getting into the crankcase as the piston goes up. I disagree with your view Shaunie that fuel would not be sucked in - as the piston goes up the pressure in the crankcase would I think be below atmospheric, and as the inlet valve would be closed the pressure in the carb and inlet manifold will be returning to atmospheric, so there wouId be fuel mixture pasing into the link tubing to the crankcase in the ASP's system. I may try converting the ASP temporarily by blocking the inlet manifold hole with a suitable small bolt and venting the crankcase breather. Has anyone actually done this conversion on a four stroke with the crankcase breather connected to the inlet manifold?

Edited By GrahamWh on 06/11/2014 13:29:32

ken anderson.06/11/2014 13:43:52
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read one time of someone fitting a 35mm film cassette inside of their model cowling to catch the residue oil from the breather tube/piping......

ken Anderson....ne....1 ....... 35mm film dept.

DelRay06/11/2014 14:59:34
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Ken,

Along those same thoughts, I installed a modified fuel tank I had from an old c.l. model.

I soldered two tabs to it for firewall mounting and two other inlet and outlet pipes with a piece of fuel tubing to connect them.

This serves as a visible level indicator and a means to drain the collected oil from the tank.

Probably "overkill", but a fun project and it works.

Jim

Martin Harris06/11/2014 22:15:16
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My understanding of the maximum length of the breather pipe advice is that it is based on the need to expel positive pressure in order to draw oil-laden air through the cam follower area.

I found this information which originated from Saito while puzzling over the purpose of an air pump built in to the rear casing of a 180 degree Saito twin. Due to the fact that this layout means there is a constant volume under the pistons there is no pumping action.

I would guess that the relatively small amount of gas introduced by blow-by would not allow a net outflow with too long a pipe, resulting in a lack of effective oil-laden air flow past the cams. It was certainly an important enough consideration for Saito to add a fair amount of extra engineering!

Edited By Martin Harris on 06/11/2014 22:17:24

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