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To Paraphrase the great bard; Why-for this hole in my Crankcase ?

Submitted by grant_macneill on Sun, 08/10/2017
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As I install my inner primary case, I notice a 3/16 diameter hole in the underside of the engine, centered directly under the drive side main shaft. The hole runs up to the shaft

It is 3/16 diameter or 4.76mm and feels smooth boreâ? though when I look at enlarged photographs it has a hint of a thread on it; possibly a stripped out 3/16 thread or perhaps leftover spiral tracks from the drilling process at the factory?

This is a pathway for oil to exit my crank case. My crank case breather is in good order so I donât need any more ventilation and my instinct is to plug it.

Question: what was the purpose of this hole ?

AND

Can I plug it ? Either with heat resistant silicone or tap a thread and cap it with a small bolt.

Thanks in advance

Grant MacNeill

Toronto

Attachments IMG_2014.JPG img_2013-jpg

Submitted by richard_payne on Sun, 08/10/2017

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My understanding is that it's a drain for any oil that has made its way past the bearings (there is a small groove on the inside).

If you block it then any excess will enter the primary chaincase and contaminate the clutch rather than drip onto the road where it belongs !

They don't normally loose much / anything there.

Submitted by grant_macneill on Sun, 08/10/2017

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Previously richard_payne wrote:

My understanding is that it's a drain for any oil that has made its way past the bearings (there is a small groove on the inside).

If you block it then any excess will enter the primary chaincase and contaminate the clutch rather than drip onto the road where it belongs !

They don't normally loose much / anything there.

>>>>>>>>>>>>

Thanks Richard,

I have the same brand and weight of oil in the chain case as the engine, so should I worry if any makes its way through to the Primary ? (they called it an oil-bath after all...grin)

I know it is a mug's game but in the interest of leaving less oil on the pavement I'm all for plugging any unnecessary outlets.

Can you think of any damage this might do?

Grant

Submitted by paul_knapp on Mon, 09/10/2017

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Hi Grant, that hole on the underside of the drive side bearing boss is a crank case timed breather. there is a drilled hole at right angles to the axis through the drive side axle which links up to another drilling the length of the axle venting inside the crank case. On the inside of the boss there is a short groove linking the hole which coincides with the axle hole as it rotates past, exhausting case pressure on the down stroke, then is closed off by rotation as the piston is on the upward stroke. If you look carefully, you will see that it vents behind the inner case, not into the primary case.

I definitely wouldn't be interfering with it as the amount of dribble I get from that area is negligible. The 'thread' you see, probably is the spiral marks from the drill.

Paul

Submitted by grant_macneill on Mon, 09/10/2017

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thanks Paul

I have a similar timed breather on my 50s BMW timed off the cam shaft. Makes total sense to me now. Plugging it would lead to leaks elsewhere (grin). I'll leave it as it is

thanks for solving the mystery for me

Grant

 


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