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Crankcase blowby

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cbclemmens
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Crankcase blowby

Post by cbclemmens »

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My R75/5 (44K miles) is blowing a small amount of oil out at the base of the right jug. It is a miniscule amount - maybe a pint every 2,000 miles. But it leaves a stain on the engine and gets the air intake dirty.

Does anyone know any way to mitigate this without pulling the jugs and installing a new base gasket? I really don't want to tear it down as it is running very well.

I think I already know the answer.

Craig

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schrader7032
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Re: Crankcase blowby

Post by schrader7032 »

Have you checked your head torques lately? Could be that the squeeze at the base is not what it should be. Do you have any knowledge of a top end and how the surfaces between the engine and cylinders were prepped? There should be a thin layer of a non-hardening sealant.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

weh8127
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Re: Crankcase blowby

Post by weh8127 »

My R75/5 was up for rings and valves at 50k. The pistons were fine and the heads got upgraded valve seats. It also needed a new timing chain, not unusual for the /5 at that mileage. Maybe its time for you do a look-see at yours, too.
Bill Husted
Barre, MA USA
1963 R60/2 w/ 1955 Steib S500
1973 R75/5

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cbclemmens
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Re: Crankcase blowby

Post by cbclemmens »

Thanks for your ideas. I am the second owner, the first owner (a neighbor) had it in his basement for 20 years or so until I bought it in '08. It has never been torn down. But I certainly could retorque the head bolts, without tearing it down. Thanks for the suggestion, Kurt. Like I said it is running really nice and as long as it is running good I don't want to tear it down and risk screwing it up. But I can retorque the heads without a full tear down. I will try that.

Craig

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cbclemmens
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Re: Crankcase blowby

Post by cbclemmens »

oh, I forgot to mention that it has 160 lbs. compression, so I think the rings and valves are ok.

Craig

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cbclemmens
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Re: Crankcase blowby

Post by cbclemmens »

So I had to pull the left head to install a heli-coil in the spark plug hole. I re-torqued the right head as well. I started at 20 ft.-lbs, then 25 and finally 30. Nothing moved.

Craig

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cbclemmens
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Re: Crankcase blowby

Post by cbclemmens »

Well, all of a sudden oil started coming out everywhere, both base gaskets and the pan gasket. The air cleaner is jet black and oily. I checked the compression and it had dropped from 160 PSI last month to 100 PSI today.

I ordered rings and gaskets from Max. Looks like my R75/5 will be down for a few weeks. It might be longer as I'm having minor surgery next week.
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Craig

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srankin
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Re: Crankcase blowby

Post by srankin »

I don't have first hand experience with your bike but I believe there is a poppet valve on top of the engine that releases pressure in the block. LOL, I am writing only rumor but I thought I had heard that the older versions of this valve could stick and cause pressure problems in the engine.

Makes sense, if there is excessive internal pressure and it is not vented, the pressure has to go somewhere and that will be the path of least resistance.

Check out snowbum's website, I bet he has written about this topic. Good luck, St.
Owner of a 84, R80RT and 78, R100RS

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cbclemmens
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Re: Crankcase blowby

Post by cbclemmens »

I took my heads and set them upside down on a workmate which was opened about 2" so the heads could sit flat. I put the plugs back in and filled the heads with kerosene. They were still full after 24 hours. That tells me that the valves are seating ok, and I can get by with just putting in new rings.

Does anyone see a flaw in my thinking?

Craig

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srankin
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Re: Crankcase blowby

Post by srankin »

LOL, this holds water or Kerosene, but did you check the valves visually for wear, I am assuming you did and they looked fine.

With good compression, and properly torqued heads and other good stuff, I hate to repeat myself but I still question the pop off valve on the blocks shape.

I mean I am assuming you are not blowing oil out the exhaust but as written, it is seeping out the base of the jugs. I would be concerned with rings, valves and such only if I had low compression, blue smoke, black oily plugs or some hint the oil was ending up being burned. Pushing oil past the seals makes me think the block is not venting off the pressure.

Then again that is me and my opinion. During the winter there is nothing to say things can't be torn down, checked and rebuilt. LOL, it gives me something to do when I can't ride. St.
Owner of a 84, R80RT and 78, R100RS

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