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Thread: Oversize pistons and increased pressure r69s?

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    Oversize pistons and increased pressure r69s?

    I have had a problem with oil misting out of the breather tube. My compression is fine in both cylinders. I have been at a loss, but I just came up with this idea: the pistons are on the last overbore, and therefore are bigger than what the bikes internals were designed for. Could this be creating greater internal pressures, and therefore more oil out of the breather?

  2. #2
    FWIW oil mist comes from the breather before you go to max oversize pistons, too. Third oversize on my bike gets about the same amount of mist as 2nd oversize gave me. At second oversize my bike compared similar to another bike only at first oversize.

    My rule of thumb is that mist is fine --- it keeps the oil pan, center stand, and bottom edge of the rear fender from rusting I'll not worry about it until the oil production gets so bad as to actually drip.

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    Quote Originally Posted by 166926 View Post
    My compression is fine in both cylinders.
    Curious...what are the compression numbers? You could also try a leak down test to see how much air is getting by the rings.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    Quote Originally Posted by 20774 View Post
    Curious...what are the compression numbers? You could also try a leak down test to see how much air is getting by the rings.
    It was over 150 on each side, IIRC 160... I don't exactly remember the numbers, and I recall the engine was cool, so it should be higher when warm.

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    That seems a bit high for a cold reading...since it is likely to go higher when warmed up, that's definitely too much. For a 9.5:1 compression engine, the general range should be about 150-160 when warmed up. That could be source of your misting.

    What's the maintenance background on this engine? If the combustion chamber has a lot of deposits, that would artificially increase the compression number.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    Quote Originally Posted by 20774 View Post
    That seems a bit high for a cold reading...since it is likely to go higher when warmed up, that's definitely too much. For a 9.5:1 compression engine, the general range should be about 150-160 when warmed up. That could be source of your misting.

    What's the maintenance background on this engine? If the combustion chamber has a lot of deposits, that would artificially increase the compression number.
    As far as the heads go, the insides are clean. I had the meltdown of the other pistons, so when I took apart the engine, I cleaned out the heads. Fortunately, the melted piston bits came off in one piece.

    Again, as far as the numbers, I only remember them being in the 150-160 range; come to think of it, I am not sure that this was a cold test or not. How much higher would it get if it was on a warmed up engine?

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    Quote Originally Posted by 166926 View Post
    How much higher would it get if it was on a warmed up engine?
    Honestly, I don't know that I've heard anyone offer a rise, or a percentage. I'm guessing it would be 10-15%. Even 10% seems like it would begin to create some blow-by issues. Not to mention, excess pressure on the rings which then affect the ring lands on the pistons. Maybe take the time to get a hot reading so you really know what you're dealing with.

    Or as Marco says, enjoy the free undercoating!
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    I'd enjoy the undercoating if the bike was black!

  9. #9
    You may be seeing more than I do.



    I wiped it down before taking that picture, but you can see how the center stand, for example, can get black over time.

  10. #10

    White really shows it all!

    I have a couple questions

    How many miles since the new top end?
    Did you clean out all the crank ventilation orifices when you fixed the melted piston?
    What caused the piston to melt down and has that issue been corrected for sure?
    What octane gas are you running in it?
    Ethanol or non ethanol and are you putting in any additives?

    You might need to lower the compression just a little, I think they have base gaskets available to do just that. Vech over there at Bench Mark Works might be able to help you out with what you should have for compression and any ways to correct it if its a little to much.

    Both my R 60US and '65 R 69S misted and dripped a little until they logged about 1,500 miles and an oil change so I might think its not out of the ordinary. Dover White R 60 and a Bristol Grey R 69S, I kept a rag and cleaner handy during the break in.

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    Quote Originally Posted by 69zeff65 View Post
    I have a couple questions

    How many miles since the new top end?
    Did you clean out all the crank ventilation orifices when you fixed the melted piston?
    What caused the piston to melt down and has that issue been corrected for sure?
    What octane gas are you running in it?
    Ethanol or non ethanol and are you putting in any additives?

    You might need to lower the compression just a little, I think they have base gaskets available to do just that. Vech over there at Bench Mark Works might be able to help you out with what you should have for compression and any ways to correct it if its a little to much.

    Both my R 60US and '65 R 69S misted and dripped a little until they logged about 1,500 miles and an oil change so I might think its not out of the ordinary. Dover White R 60 and a Bristol Grey R 69S, I kept a rag and cleaner handy during the break in.
    1. Maybe 250-300 miles. I'm terrible at remembering these things! I did another oil change, re-torqued everything, and re-set the valves just a few weeks ago.
    2. Uh Oh...
    3. Not sure about that either. Long story, but I'm pretty sure that I bought a bike already knackered. Check my earlier posts about the saga of this bike. Short form is I bought the bike without ever starting it or seeing it run. Beautifully restored. Had a sheet with all the names of the persons who suppsoedly did the work on the bike. It turned over, I never checked compression before I bought it. It was 3 miles from the house, and was the bike that got me into motorcycles in the first instance. I sold an R50/2 I restored to get this bike, along with 2 other bikes. Bought it at the beginning of winter, didn't try to start til spring, and then, it never started for love or money. Went on a ride, and ended up with a loud "TING TING TING TING TING", oil barfing out of the breather tube, and bike dying.

    Now, I think the timing and fuel mixture is on, and seems to properly start...
    4. 93 octane
    5. Non ethanol, no additives (bike was sold to me supposedly with "hardened valve seats")

    I've got to get more miles on the bike, to be sure, to make sure that everything is all set and seated in. I'm still gun shy about the bike, don't want to get stranded too far away. But I'll keep fingers crossed on this bike.

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