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Thread: Valve Guides

  1. #1
    Atomic City Boxer 154048's Avatar
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    Valve Guides

    Greetings,

    Would worn valve guides lower compression?

    (I am lower on my 1980 100 right side at 60K miles.)

    I am smoking a bit on deceleration and wonder if the two might be related...

    Thanks
    Steve in Santa Fe
    1980 R100RT
    2005 DR 650

  2. #2
    Administrator 20774's Avatar
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    Not an expert, but I would think the biggest affect of worn guides would be oil getting past on deceleration, resulting in smoke. The presence of oil in the combustion chamber, over time, will begin to increase compression because the deposits take up room.

    Besides, when the valve is open to expose the guides to the intake/exhaust flow into the chamber, you have effectively 0 compression. At the point of maximum compression, the valves are closed and the guides are not in play.

    In order to determine where the compression loss is coming from, you need to do a leakdown test.
    Kurt -- Forum Administrator ---> 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!

  3. #3
    James.A
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    Compression loss comes from 2 places. 1) mis-adjusted valves, or,...2) worn out rings.
    Kurt is correct. Most of the time when I find something like this, it is worn out rings, and when the compression value is deviates more than 10% from one side to the other, you'll find a cylinder ovalled out.

  4. #4
    Registered User donbmw's Avatar
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    Yes worn guides can have an effect on compression. This coming from a aircraft mechanic.

    Don
    1975 R90/6, 1980 and 1982 R65, !959 TR3A Triumph Car

  5. #5
    MOA #24991 Pauls1150's Avatar
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    All of the above are definite possibilities, but also:
    I have to agree with Don; but if the guides (and/or seals) are worn, you probably have valve problems too.
    1980 and 81 were when the factory transitioned to the "lead-free gas" valve seats - these distorted from heat and impact (especially on the exhaust side; no lead coating to soften & lube the surface) and caused retraction & mushrooming of the valve as the seat wore. If you've noticed that your clearances tighten up quickly, that's what's happening.
    Happened to my '81 R100S... warranty took care of the first head (the right side, btw; thank you, Marty's & Jehd), but when it started to happen to the other side, I sold the damm thing. Too bad, that was a fun bike...

  6. #6
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    Torquing the cylinders and heads to 25 ft/lbs would be a good idea before doing a compression test. Generally, loss of compression is from poor sealing of the compression ring(s) provided the valve clearances and the head torque is to specs. That can be diagnosed (to an extent) by observing if compression increases after squirting oil into the low compression cylinder. A burnt valve can also cause compression loss along with having valve guides worn to the point a valve seals poorly. If squirting oil into the low compression cylinder does not raise the compression, look to burnt or poor sealing valves or a leaking head gasket/warped head. Good luck!
    Stan

    AH# 13238

  7. #7
    Administrator 20774's Avatar
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    Quote Originally Posted by Pauls1150 View Post
    1980 and 81 were when the factory transitioned to the "lead-free gas" valve seats - these distorted from heat and impact (especially on the exhaust side; no lead coating to soften & lube the surface) and caused retraction & mushrooming of the valve as the seat wore.
    I've read a lot of what Oak has written on this. It was the 1981 models, some built of course in 1980. The seats and likely the valves were considered "unleaded". The problem was that the seats didn't conduct heat into the heads well at all...wrong choice of metal. All of the heat had to go somewhere, so it was transferred to the valve face. The valve face began to tulip and bend, sinking somewhat into the head. The result was the same...a tightening up of the valve clearances. Beginning with the 1985 models, BMW got the metallurgy right.
    Kurt -- Forum Administrator ---> 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!

  8. #8
    Benchwrenching PGlaves's Avatar
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    To specifically answer the OP question, yes and no.

    Worn valve guides can allow valves to wobble in their bore, and the wobble can cause accelerated valve head wear. This wear can cause a drop in compression.

    Conversely, worn valve heads can allow valves to seek to wobble in the seats and this puts side stresses on the valve stems which can accelerate valve guide wear.

    But worn guides around the stems of sound valve heads and seats won't cause a loss of compression.

    While I normally think of 60K miles as early for head work even on Airheads, valve seat recession on an '80 model due to unleaded fuel is a distinct possibility.

    You have done a compression test. Now do a leakdown test and listen to where it is going. In the exhaust is exhaust valves. At the intake ports is intake valves. Into the crankcase is rings. Go from there.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://www.bigbend.net/users/glaves

  9. #9
    Atomic City Boxer 154048's Avatar
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    Excellent advice all...Thank You.

    I'll report back.
    Steve in Santa Fe
    1980 R100RT
    2005 DR 650

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