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Thread: Poor Compression After Top End Rebuild

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  1. #1
    Luddite Looney wecm31's Avatar
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    Jun 2009
    Eganville, Ontario

    Poor Compression After Top End Rebuild

    This time last year I checked the compression on my bike and found it to be 125 psi L/ 130 psi R. Those numbers combined with a chipped exhaust valve inspired me to redo the top end. Heads and cylinders went off to a BMW dealer of note, well experienced with airheads and performing in house work. Cylinders were checked, heads had the seats milled and new valves installed. Exhaust valves had new guides installed as well. I resinstalled the heads with new rings, careful to orient the gaps per Oak's manual. Fired the bike up for the first time in the spring with no oil in the cylinders and held rpm at ~ 3000 for a minute. Then went for a 60 mile ride and dumped the oil. Ran a second course of dino 20W50 with some break in additive (ZDDP stuff mainly) and after 250 miles changed that to my preferred 20W50 semi syn (Brad Penn).

    Bike ran fine, seemed a bit peppier than last season, so I thought. Didn't campaign at all this year, other issues both bike and nonbike kept me close to home. With only 1200 miles on the season, changed the oil out this week and tucked the bike into storage (snow has arrived in the Great White North).

    Here's the kicker....did a compression test after my last ride (carbs off) and both sides are exactly the same at 120 psi. WTF???

    Did I not run dino oil long enough? Have I not given it enough time? How long should it take to seat the rings??

    Anything else I could be pondering?

    Bike is tucked away till spring, won't be investigating until April now....
    Last edited by wecm31; 11-24-2012 at 04:02 PM. Reason: spelling
    Gerald P
    The last thing I want to be is just like everyone else...
    1985 R80RT
    1969 Sport Fury Convertible

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