Airhead Stud Repair Advisory

Discussion in 'Airheads' started by Bill Harris, Nov 28, 2011.

  1. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    There is a potential for getting drill cuttings into an oil passage when repairing the pulled or stripped cylinder stud on a BMW Airhead using Helicoil, Timesert or Bigsert thread repair inserts. (whew, that sentence takes care of all the keywords for the search engine)

    Thanks to WireWrkr for sending me a piece cut out of an R65 crankcase, this answered a long-standing question I had.

    When I did a Bigsert repair of a thread repair I did in the '80's on the left cylinder, upper left cylinder stud, to my surprise I saw what seemed to be an oil passage at the bottom of the drilled hole for the stud. I didn't recall the oil passage on the first repair, but that was "routine", drill to the same depth as the first hole, tap and repair type of operation. And this time I didn't drill any deeper (look at the attached image, you can still see the bottom of the original factory drilling) so I didn't think that I drilled into an oil passage. I figured that it might have been blind drill hole for the attaching bolts if the timing case cover. Nonetheless, I devised a vacuum device to suction oil from that oil passage into the drilled out stud and it came back with only a few small flakes of swarf. I packed the bottom of the drilling with a thick grease, tapped to hole and finished the repair.

    I asked online if there was an oil passage in that area, but didn't get an answer so I assumed that there was not a problem.

    But the crankcase cutoff answered my question immediately. There is a diagonal oil passage that comes up from the bore of the front main beariing cap and intersects a horizontal oil passage that feeds to two top cylinder studs that feed oil to the rocker arms. This oil passage is right at the bottom of the upper left cylinder stud thread, as designed and as it comes from the factory.

    Shown below is an annotated photo of a crankcase cutout from an R65 of the left cylinder in front of the oil dipstick. Refer to the oilsketch.jpg image for more info.

    Yellow-- oil supply passage from the front main bearing cap to the oil-supply studs for the rocker arm oil suply passages in Green.

    The area of concern is shown with the Red-Yellow splat near the top left cylinder stud.

    Red-- oil supply passage from the front main bearing cap to the rear main bearing and the oil pressure switch.

    Oil is supplied to the front main bearing cap from the oil filter and oil pump.

    Like I said, this repair is done frequently and you don't hear of a this oil passage or of this problem, but nonetheless it horrifies me as that you are drilling near an oil passage pointing downward to the front main bearing and generating a lot of drill and tap cuttings. I see the potential for scoring the mail bearing insert and/or crank journal.

    I would recomend that when this upper left, left cylinder stud is repaired that the bottom of the hole be packed with a thick grease to preclude any possibility of drill cuttings entering that diagonal drillway and making sure that the grease is removed from the passage before completing the repair. If you remove the oil pressure switch you can blow out this branch of the oil passage with compressed air or "backflush" it by pressurizing it with oil.
    [​IMG]

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    #1
  2. DoktorT

    DoktorT BigBrowedNeandereer

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    I would also follow up with removing and flushing the grease after surgery. The grease itself can block the passage. I have seen engines assembled with so much assembly lube all the oil holes are packed. Knocking rods before you can get it tuned and test driven.
    #2
  3. wirewrkr

    wirewrkr the thread-killer

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    How about pressurizing the oil system with oil, (essentially back-flushing) through the oil pressure switch? If the rods are intact, then the open oil gallery at the stud will be the least path of resistance? I haven't looked at the oil schematic lately, so I'm not sure. What say you Bill?
    Anyway, it's just a Monday morning idea.
    #3
  4. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    Pressurizing from the oil sender is a fine idea. I did the vacuum because I had a vacuum pump and it seemed expedient. Sucking the bad stuff out seemed better than pressurizing the unknown.

    Attached is a diagram from the factory manual of the R-bike oiling system. The oil pressure switch is #10, and the oil passage in question is the lefthand #14.

    You might even be able to blow it out with compressed air at the pressure switch threads, though I do like the idea of pressurized oil.

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    #4
  5. bmwrench

    bmwrench Long timer

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    I'm a bit puzzled by the photo you posted. I have never noticed a blind stud hole, but have a bare /5 case at the shop, which I'll be looking at tomorrow.

    As you can see by the schematic of the oil circuit (/7) you posted, the oilway for the studs/rockers is drilled perpendicular to the timing chain cover. /5 motors had an oil gallery for the cylinders-that's what that groove around the cylnder spigot is for-, but I don't remember it connecting to the stud oiler. You can also see at 12:00 in your photo the oil hole. It would be quite difficult to get any chips into it.

    Whatever the outcome of this, it should convince anyone who believes otherwise that BMW changed things constantly.
    #5
  6. _cy_

    _cy_ Long timer

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    hmmmm... guess that means don't strip out your airhead studs!!
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  7. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    Do look. When I first encountered this oil passage in a 2009 repair on my bike, I was puzzled and finally concluded that it was juat an anomaly with mine. I do a little bit work on BMW bikes, but haven't had the occasion to look that deeply into another engine. When I got that "cutout" from Wirewrkr, the first thing I did was look at that "upper left" stud thread and start poking a wire around to see where the oil passages went.

    The hole from the oil gallery for the cylinder oiler comes out on the top of the crankcase cylinder bore, is about 1/16" (1.5mm) dia and feeds oil to the annular groove around the spigot. Oil holes in the cylinder wall at 12:00 and 6:00.
    #7
  8. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    It's inevitable. That particular stud, the top left of the left cylinder, is the most prone to stripping. When you look at the hole, that oil passage is real obvious (tho it took me 3 years to get a confirmation) and "good practice" dictates that you don't get swarf in an oil passage. I have taken extreme care in blowing out potential chips in the angled passages at the top of the
    repaired threads and taken pains to make sure that the oil flow was not cut off to the rockers.
    #8
  9. bmwrench

    bmwrench Long timer

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    I inspected three cases today-a /5, a /6, and a /7. They all have the stange drilling I noted in Bill's photo. It took a while to realise why it's there; because the left cylinder is the forward one, the angle of the oil drilling cuts through the stud hole. The way it appears through the stud hole makes it looked curved, but this is an optical illusion.

    Fortunately, I have never had to repair this hole, but if I ever do, I'll be taking Wire's advice and blowing it clear via the oil pressure switch hlole. If air is used, pre-oiling via the same hole will protect all the bearings that were blown dry. Ideally, one would pressurise via the oil filter feed hole, but that is not always practical

    A note about torque vs, stripping stud holes: even if the studs/nuts are never over-tightened, the pressure put on them by combustion and cylinder expansion far exceeds the torque used to tighten them. John Healy, a noted Classic Triumph guru says (roughly) that cylinder and head torquing is only to seal the cold engine and that expansion will do the rest.
    #9
  10. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    Good post, 'Wrench. The more I think about this the more I think we've dodged the bullet on a potential disaster. I'll post more later.

    I added an annotated image of the R65 cutout piece to the initial post.
    #10
  11. Melrone

    Melrone Long timer

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    I can say from personal experince the tinesert works better than anything i've ever used.A few years back the top left stud on the left side stripped.After we opened it up I found 2 heli coils already in there.Not good.A friend ofmine and probally one the best Airhead mechanic I know bought a tinesert kit for Airheads.It wasn't cheap,but was one of the best kits i've ever seen.The kit has a jig.spacers.and both drill bits needed.The jig slid on the other 3 studs slide spacers over the other studs used the nuts provided locks the jig in.We drilled & tapped out the old slot pulled the jig out.Screwed in the Tinesert with High strengh Locktite & went to lunch.When we came back we reinstalled the jig.The second drill bit is for the oil passage the second hole on the jig is for that purpsose only.This kit was fantastic.The 92 RT/EML is still on the road running great.I will never look at using Heli coils agin.

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  12. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    I edited the opening post to clean it up a little and incorporate later comments into it. Please mention it and feel free to link to it from other sites.

    The distressing thing about this is that when repairing this "upper left, left" cylinder stud thread, typical "good practice" would be to blow out the blind hole with air and/or rinse it with an aerosol brake cleaner. Both actions would potentially introduce drill cuttings back into the annular oiling groove in the front main bearing insert. Hopefully when the engine is first started and the oil system is pressurized the oil flow would tend to flush any cuttings into the rocker arms and out of the pressure loop, but there is always the potential for swarf to get intro trhe front main bearing or the front cam bearing or as far back as the rear main bearing.

    We've seen scored main bearings from time to time and generally assume that it's the "$2000 O-Ring", but some of it may be due to this problem area. Pro-Wrenches, keep an eye open for repaired "upper left, left" studs and scoring in the front main bearing. I'll betcha we'll see a correlation.

    On my part, I have a timing chain and crank timing sprocket replacement in the late Spring and I'm contemplating pulling the front main bearing carrier and taking a look. A PITA to be sure, but I gots ta know...
    #12
  13. jimbee

    jimbee Airhead Intermediate

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    should we tell snowbum? :y0!
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  14. supershaft

    supershaft because I can

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    Great topic. I have seen a number of engines F'ed up for stud repair chips going through the engine. Glass beads too but that is another story. It can usually be traced back to a pile still on top of the offending stud's rocker arm bearing. I don't understand what is unique about the left front versus the right front Bill? The rear studs on both sides can harbor a bunch of chips too. I vacuum them after a repair.

    I recommend coil inserts (Helicoil is but one brand). That is what BMW uses. They are superior in design and function if installed correctly. I have never seen a factory installed coil pull. They are most often in the front and back of cases and cam chain covers if you look.

    Pulling studs is inevitable? I strongly disagree. I have never pulled one on my own bikes. Out of the hundreds of airheads I have torqued down the cylinder studs on, I have only pulled a couple on customer bikes but that was most likely due to them being over torqued before hand since you could see the stud being further out than normal.
    #14
  15. wirewrkr

    wirewrkr the thread-killer

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    I'll second that, Billy, not yet for me either. Same goes for torquing to the correct values etc.
    When I was a VW and Porsche mechanic for nearly two decades that's a whole different ball of wax.
    Timesert had a kit with a heavy steel guideplate, predrilled and pretapped. It was so bad we just started doing it on every stud on every early VW case when we over hauled them.
    I thought back on it recently, I believe I have done over 200 VW engines in my short career. In the California Central Valley in the 80s, we could hear the popped head studs coming down the street. The first words out of the customers mouth was usually,"I think I need a new muffler"..............
    #15
  16. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    Jim, 'Shaft, 'Wire--

    This is going to be a lively topic. It's beyond "use a drilling guide and clean out the rocker oil supply holes".

    I've posted a link to this thread on The Airlist and have already gotten responses. I'll bet that 'Bum will get involved-- I won't know right away since I'm on Digest Mode there. I've notified other "Guru's Of Note on this topic, too.

    Dis be a can o'worms...
    #16
  17. supershaft

    supershaft because I can

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    Let us know what they say. Personally, I don't think there is a can of worms here if you vacuum clean the oiling system. It works perfectly as far as I can tell. The chips are super light weight and are easily picked up with a good shop vacuum. Flushing and blowing isn't the answer here.
    #17
  18. wirewrkr

    wirewrkr the thread-killer

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    Why exactly does everyone wait to hear what snowbum says?
    I'm not ever sure, every time this kind of stuff comes up.
    Luckily for me, I was never much into hero worship, religion, government etc.
    Bill, I think that you're heading down the right path with the information and tools that you currently have and you seem extremely savvy in the way of mechanical systems. Go forth and prosper I say!
    Damn the torpedoes, full speed ahead.
    #18
  19. robtg

    robtg Been here awhile

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    +1 There is so much crap from the so called "gurus" just think for yourself and use common sense and make it happen.
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  20. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    Posting at certain places one really has no idea what to expect-- I just tossed it out there for "peer review". :eek1

    Posting here, I incorporated enough keywords so it'll show up in any Search relating to Airhead cylinder stud repair. I don't think that the oil passage by itself is a problem, but ignoring or not being aware of it is THE major problem.
    #20