Starter sprag clutch problems in classic K bikes
K bike section, article #1
kstartersprag.htm
©
Copyright, 2011, R. Fleischer
REFERENCES:
A description of starting problems, starter sprag clutch,
various fixes, photos, etc., were in an article entitled "Starter
Clutches In K Series Bikes", published in BMW Owners News,
authored by Paul Glaves, in the December 2004 issue.
I
HIGHLY
recommend you obtain a copy of that article.
Paul's article, as well as mine, below, lists some possible easy
fixes, such as oil additives, etc.
MY article, below, will not cover some of the things in Paul's article.
What are the SYMPTOMS of a
starter sprag clutch acting up?:
Starter spins but
the engine does not turn over, or poorly. Screech noise
(maybe). Intermittent, perhaps. Can often start the
engine by push-starting, pushing the starter button AS one lets
out the clutch, to start the fuel pump. May vary
with temperature conditions. Sometimes one can
temporarily 'fix' the problem by having the bike in gear and
trying to move the engine backwards.
What follows is edited commentary from exchanges I had on an Internet mailing list. I have expanded my commentary after editing the originals.
Part 1; Noticeable mechanical damage;
you have done the large amount of labor and have the sprag clutch
on your workbench:
Sprags not spragging? Look at them carefully. You may see a "polished" flat where it runs on the inner cylinder
lightly pressed there by the springs. When the springs get gunked, they end up exerting less force and so less friction
for it to "lift itself" over the high point to start
wedging.
Part 2: You have the sprag clutch on the workbench and there
is NO noticeable mechanical damage nor gunking-up:
The starter clutch is a constantly running device that is
designed to transmit force in one direction, and it is not
supposed to have any appreciable grabbing engagement until the starter
rotates, which is supposed to cause the sprags to move, and that
movement is
to LOCK the clutch up, so the starter rotates the engine. When
the starter is NOT powered, the starter sprag clutch is
free-wheeling, with no appreciable friction in its innards. This clutch is a ONE-WAY device. One direction, it
locks up, other direction, it rotates freely, with no coupling
between the two main parts other than quite minor friction except
in the oil film contact involved. I have seen TWO of
these sprag clutches (of the later types too!...not the earliest
K bike ones that were heavily modified by the factory for later versions) that have failed, withOUT any apparent real wear on
any part. I have ONE of these in the shop. I was so curious
about it, that I paid the owner to send it to
me. I have compared it to a brand-new one, with no good conclusions, beyond the fact that dimensions have not changed
enough to mention. My suspicions are STILL that it failed due
to unseen-by-the-eyeball microscopic surface glaze, or other
type of surface change... on the inner friction 'drum' part
(and likely also the sprag parts that contact it). I have
not proven this....might be able to at some point. The owner told me he had tried both Seafoam and
CD40 additives. Even if I could prove what happened, it likely would not
help us all very much (??).
Anecdotally, no one that has used Mobil 1 a TRUE full synthetic,
has had a problem...that has been reported to me. It is entirely
possible that OIL is THE > major factor.....or not. I do NOT want
to imply that wear is not a concern. I HAVE seen worn starter
sprag clutches; and the springs, if gunked-up enough, could cause
reduced tension. My suspicion is that it was also the
OIL that enabled excessive wear, when that is seen,
but I cannot be 100% sure about it. The only way I can get a
handle on that, is maybe to actually be present at some
teardowns, and MEASURE side-play, and so on. I am a bit
hesitant to state that it could be a problem, because there is
no indication of sideplay wear on the needle bearings in the
starter sprag clutch here....that I can see...nor on the ones I have
looked at in that particular regard (two of them). Keep in mind that
a starter sprag clutch works by friction, and it is not
supposed to be friction of metal to metal, but oil film to oil
film. Most do not understand how oil films work. There is an
in-depth article on my website for anyone who wants details. My guess is that there is more than just one thing at work with
the starter clutch problems. I suspect basic failure modes; OIL film failure (the
result is mechanical damage as in part 1 above, or failure due to
a glaze or gunked springs). I am beginning to think that all the
other failures, which manifest themselves as WEAR failures, are due to a combination of oil, perhaps side
forces, and maybe there is some sort of production
tolerance...and maybe contaminant/protectant problem at work too. I am only too well-aware that MANY have NEVER had a problem
with the later starter sprag clutches, and have used fairly
conventional oils. My mind jumps to the same situation with
camshaft and followers, in the Airheads. Many got HUGE mileages
without problems; others, even running the same oils, had
problems. I have personally witnessed these engines apart. WHAT
was the answer? Seems to be heat treatment variations. For
the Classic K bikes, I think we have a combination of effects,
and anecdotal evidence TENDS to point towards using a REAL
synthetic oil (Castrol Syntec is NOT!), like the Mobil 1 I have
been recommending. It may well be that the higher amounts of
ZDDP in Mobil 1 is helping with the WEAR failure modes. In
addition, in what APPEARS to be the case, Mobil 1's formula does not
leave the surfaces such that the sprags do not properly grab.
Until someone does a serious laboratory analysis, right down to
the surface's molecular analysis, I doubt we will know much more.
Because of the huge amount of labor involved ,...not to mention
the $$$$$ parts cost...I personally use ONLY Mobil 1 in my K
bike, from WalMart, in 15W50, it comes in 5 quart jugs. I am simply
using the oil that the majority of information I have points
towards it being much less of any problem. I have nothing
against someone using the particular Mobil 1 20W50 oil that has even higher amounts of certain additives
(V-Twin version) but I think that the K
bikes do not require the thicker oil. Maybe
the 20W50 is a bit better if someone rides a lot in the hot
Southwest, where temperatures can exceed 100°F relatively often.
I don't think the Mobil 1 20W50 is warranted otherwise. I leave my Mobil 1 in my K11 for the entire
year; unless I have put more than 8,000 miles on the bike. It
really does not need to be changed often, particularly if one
does not do a lot of very short commutes; even more particularly in
cold weather. I also note that analysis of my oil at changes has
shown no wear to speak of, so I quit paying for analysis. My
K11 also works hard; so is hard on oil...it hauls a big
sidecar. I do change the filter at the same time, but I well
understand that I could, without the slightest harm, go for an
extra oil change between filters. The in-depth analysis by a
lab showed that the additives in Mobil 1 hold up quite well
over time and mileage. You get, sometimes, what you pay for. I also use nothing but BMW filters. I have no problem with
folks using aftermarket filters, but find them highly variable
internally, so I don't use them. NOTE!....K bikes seem to
use less oil, if the oil is a quality one, and if the oil level
is NOT maintained all the way to the top of the sight glass. I
like the middle.
Part 3; fixing an original late model starter sprag clutch (of course, you have to remove it first, not a simple job!):
It has
been reported to me by a trusted source that after removing a
starter sprag clutch that has failed or gotten intermittent, but
exhibits no signs of bad mechanical wear, that it can be repaired
by a very thorough cleaning in strong solvents. I have, myself,
done this to one, and I also used about a 2000 grit paper in
'polishing' the contacting surfaces; with no further problems
from the starter operation; so it was nice to hear that someone
had used JUST solvents to effect cures. I would recommend
the polishing by super-fine grit paper, as I did, plus VERY
thorough solvent cleaning (and scrubbing by turkish toweling with
the solvents). I used a 50-50 mixture of MEK and Acetone.
Rev:
04/29/2010: Update slightly; +clean up excessive verbiage,
etc.
© Copyright 2011, R. Fleischer