Nobody likes one of those “head-scratcher” comebacks, though
any honest mechanic will probably have a few tales that they are willing to
share. Of course, when the comeback
winds up being no fault of your own, then the telling comes a little easier,
accompanied as it is with a sigh of relief.
But to shift into reverse for a moment, in case any readers
are uninitiated, a “comeback”, in the mechanical repair game, refers to having
a customer returning with the same (or closely related) problem which was
recently “fixed.” A head scratcher, on the
other hand is something that is confusing, mysterious or hard to understand.
One of the first useful things most “techs” (as they are
called these days) learns is that when attempting to find a problem with a
bike, the first place to look is the last part of the bike that was worked
on. That rule of thumb is valid whether
said previous work was done by the owner, another shop, or the ace mechanic you
consider yourself to be.
The tale of this particular head-scratcher is presented here,
not only for its educational value in general, but also in the hope that it
might save someone else from chasing an elusive problem.
Earlier this year I had occasion to do a stock valve job on
a set of late model Twin Cam heads. The heads were carried in, by a customer
who we will call John. We’ll call him John for two reasons. One is that “John”, being a very common name
will grant the customer a certain degree of anonymity, and secondly because
that is his name. While not a
professional, John is also not a stranger to R&R (Removing and Replacing)
top ends. The reason the heads were brought in was basically as preventative
maintenance due to about 70,000 miles on the clock. The top end was off to install some big-bore
cylinders and pistons, so John decided that it would be worthwhile to be sure
the valve job was up to snuff at the same time.
The job appeared to be about as straight forward as they come; the
exhaust guides were bell mouthed past service wear limits so I replaced them,
sized them and cut all four seats. John
also brought in a new set of stock valve springs and guide seals purchased from
the local Harley dealer where he had acquired the cylinder and piston kit. He
requested that I install them as part of the valve job. I wrapped the job up in short order and life
was good.
Then, a few weeks later, John returned with the rear head. As an aside, let me tell you how much I
appreciate this type of customer. Most
of us who have been working on bikes for a few decades have had to deal with
the customer who returns with the attitude that you have screwed up his bike
and he’s not going to put up with it (after all, his bike was perfect before
you fixed it)! John was the opposite of
that. He explained what had taken place,
described the symptoms and calmly asked what I thought might be causing
it.
The symptoms were that once the engine reached full
operating temperature, it would begin smoking profusely from the rear
head. John reported that his first
thought was that he may have inadvertently installed a piston ring upside down,
which only goes to show that the guy is no hack, since that also would have
been my first suggestion to check. However, the rings were in correctly, and
there was no sign of oil leaking in the head gasket area. Yes indeed, it did seem as though a close
re-examination of the head was in order.
Which I did promptly.
But here is where it gets to be puzzling. The valve seals were still in place. The valve to guide clearances were good:
right where I had measured them when freshening the valve job. I put lacquer thinner in the spring pockets
to see if I could detect any porosity: none detected. I even went so far as to give each guide a
solid rap to ascertain that they were not loose in the head: they were
not.
Keeping in mind that the engine in question did not have any
symptoms prior to pulling the top end for the big bore kit, there was only one
other possibility concerning the head itself that I could think of. If the rear
head had porosity between the spring pocket and the port one would have
expected that to show up pretty early in the life of the bike (and remember
this one had about 70K smoke free miles in its history). However, for quite some time now, Harley
heads have been powder coated before they are machined in the manufacturing process.
Because of this practice, the inside of
the ports (and to a lesser extent the spring pockets) leave the factory with a
nice thick durable coating. Of course,
when cleaning up the heads for a valve job, media blasting removes the carbon
and along with it the powder coat. What
do you suppose the chances are that by removing the carbon from the port, I had
also removed the coating which had been sealing porosity, and in this case
porosity which only opened up at operating temperature?
If the scenario of a porosity which only opened up at
temperature sounds bizarre, I might have agreed had I not seen it in the past. Way
back in the early 1980s, the dealership I was employed by had a new Superglide,
still in its 90-day warranty, return with similar symptoms. Under the tutelage
of the dealership owner/ head mechanic (and when is the last time you heard of
anyone holding that dual title?) I put the head on a burner of the shop’s
electric stove with oil in the spring pocket.
Sure enough, once the head got enough heat into it, the oil started
dripping through into the port at a brisk pace.
Rare? You bet; but if I ran
across one 35 years ago, why not another one now?
Since the architecture of the Twin Cam head did not lend
itself to holding oil in the spring pocket while heating on a stove, instead I
chose to coat the interior of the spring pockets and oil drain passages with a
product called “Gasoila Hard Set” to seal any potential porosity from the top
side. This had the added advantage of sealing
off any possibility of oil getting between the head and the outside of the
valve guide. Still, it was a bit of a
shot in the dark since I was unable to actually pinpoint any problem.
That of course still left the rest of the top end as the
potential culprit. Though John had
checked to be sure he had installed the piston rings in the proper orientation,
he took my advice and replaced the new rings based on my theory that it was
better to spend a bit more money than to chance spending a bunch more time later. Of course if the end result was success, then
we might never know for sure whether the heads or the rings were at fault, but
that was something we were both willing to live with.
So that is what the John did. He once again installed the top end, with new
rings & gaskets along with the (hopefully) repaired heads. And all was well with the world, …at least until
the engine once again reached operating temperature! Whoa! Or maybe even Woe! What in the world could possibly be going
on? At this point he made a trip to the
dealership and described the conundrum to the service techs, but none could offer
any ideas which had not already been addressed.
Now what?
Well, after a few more test rides to be sure it wasn’t just
some left behind oil in the exhaust pipe causing the smoke, John received his
first solid clue. Pulling the “smokin’-hot”
bike into his garage (well, …”smokin’-warm” to be more accurate), he started to unscrew the oil reservoir cap
only to be greeted by a sound just as if he had removed the valve stem from one
of the wheels. Suddenly everything
pointed to the breather system. And just
as suddenly John thought of the new breather assemblies which came with the
kit, the ones he almost didn’t change out but at the last minute decided he
might as well install since he had them. You know, the stamped steel ones that
come pre-assembled. Yeah, those.
Sure enough. As it
turned out, the umbrella valves in both of them were installed upside down,
effectively sealing the engine and turning the crankcase into an air compressor.
I must admit to being a bit impressed by how good a job Harley has done with
sealing these Twin Cam engines, because evidently there was not a hint of oil
weeping from any joint, seal or gasket.
The whole situation reminded me a bit of a stroker Shovelhead
motor that I built for a customer back in the late ‘80s or early ‘90s. Harley had recently started to install a
little squiggly plastic “oil separator” into the breather pipe passage between
the cam cover and breather trap. When I
built the Shovel motor I decided to try one since some oil carryover was not
entirely unusual on strokers. Perhaps Harley’s latest technology would help, I
reasoned.
The results could have been a disaster. The motor “wept” oil from the base gaskets,
the tappet block gaskets, the pushrod tubes, and nearly every other place you
can imagine. The customer was not
impressed. When he brought it back, I
immediately decided that the new-fangled oil separator was too restrictive, and
removed it. The only question I had was
whether the oil leaks, once started, would go away when the added restriction
was removed. I thought it worth a shot, though
the customer was convinced that it would not, and perhaps suspected that I was trying
to dodge my responsibility to stand behind my work. But he doubtfully took the bike, and sure
enough, a couple weeks later came back to say that the engine had completely dried
up – no oil leaks or weeping anywhere. I
felt like I had dodged a bullet and never put another one of those separators
in a motor and removed them from any motor I found one in. Evidently Harley came to a similar
conclusion, because they did not make it into many engines.
So, I guess the moral of the story is beware of your Harley's
breather system or it might turn into a real head scratcher.