Larry Flesner wrote:
>Sounds like an engine tear down to me. I'd hate to even run that
thing again without a flak vest.<
Thanks to all for the feedback on the ominous internal noises and ideas
as to what it could be, and the agreement that an immediate tear down
was prudent. Really I was giving myself a sanity check when I posted
the noise video, given that I actually tore the engine down Friday,
pressure washed the case halves and cleaned parts Saturday and Sunday.
I thoroughly inspected everything that I thought could possibly be
responsible for that noise....and found ABSOLUTELY NOTHING WRONG! At
least nothing that I could find, FWIW. All gears were pristine, no
cracks, no slop anywhere, and nothing broken anywhere. The bearings
look practically new, as do the pistons and cylinders, as they should
since I replaced the pistons, cylinders, valves, guides, and exhaust
system only 154 hours ago, June 5th, 2021!
The only thing that I can imagine would be causing that noise would be
the slight side to side movement of the connecting rods on the rod
journals of the crank, and I see no reason why they would have an
alternating side load on them to cause that....well, maybe because it
was rotating the prop back and forth! And they DO have exactly that
same frequency noise, so I measured the side-to-side clearance. The
tightest is .008", while the loosest is .012". The manual says .020" is
the max, so I'm barely at half the allowable tolerance. And these are
ARP rods made for Type 1 VWs, which are pretty high quality. And maybe
it could be "piston slap", but all pistons are properly installed, not
backwards.
I even checked for loose magnets in the generator rotor but found none.
I've ordered a bunch of gaskets and seals, new pushrod tubes, bearings,
and all that standard stuff and have disassembled and pressure washed
the heads, and today will walnut hull blast them and pressure wash
again, install new exhaust valves (I had four new spares on hand).
Before I reinstall the engine, I'll put a prop on it and give it the
same test as before to see if it still makes the noise. At this point,
I'm thinking I'll reinstall it and fly it whether or not it makes that
noise. Given that the last flight was uneventful with that going on,
and after I noticed the noise and the oil change and the valves were
adjusted, it ran up just fine in front of the hangar, and still made the
noise afterward, I'm just thinking it's a noise it's going to make. All
of my last five flights were something like 3-4 hour flights each,
including Oshkosh and the KR Gathering. If nothing else, your replies
to my post tell me that I wasn't an idiot to tear it down for a "look
see", so thanks a lot for that! I should have it back together after my
new Revmaster stainless steel exhaust system arrives in two weeks or
so. So if nothing else, the noise may have prevented an inflight fire
from the compromised exhaust system. How's that for a bright side?
And then late yesterday (after I wrote the stuff above), an offline
plausible explanation from a good engineer buddy, motor head, and KRnet
lurker, who hypothesized some possible culprits, such as a slightly worn
aluminum cam gear, and maybe cam thrust bearing surface as well. The
gear LOOKS fine, nothing irregular, but replacing it is pretty easy
right now, so I will. I just happen to have a new one on hand already.
Today I reinstalled the cam with the old cam thrust bearing in place,
and indeed the end clearance was .011" rather than the .004-.005" range
given in the manual. And it makes a noise that sounds identical to
what's in the video. It's worth considering that during engine
operation, all of these engine parts are being driven in one direction
and are under a load, and therefore there's probably no "slack" to cause
much in the way of noise or further wear. That would also explain why
it runs just fine, but makes weird noises when rotated by hand, which is
likely harmless.
I already have a new bearing set, as well as four new stainless
exhaust valves from Revmaster, and will swap those out, as well as
cleaning up all the intake valves and refreshing the valve seats. Last
time I checked compression all four cylinders were either 112 or 113
PSI, which is an unheard of narrow spread. The compression ratio is set
so low because I deliberately went "low compression" with this engine
for reliability reasons. There's something about "dead-stick" landings
that doesn't appeal to me! And this slippery little KR2 will still do
160 MPH wide open, while getting incredible fuel economy.
As I put it back together, I'll have crank and cam in one half of the
case, and can test for noise before I even close the case, so that'll be
some interesting data points as well, such as gear lash using old gear
and new gear cam. That may very well tell me exactly what the noise is,
and whether or not it's a real reliability issue, or just a nuisance
noise when rotating the prop. I'll let you know how it goes, but it'll
probably be a week or two before I get to that point.
Thanks for the validation that tearing it down was a good idea! And
honestly, my money was on a broken cam or crank too, given the tone of
the noise. See below message from the "lurker" with excellent
suggestions. Again, thanks a lot for all the input! See the most
insightful message with several options below:
Watched and listened to your VW noise video.
_/I'm not experienced with those engines/_, but by
estimating the degrees of rotation and comparing
the number of clunks heard, it would seem to relate
to the number of valves/lobes on the cam.
(8 valves, 4 cyls, 360 degrees. It seemed to be
4 clunks per 45 degrees in the video = 32 in one
rotation.) Hard to be sure about the relation due
to no degree wheel, just listening and watching.
The valve springs put some load on the cam lobes
during rotation, both while lifting and releasing,
which is transferred to the cam gear if rotated by
hand - - - and the cam gear meshes with the crank
gear, right? Those two gears are next to the removed
distributor and resulting hole for sound to exit, right?
(or are the gears at the opposite end of the crank and
I'm all wet about the location coinciding?)
If (BIG IF) I am right, then gear lash is what you hear.
Maybe too much from worn gears though, or no oil
at all on them now from sitting idle too long. Are VW
cam drive gears known to wear?? Or are the gears
known to become loose on the shafts?
Whatever, more investigation before flying is a good
idea. How, I don't have any input, unless you might
squirt some oil onto those gears via the distributor
hole in the case?? Again, I'm not that up on VWs.
OR could be camshaft end play clunking
from the same source pressure when turning
the crank. Doubtful the crank is the noise
source but when the gears have slop, just
could be involved too.
Best guess: cam movement fore/aft due
to worn thrust bearing face. Next best:
drive gears have too much lash - wear
in the softer gear teeth??
Thanks....
Mark Langford
m...@n56ml.com
http://www.n56ml.com
Huntsville, AL
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