Guzzi Saga continues

 

Well, the Guzzi saga took a turn for the worse.

On Labor day weekend I was headed for a Guzzi Rally near Steubenville, OH.
30 miles from home on I-71 the bike gave a light-sounding but nasty rattle
from the LH cylinder; I pulled the clutch and the motor stopped.  Seized
sold.

Removing the barrel revealed a seizure on 3 points of the piston, with big
chunks torn from the nice new nikasil plating.

With much difficulty, due to the
smeared piston material being smeared over the wrist pin hole, I managed
eventually to remove the seized piston.  Ugly Piston and Barrel Pictures are
at http://flickr.com/photos/9905235@N08/sets/72157607210357563/show/

The broken metal you can see around the wrist pin
hole is secondary damage during removal, as is the ding in the skirt.

Observations:
- I was worried about where the big chunks of nikasil missing from the bore
had ended up.  The good news is that they appear to be embedded in the
piston, which means they're not scattered throughout the lubrication system.
Looking inside the cases there's no sign of debris dust, which means that a
good cleaning of the sump will hopefully suffice.
- The excess heat that melted the pistons to the bore is only evident on the
sides of the piston where the seizures happened; the crown of the piston
shows no sign of being overheated.. This really makes me wonder if the
plating gave way first.

Managed to source a pair of NOS pistons from Harpers; Curtis told me on the
phone he only had one "B" piston, but a pair of "A"s, which didn't worry me
as I'm going to get the bores re-done anyway.  However, when they arrived
today, they were "B"'s (and cheaper than Curtis quoted).

I THOUGHT I had found a smoking gun.  When I assembled the motor, I was
struggling to decide which way around the pistons went.  After some
discussion on the Guzzi Loopframe list, the conclusion was that it didn't
matter on a V700
(I found no arrows on the pistons).  What was interesting, however, is that
the new pistons ARE clearly marked with an arrow labeled "SCA", which I
assume
stands for "Scarico" or "exhaust".   When I checked the position of the
arrows relative to the "B" and "guzzi" stamps on the old and new pistons at
first I thought "Aha!" - based on those markings I had fitted the piston the
wrong way around.  However, when I looked at the casting "Mondial Piston"
inside the pistons, the "B" and Guzzi marks were on the other side of the
piston on the old piston.  So I set to with the wire brush and sure enough
there WAS an arrow, much smaller and shallower, with a faint  "S" next to
it.   And I had fitted that side towards the exhaust - so I guess that isn't
the root cause.

So - this leaves a few possible root causes
- Too tight a clearance. When I got the barrels back from the plater,
Langcourt, the
diameter was marked on them, and it was correct (per the manual) for a "B"
piston.  I'm going to try to get the damaged barrel measured in the "Good"
section to check the diameter and that there's no obvious ovality or taper
- Bad plating such that a piece let go, embedded in the piston, thereby
taking up all the clearance and causing the local overheating that caused
the seizure in 3 separate locations.  I don't know how to prove this one way
or the other, but 3 separate areas letting go together seems unlikely
- Lubrication failure.  Oil (Rotella T) was less than 1000 miles old, was
not contaminated with gas, and was not low.  There was evidence of oil even
in the seized bore.  And no sign of any bottom-end noise which would be
immediately evident if oil pressure was lost.  I can't do much other than
remove the oil pump, check it out and check I get good oil pressure before I
fire it up after it goes back together
- Overheating due to timing.   I'd just replaced the points, condenser and
advance springs.  Timing, static and with strobe, looked good.  93 octane
gas. However, the ambient temps were up in the 90's and the bike WAS pinking
fairly heavily shortly before the event, and I'd backed off the gas to get
away from it a few miles before.  But, OTOH, I would have expected to see
some sign of piston burning or melting  up around the crown or squish band,
and there really is nothing.
- Overheating due to mixture.  Despite new needles, set in correct position,
all new jets the bike seemed a little "flat" at mid throttle.   OTOH, the
plugs were a perfect tan color, not white at all (and comments above on
piston crown also apply)
- Distorted piston?  If it wasn't before, it is sure to be now (removal was
fairly brutal)

So, unless the bore measurements tell me something, I'm kinda stuck.  So
here's my go ahead plan:

- Have BOTH cylinders re-plated by another source (Millennium, who come
highly recommended by several Guzzisti)
- New pistons, rings
- Check oil pressure
- If pinking evident, retard timing to eliminate
- More careful break-in.   I really didn't do anything other than take it
easy for the first 500 miles or so when I put the old pistons in the new
bores last time

Can anyone suggest anything else?

Right now, the rebuild is on hold.  Ann has decreed we WILL get the new
kitchen we've been talking about for 10 years or more before any more money
is spent on the bike.   She has a point, and knows I won't wait 10 years to
get it back running this time!

Bob

 

The Original Redbeard #37 wrote:

Aw, that's ugly.  Aggravating in the extreme.  I can't think of anything
relevant that might have caused it that you didn't and wouldn't have
thought of some you did.  Is it just me, or does the damage in pic 2442
look almost like theres a void behind the plating?  Could well just be
me, but the way the plating gave 'way there almost looks like a blister.
  Dunno how that could be; it just looks funny to me.

Sorry the bike broke.  On the plus side, you're getting a new kitchen
out of it.
--
spunky hussein HatesTradingBikesForKitchens tuna

 

On 2008-09-10 05:45:52 -0700, spunky hussein tuna


For the pieces to stick to the piston like that I would think that the
piston would have to already be melted.
Was there enough side clearance on the rod to provide oil to the cylinder?
Did you axe the plater what happened? Send him a link to photos... Or
the new plater?
Was the cylinder finished after plating (honed with diamonds) to insure
it was straight and round?
--
Ryder Rick
<  >

 



Ryder Rick wrote:


 That's what I was thinkin' looks like the hole wasn't round like it's supposed
to
be.



 nobody #1

 


Chicken or egg?  Hard to tell if the plating de-laminated and caused
the piston scuff or the piston scuff caused the plating to de-
laminate.

Hoppy
BS7  AH85  SENS  SLOB16