I finally got the bad piston out of my MGB. If you recall, I was
planning to drive my '74 Roadster from Dayton to Myrtle Beach and back
through the Smoky Mountains back in June. Two days before I was to
leave I discovered I was going through oil rather rapidly and further
inspection revealled that the #4 spark plug was covered with oil and black
crumbly gunk. I went on vacation anyway, though I took the ever
reliable Mazda instead. I got the head off a couple weeks ago which
revealed black crumbly gunk on the top of the piston, but no noticable
damage aside from a slight notch that almost looked as if it belonged
there.
Well, last night I finally got the oil pan off so I could get
the piston out. What a sight! The top ring no longer exists except
for two 1/4" segments. There are some big junks missing from the side
of the piston and some nicely melted areas. The second and third
rings are complete, as is the oil ring, which would explain how I
still had compression of about 130 psi. All in all, a lovely
mess. It looks like I need a new piston. The bore seems to be okay,
I couldn't see any gouges or scratches (although it was 1 am by this
point so I need to look more closely.) There is a slight ridge at the
top of the bore, but not enough that it impeded the piston on its way
out the cylinder.
I'm not sure when all this piston damage happened, although I have a
few theories. Remember last summer when I posted about all of my
backfiring problems? I suppose this could have begun the damage,
although I had the head off after that and everything looked okay.
My second theory is that when our local MG club took a 250 mile tour
to a state park, I was getting some pinging while under light
throttle, but it would go away under no thorttle or heavy throttle. I
retarded the timing a bit after that and it seemed to go away.
Anyway, the damage is done and now I need to decide whether to pull
the engine and get the cylinders bored out and buy new pistons the
next size up or just find a used piston and buy new rings. (I did
have compression of 160-180 before blowing this piston.) If I do
this I may as well throw in some new bearings too. Hopefully, I
didn't damage the head during all this, as I just had it rebuilt last
fall.
This leads me to my next problem, I don't know what year piston this
is. The car is a '74. When I bought it I was told it had a '72
engine in it. When you look at the casting date on the head, it
says 4/69. The rods are the early type which have the bolts that
come in from the bottom (I'm not sure what years these were made,
until '66 maybe?). How is the number of rings defined? Does it count
the oil ring when you say a "4 ring piston"? If so, I have 4 ring
pistons, if the oil ring doesn't count I have three ring pistons.
There is no engine plate #, so that is no help.
One possible source for a piston is my parts car which has an 18GK
engine made in '71. Would I need to swap all four pistons to get this
to work, or could I just swap one? I suppose the balance would be a
problem with different piston types in there.
The amazing thing is that depite this piston being burned to a crisp,
the car was running great right up until the moment I took the head
off.
I think I am leaning towards just getting a used piston and getting my
car back on the road again for now, since there still are over two
months of roadster weather left, so any advice along these lines would
be appreciated, i.e. how to identify my piston so I can get the right
one.
Thanks,
Bob Jones
bobj@meaddata.com
p.s. Mark Dodd, I'm planning to bring the piston to tonight's meeting
to see if I can get some help IDing it, so if you want to see it you
should show up.
p.p.s. Roger & Scott, I decided not to buy the '66.
p.p.p.s Scott Paisley, when is it again you planning to drive out west?
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