At 06:52 PM 5/31/02 -0600, David Councill wrote:
>I finished disassembling my engine from my 71BGT (ironically a 72 B
>engine) ....
>....
>I can only guess the mileage on the engine at maybe 130 to 150 thousand
>miles ....
>----
>I have two pistons (#3 and #4) which seem to be missing a small portion of
>the top ring and there is a tear or cut in the side of the piston. ....
>pictures....here: http://www.imt.net/~dcouncil/mg/piston.html
>
>#3 shows the cut or tear, a loss of 10-15% of the ring, and another curved
>indentation below that. #4 has a much smaller tear but a similar ring
>loss. .... Pistons #1 & #2 are shiny (not black) in those areas. There are
>no visual markings or scorings in any of the cylinders (just a slight
>ridge is all).
>....
I think I have the answer. BTDT. Remove the rings and measure the width
of the rings inside to outside. I believe you will find the top rings are
worn considerably more than the others.
I suspect your engine was running hotter on #3 and #4 cylinders, perhaps a
resullt of a lean mixture on the rear carburetor (and possibly agrevated by
too much spark advance at road speed but less likely). Running hot tends
to burn pistons and eat rings especially the top ring. When the ring wears
on the outside it expands in diameter to maintain contact with the cylinder
wall. This results in an ever increasing ring gap. If you have 0.030" of
wear on the top ring, the end gap increases to about 0.100".
Hot combustion gases leaking through this enlarged ring gap burn the top
land on the aluminum piston, very much like taking a hot torch to it. It
may also burn the bottom side of the ring groove in the same
vacinity. When the groove gets wider (top to bottom) the ring loses the
mechanical support, and the ends of the ring start to flex vertically and
eventually break off. If you're lucky the small loose ends of the broken
ring may gradually burn up and disappear. If you're not so lucky the
broken bits of the ring may get caught between the piston and the cylinder
wall and score the wall badly, resulting in increased oil burning. I had
one case where the top ring wore down to about half of its original radial
width, then caught and jammed up the piston so it broke off the top of the
piston at the top ring groove.
I think this condition is actually fairly common in these engines, probably
due to misadjusted carburetors. In my MGA I had a lot of top ring wear on
a new engine during my Alaska trip, but I figure that was due to lots of
ingested dirt from 5000 miles of gravel roads in Canada and Alaska, and
blame it on the bug screens on the air intake (not real air
filters). Since you're rebuilding the engine anyway, don't worry about it
any further. You will most likely be having the block rebored and be
installing new pistons and new rings, so you get a fresh start. In the
future just pay more attention to getting the carbs adjusted properly.
Incidentally, congratulations on getting 130K-150K miles out of your last
rebuild. Give that man a cigar!
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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