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engine disassembly curiosity

To: mgs@autox.team.net
Subject: engine disassembly curiosity
From: David Councill <dcouncil@imt.net>
Date: Fri, 31 May 2002 18:52:03 -0600
I finished disassembling my engine from my 71BGT (ironically a 72 B engine) 
which I will put in my 72B when I finish rebuilding it. I have only done 
two engine rebuilds, both 18V engines. This particular engine was my second 
rebuild which I did on my own after having help on the first one (a 
different engine) about four years earlier.

It was a successful rebuild as I did this last one in 1983. Outside of 
premature cam failure a year later, the engine ran quite well over the 
years, initially under extensive daily driving (104 miles a day round trip 
work commute the first four years). It had started to leak a moderate 
amount of oil but no sign of any oil burning when the car met its end in an 
accident almost two years ago - my son was driving and a girl ran her van 
into the side of the car, her fault.

I can only guess the mileage on the engine at maybe 130 to 150 thousand 
miles as the speedometer was not working during a few stretches over the 17 
years the engine was in service. So I was quite curious how the engine 
would look when I took it apart. There was no noticeable bearing wear (all 
grey). But the cam had one lobe with significant wear, the intake lobe for 
the #4 cylinder. All the cylinders look fine subject to the machine shop's 
inspection next week.

But two pistons showed some unusual wear, reminiscent of  the engine 
failure that preceded my very first engine rebuild (1979 - an interesting 
story in itself) where one piston was missing a chunk off the side and a 
few rings.

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. #3 was 
the worst one - I have attempted to get clear pictures which are 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. 
It was hard to get good pictures because the top of the pistons are black. 
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).

As I mentioned, the engine gave me many miles of service and showed no 
problems when I took it out of service, outside of the oil it left on the 
driveway from the rear engine seal leak. So I'm just curious as to what 
caused this wear as the engine would eventually have failed due to this 
unusual piston wear. Any ideas?

David
67 BGT
72 B restoration project

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