I would say, largely because of the way the pieces have broken off the top
ring land, that there is little chance that that damage is attributable to
anything but detonation. The other marks in the pistons are probably the
result of the broken off pieces hammering around before they went out the
There appears to be a cracked piece just hanging on in this picture.
I have seen similar damage several times particularly on MGBs and would
suggest that you check your distributor advance curve carefully both before
and after you have the engine rebuilt.
I'm not convinced that the fuels that are available today, particularly in
CA I understand, are good enough to handle the amount of advance that the
original distributors can produce.
Detonation can be quite serious at highway speeds but completely inaudible
to the driver. You can buy knock sensors which will turn on a light when
detonation is occurring; might be a good investment.
From: firstname.lastname@example.org [mailto:email@example.com] On
Behalf Of Freese, Ken
Sent: Tuesday, January 09, 2007 10:17 AM
Subject: Pistons damaged again
I cleaned up the compression reading columns.
Saturday, I removed the head from my BJ8 to have steel valve seats
installed because of #4 exhaust valve recession and low compression. I
figured #4 was now burned enough, I should get it fixed. After putting
on the hardtop on New Years, I could hear rougher running.
Now I see that #2, 3, and 4 pistons have 1/16 to 1/8 inch notches
missing and I can see the top ring on #4. Preliminary inspection of the
notches don't show smooth melting. So, new set of problems. I figure the
1. Swallowed debris
3. Defective pistons
A - oil never filtered only changed for about the past 40000 miles due
to faulty spin on filter adapter.
B - last rebuild in 1982 and 70000 miles ago due to melted pistons
Compression 1997 Compression 2004 Compression 2007
165 130 110
155 135 90
150 145 70
150 90 70
155 130 115
175 150 145
Any thoughts on the cause until the pistons are actually out?
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