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MGA engine fried in 25K miles

To: mgs@Autox.Team.Net
Subject: MGA engine fried in 25K miles
From: Barney Gaylord <barneymg@ntsource.com>
Date: Fri, 07 Aug 1998 03:54:14
Well gang,

Just getting around to unfolding an engine story from last fall.

In the spring of '97 I installed a fresh engine in my MGA 1500, rebored,
new pistons & rings, reground crank, and all new bearings throughout, and a
no-lead head.  I gave it a good workout for a couple of months, took it
racing a few times, put a couple thousand miles on it, and then headed off
from Illinois to Alaska on Brit Run To The Sun (by way of San Antonio,
Texas).  It the beginning it was getting about 1000 miles to a quart of
oil, which is pretty good for an early B-series engine.

When the engine had only gone about 4000 miles, it lost compression big
time.  One morning the readings were 50-70 psi.  Seems like the carbs were
running very rich, having been transplanted directly from the old engine
without adjustment.  I guessed that the valves were getting badly carboned
up and leaking.  Not wanting to interrupt the trip for all involved, I just
adjusted the carbs to the extreme lean side and continued on the road,
hoping that it would burn the carbon out.  Appearantly it did, because by a
few days later in Cache Creek, BC, the compression readings were back up in
the 120-135 psi range.  Maybe that 85 mph run through Montana was good for
it.  So I adjusted the carbs back to normal.

A week and a half later it got another oil change in Fairbanks, Alaska,
having used just 3 quarts of oil between oil changes.  It was getting an
oil change every 4000 miles, which at that time was about every 10 days or
so.  The car had just done several hundred miles of gravel roads in Canada,
mostly in Yukon Territories, so far doing quite well.

Then we ventured up the Dalton Highway (the "Haul Road") along the
Trans-Alaska Pipeline all the way to Deadhorse and Pruhdoe Bay on the
Arctic Ocean.  By the time we got bact to Fairbanks a few days later it had
crawled along another 1000 miles of quite dusty gravel road.  Shortly
thereafter it started using noticeably more oil, going about 700 miles to
the quart, but still not too bad.  Over the next few weeks there were more
gravel roads, and by the time we got out of Alaska and back through Canada
it had taken to using a quart of oil with each tank of gas, about every 250
miles.  And it was obviously blowing past the pistons and out through the
vent tube on the tappet cover, as it would continuously soak the bottom of
the car in oil and drip all over wherever it stopped.  I was already
assuming that all that road dust had eaten the new piston rings.

We muddled through the rest of the trip and for a few weeks thereafter by
pouring in oil with every fill up and putting up with the ugly oil
droppings wherever the car stopped for a minute or more.  By mid October I
had pulled the new engine and put the old engine back in (with a little
touch up).  The old engine was a little low on compression, but at least it
wasn't blowing oil.

Well I finally got around to tearing down the new engine, and as expected
there were problems with the rings, but at least the cylinder bores were
clean, so it's a home garage job.  #1 piston had the top ring completely
carboned up and stuck in the groove.  #2 piston was cleaner, and the rings
were not stuck, but it had a 1/2" long section of the top ring groove (near
the exhaust valve) noticeably widened.  That was a bit of a puzzle because
the ring itself looked okay.  Must have been overheated a bit.  I was
almost considering keeping that piston, until I saw the other two.

#3 and #4 pistions were almost twins.  On the top edge of the piston near
the exhaust valve there was a small crater about the size of a fat pencil
lead, but still a clean cylinder wall.  When the pistons were out, there
were sizeable sections of the top ring missing, with the rest of the ring
carboned up and stuck in the groove.  Near the exhaust valve the top ring
groove had been badly burned out over a distance of about an inch, about 3
times wider than original and ugly black and ragged.  Through all this, the
tops of the pistons were all fairly clean (once a little carbon was rubbed
off), and did not look like they were overheated at all.

For a wear reference I was measuring the radial thickness of the piston
rings, starting with #1 which still had 3 good compression rings.  All had
excellent looking 4-piece oil wiper rings.  All the compression rings were
the same radial width to begin with, and I'm guessing that they were
originally close to .118" wide (3.0 mm).  The third compression rings are
now .105" wide.  The second rings are now .100 wide.  The top rings are now
only .075-.080" wide.  It looks like those rings sustained a tremendous
amount of wear from ingested dirt, especially the top rings.

It appears that when the rings get down to about .080" wide they can break
from increased stress.  Once there is significant blow-by and a radial
break in the top ring, hot combustion gasses will blow through that gap
like a blow torch, melting the aluminum piston in the vicinity of the ring
gap.  The end result is that I not only need new rings, but a new set of
pistons as well.  Maybe I can keep #1 for a spare.

Incidentally, the cylinder head looks to be in very good condition,
probably doesn't need any work at all, but I may give it fresh valve seals
on general principals (they're cheap).  Three cheers for the hard seats and
stellite exhaust valves.  And with the regular oil changes, the cam and all
the bearings still look like new (wheew, lucked out there).  Oh, George?
Ready for another parts order?  With little luck I'll have the new engine
back in the car within a week, and the old 100,000 mile engine back on the
shelf for an emergency spare.

Chin up,

Barney Gaylord
1958 MGA with an attitude


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