Engines can have head gasket failure in two ways:
- They can leak oil or coolant onto the ground, with no internal failure
- The fire ring seal can fail, allowing combustion gases to pressurize the cooling system
Of those two failures the second is by far the worst because
it’s usually associated with overheating and sometimes catastrophic engine
failure. The common dealership repair is
usually to slap a new head gasket in place and send it down the road. That works for some engines. But on others, the result is a repeat failure
– a month, a year, or three years later.
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| A blown head gasket. Area in the red box is the coolant passage. The half circle is the combustion seal |
Most of the cars we see are in this latter category. For example, we get quite a few Land Rover
engines that have a “history of head gasket replacements.” No engine should have repeat head gasket
failures. When that happens, some repair
step is being skipped, or there’s another root cause.
Almost every engine we see needs some additional machine
work when the heads come off. That makes
me wonder if the shops who are just slapping in a new gasket are ignoring that
and trusting luck, or if we just see the “repeat offenders” because we are more
a shop of last resort when the local mechanic could not fix the car.
How can you tell if a gasket alone will fix your
problem? You measure. Read on, and I’ll show you how we check for
problems and what we do when we find them.
The first things we do are tank clean the head, and then
blast it with walnut shell grit to get it as clean as new. Next we measure the
head for flatness with a straight edge.
When aluminum heads are removed from an engine, they are usually
slightly out of flat. In addition, they
can be corroded. Sometimes we see damage
from coolant gone bad, and we occasionally have to weld that up. We also see cracks on some engines. Cracks are typically repaired by welding
too. When all that is done, the surface
of the head is machined flat. If the engine
is a V6, V8, or V12 we surface both left and right heads equally so as not to
cause a compression imbalance.
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| Repaired cylinder heads for a Bentley (c) J E Robison Service |
If an engine has more than 75,000 miles the heads will
always benefit from freshening up. In
that, we reseat the valves, check the guides and fit new seals, and clean the
ports. We check for valve seat damage, which will lead to burnt valves.
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| A burnt valve as seen in the head (c) J E Robison Service |
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| Burnt valve removed from the head (c) J E Robison Service |
If the owner wants his engine blueprinted we will also
measure each combustion chamber’s volume and increase the size of small
chambers to match the volume of the largest chamber. This is typically done by grinding material
away and by recessing the valves deeper into the seats.
The heads are the easy part.
Now we check the block. We use a
straightedge to measure the deck surface for flatness. Sometimes we find broad warping, while other
times we find depressions or valleys.
The head gasket can take up a few thousandths of warp, but a block with
5 or 10 thousandths has to be taken apart and repaired. The reason:
major warpage of the deck often means the bearing journals are warped
too, and if that is ignored the engine will have a lower end failure at some
point.
Significant deck warping is a sign of major
overheating.
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| Illustration of a low spot in an aluminum block deck (c) J E Robison Service |
The next thing we look for is out-of-round in the
cylinders. We measure the cylinder bore
front to back, and inside to outside.
The difference between those measurements is called “egging.” We don’t want to see more than .002
inch. More than that and the piston
rings won’t seal well, and the engine will use oil and possibly lose compression. Egging is another sign of overheating.
We also make those measurements at the top and bottom of the
cylinder. Again we don’t want to see
more than .002 difference. Top to bottom
difference is called “taper.” When a
motor has too much taper it may know, and that’s a sign it’s worn out.
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| Measuring cylinders for taper and egging, BMW V8 shown (c) J E Robison Service |
Finally, we test the studs or the head bolt threads,
particularly on aluminum engines. We
look for evidence of stripped or pulled threads, and we repair any damaged ones
with inserts. An insert repair will be stronger than the original in most
cases.
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| Repairing damaged head studs with oversize inserts Bentley V8 shown (c) J E Robison Service |
If the block is out of spec in those areas it should be
removed and overhauled. We can fix
dimensional errors like that in most cases.
The other thing we look for is corrosion damage.
If you think this sounds like a lot of work compared to
slapping in head gaskets, you are right.
The gasket slap is a strategy for dealerships working on factory
warranty (where it just has to last to the end of warranty . . .) and new cars
(where corrosion and wear are seldom issues)
If you work on older vehicles, or if you want your repairs to be at
least as good as original (as opposed to almost as good as original) this is
the only path to take.
Good luck
John Elder Robison
Robison Service has provided independent service, repair, and restoration for BMW, Land Rover, Jaguar, Mercedes and Rolls Royce -Bentley owners all over New England for over 25 years. Founder John Robison is a long time technical consultant for the Rolls Royce and Bentley Owners Club. Our company is an authorized Bosch Car Service Center. We also service Mercedes, Jaguar, Land Rover, Porsche, and MINI motorcars. We have flatbed transport throughout the northeast region, and we work with Intercity and other transporters for greater distances. We also offer pickup and delivery for cars in Springfield, Wilbraham, Longmeadow, Agawam, Westfield, Northampton, and Amherst.
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