Wednesday, November 26, 2014

Checking an engine after head gasket failure


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. 

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.


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.

A burnt valve as seen in the head (c) J E Robison Service
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. 

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.




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.





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.


Quality engine work takes time, and costs money.  Jobs done correctly last, and the price is soon forgotten while poor quality never goes away.  


If you're wondering what's below the heads, read this story about liner failures in Land Rover V8's and how we fix them.

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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