Thursday, October 30, 2014

Blown Head Gaskets in Bentley Turbo R and Azure

“My Bentley overheated, and now the engine is smoking.” As the cars age, I hear that more and more from Turbo R and Azure owners.  As the story unfolds, the narrative often runs something like this:

“I pulled out of my driveway and went down to the ramp for I-95.  I gunned it to move up the ramp, and accelerated to 80 in the left lane.  I looked down a moment later and the temperature gauge was all the way to the right.  I started to pull over, and the engine stalled.  When I started it back up, smoke came pouring out from under the hood.  You’ll have to send a wrecker for it.”

Steam comes out of the head bolt holes on a Turbo R with blown gaskets
Coolant leaking from head bolts is a sure sign of head gasket failure, and maybe more damage

People describe the problem as head gasket failure, but it’s often quite a bit more involved.  And it’s happening more and more as these fine cars age.

When you look at the motor on one of these cars you may actually see coolant coming out around the head studs as shown in the photos above.  Most times, though, you don't see anything but steam from the radiator and bubbles in the expansion tank. What’s that tell us? It means the head gasket has blown out, and the motor has to come apart.

A Bentley engine with heads removed (c) J E Robison Service
An assembled Bentley Turbo motor, vintage 1996 (c) J E Robison Service
How far apart, you ask?  It depends on how much damage the engine has sustained.  It's hard to know that until it's taken apart, and even then you may not be able to tell. The only sure cure for overheating damage if a full rebuild, which few people want to do.  So we take apart the top and see what we can see . . . 

Rolls-Royce Bentley V8 with 20 head studs on each side (c) J E Robison Service

To begin, all the turbo and intake piping comes off, and the valve covers are exposed.  The covers come off, as do the rocker shafts and exhaust manifolds.  The head bolts are now exposed, 20 of them on each side.  The first clue to the extent of damage is how tight those bolts are.  If they are still torqued that’s a good sign.  If they are loose you should expect some trouble and possibly a number of pulled studs.

Once the head is off you can often see the blowout.  In my experience the cylinders that are physically closest to the turbocharger are the most likely to fail.  The next photo shows a blown head gasket.

Bentley Turbo motor with head removed - front two cylinders have blown out
When the fire ring fails (that’s the part of the head gasket that seals the combustion chamber) the combustion gases burn through the inner gasket material and pour into the cooling passages.  Check out the closeups in the photos below.  The hot combustion gas fills the cooling system with bubbles, which reduces the efficiency of the coolant.  It also pushes coolant out the radiator overflow as the system is over pressurized.

Bentley Turbo - head gasket blown at the bottom on both cylinders (c) J E Robison Service

That is the means by which head gasket blowout translates into an overheating failure.  But that’s just the first step . . .

Closeup of burnt Bentley head gasket - blown into water jacket. The red band is part of the head gasket - it's extra sealing around a cooling water jacket area. As you see, the gasket is burned through from there into the combustion chamber seal (c) 2014 J E Robison Service

When the engine overheats the metals expand.  The aluminum block expands more than the steel studs, so the tension on the studs rises as the motor heats up.  That’s good to a point, because it makes the head gasket clamp tighter at normal temperatures.  But when the motor overheats – particularly when an area near a stud gets really hot – problems develop.

When the aluminum expands the stud stretches, and the gasket compresses . . . to a point.  Beyond that point, the studs pull out of the block, taking the threads with them.

Here’s a pulled stud, with strands of aluminum block clinging to it.

Bentley head stud with threads pulled (c) 2014 J E Robison Service

I used to think these Bentley head gasket jobs were simple.  Not anymore!  It seems like every one we've seen in the past few years has had more complications than the one before.  Most of these engines have one or more pulled studs and some have other damage.

Head stud and insert (c) J E Robison Service

A repaired head stud (c) 2014 J E Robison Service

The most common stud failures are on the top and bottom rows.  That's lucky, because the center rows studs can't be fixed without stripping the block.  If you fix studs, it's absolutely critical that you drill the repair holes straight.  If the studs are even a little bit crooked the head won't go on!  The photo below shows the alignment fixture we use to drill straight holes.




One engine this summer got hot enough in the front cylinder to melt the injector tip.  What do you do then?  A wise person would change all 8 injectors on a 20-year-old car, but that is several thousand dollars of parts.  When the fuel rail is apart don't forget the other o-rings that you can't get at when the motor is together.  Repair of these motors can get expensive fast.  You don't want to ask how bad it can get because the answer is over $50,000 as of 2014.

Before you dismiss that number as crazy, consider these costs if you drive your Turbo Bentley until the engine overheats and seizes

  • Rebuild long block engine after major failure - $35,000
  • Replace melted injectors and rail parts - $3,000
  • Replace water pump, radiator, coolant hoses and thermostat - $3,000
  • Replace other heat-damaged parts - turbo, pipes, possibly catalyst - $3-6,000
  • Labor for all this work - $10,000+
All you can do is shut the motor off the moment you see it's overheated, and hope your case is not the worst case.

When an engine has gotten hot enough to pull studs you have to look for other issues.  The first is warpage.  The heads are almost certainly going to be warped, and we would surface them as a matter of course.  The mating surface on the block – called the deck – may also be warped, but it’s not so easy to fix that.  The only way to true a warped deck is to remove the motor, strip it completely – including stud removal - and then machine it flat. It's very common for people to skip this step - "just throw some new gaskets in and per er together" - but that really just sets you up for later failures, often worse than the first one.

Here’s a cylinder head after it’s been surfaced.  The surface is uniform and smooth but we had to shave off .008 to achieve that.  If we had skipped that step the head gasket would have been overstressed in the high spots and under stressed in the low ones.  That's why surfacing is important.  

We also do a full valve job, to ensure the car runs as smooth as possible.  I often see shops skip this step too, but why?  If you can spend a few more hours here and get a smoother car, why would you not do that?

Bentley cylinder head ready to install (c) J E Robison Service
When studs pull out of the block you have two choices.  Mild damage can be fixed with a Heli-coil insert.  More major damage calls for a solid insert.  In both cases the fitment of larger inserts between the stud and the block makes for a stronger joint than original. That means we can assemble with 110% of the original torque, which reduces odds of failure even more.  Strong studs - strong engines.

What's the typical repair consist of now?
  • Remove heads, clean all parts, and check for damage. Always pull both heads, even if only one is blown.  If you don't, the other side will fail soon after.  We have learned that the hard way.
  • Check the heads for cracks and warping, and do a full valve job with guides and seals.
  • Check all the injectors and replace all fuel injection o rings.
  • Replace the thermostat, belts, and other consumable parts.
  • Use a fixture to test torque the head studs 20% tighter than stock and ensure they do not begin to give way when left overnight.  Repair any that are marginal.
  • Use a gauge to check block flatness and consider full overhaul if the warpage is more than .004
  • Replace all the hard rubber hoses in the center engine area - this is your time to get at them easily.
When you do all this work, you always wonder . . . is this as deep as the damage goes?  Unfortunately, there is no way to know, short of total engine disassembly.  It’s possible the pistons have seize damage, and it’s possible the liner seals are damaged.  There are other possibilities too, but they are less likely.  You can look for evidence, but if there's nothing to see you have to put it together and hope for the best.  

What can you do to check for other damage?
  •        Look at the cylinder walls for signs of scraping.  That could be evidence of piston damage.  If you see this, the engine should be removed and the pistons pulled.
  •        Look at the oil, and sniff it. Is it burnt?  Burnt oil is a very bad sign because it means the core of the motor got very hot.
  •        Look at the amount of warpage in the deck.  If you have .003-.005 of warping (measured with a good straight edge) that’s probably ok.  .010 of warping and you may have bigger problems that will necessitate engine removal.  When the metal is warped at the top it may be warped at the bottom, and if the crank journals are out of true the engine will eventually fail.

 Why do these engines fail?

That's an excellent question.  Here's my theory.  When the car is new the head bolts are torqued to around 50 foot-pounds.  With 20 bolts per side, this adds up to many tons of clamping force on the gasket surface. This force is increased every time the engine gets hot, and it's relaxed when the motor cools down. When the motor is overheated the pressures skyrocket.

After a few such cycles the gasket gets squeezed a tiny bit thinner, and the torque on the bolts starts to drop.  The result - lower clamping force on a cold engine.   When the motor heats up, all is still well because the thermal expansion tightens everything up.  But on a cold engine we have an incipient disaster.  When the low clamping forces of a cold engine with relaxed torque come up against the high cylinder pressures of a turbo engine under throttle the result may be a blowout.

That is why Bentley Turbo head gaskets blow out on cold engines.  Hot motors are much less at risk. It's one more good reason to warm your Turbo up thoroughly before you get into the throttle.

Bentley engine - always check the desk surface for warpage (c) J E Robison Service
We use Copper-Cote to help seal the head gasket after a blowout.  Reassembly of a Turbo R (c) Robison Service
Applicability of this article:

The information in this article applies to all Bentley Turbo R, Continental R, Continental T, and Azure cars, from the late 1980s to the early 2000s.  It's also mostly applicable to newer Bentley cars (Arnage, etc) with the 6.75L twin-turbo motor, though the teardown and reassembly of those engines may be more complicated and they are likely to have additional damage.


Bentley Continental T (c) J E Robison Service

Bentley Azure (c) J E Robison Service

Bentley Turbo R (c) J E Robison Service, Springfield, MA, USA
Give us a call at 413-785-1665 if you'd like to talk about a Rolls Royce or Bentley engine repair.  We handle all aspects of engine service and rebuilding.

And if you found this useful. . . . here is more on Rolls Royce and Bentley service:

Thoughts on buying a used Rolls Royce or Bentley - applies to Silver Cloud and newer series cars

More thoughts on Spur - Spirit - Turbo era car buying

Thoughts on restoration - applies to all cars

Evolution of the RR/B models - Silver Shadow through Arnage/Seraph - original article from the Robison Service website

Inspecting a Rolls Royce or Bentley - Applies to Corniche, Continental, Azure, Turbo R, Mulsanne, Eight, Turbo R, Silver Spur, Silver Dawn, Silver Spirit

More Things to Look For in a 1981-2000 Rolls Royce or Bentley - this is the original article from the Robison Service website

The last Crewe built Rolls Royce convertibles - applies to 2000-2002 final Series Corniche

Repairing convertible top hydraulics - Applies to 1996-2004 Rolls Royce and Bentley Corniche and Azure cars

Head gasket failures in Bentley Turbo cars - applies to Turbo R, Continental R and T, Azure, Arnage

Checking engines after head gasket failure - Applies to all cars

Checking and inspecting Rolls Royce hydraulic systems - all cars after Silver Cloud and print to Silver Seraph. Applies to all Shadow/Spur era vehicles

Case Study - brake failure in a Shadow - Silver Shadow era cars with RR363

Rear suspension gas springs - Applies to all 1981 - 1999 cars prior to Silver Seraph

Changing batteries in seat and ECUs, Applies to 1980s-1990s Silver Spirit / Silver Spur / Mulsanne /Eight / Turbo R

Changing alarm ECU batteries,  Applies to 1980s-1990s Silver Spirit / Silver Spur / Mulsanne /Eight / Turbo R

Servicing Shadow and Spur series brakes - applies to 1966 - 1999 cars after Silver Cloud and prior to Silver Seraph

Alcon racing brakes for Continental and Azure - Applies to all 1990s cars but most particularly to the final series Azure, which had these brakes fitted at the factory - a unique variant

Fixing Power Steering Leaks - applies to 90s cars with the reservoir above the alternator

Questions and answers on collector car storage - Applies to all cars


Evaluating paint - Applies to all cars


Robison Service has provided independent service, repair, and restoration for Rolls Royce and Bentley owners all over New England for over 25 years. 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 region. We also offer pickup and delivery for cars in  Springfield, Wilbraham, Longmeadow, Agawam, Westfield, Northampton, and Amherst.