Rebuilding a Bentley T2 engine - Step 1 - figuring out what went wrong

Sometimes engines wear gradually and you have plenty of warning that an overhaul is in your future.  Other times engines just blow, but even then, there are often clues if we pay close attention.  This Bentley motor had what seemed like a subtle problem - it had an intermittent misfire at idle - a shake - that wasn't repaired by new plugs, wires, or any degree of adjustment.  You would not expect that to be a symptom of incipient engine failure, but in this case, it was.

We have seen a lot of failed Bentley engines in our day. This one is a bit unusual.  I've written about engines that developed clogs in the cooling passages in the block, leading to failure of the driver side rear piston. But that's not all that can happen, as this 1980 vintage engine shows.

Before we get into damage let's look at one of the causes.  Owners create sediment inside their engines by mixing different coolant formulations (for example, adding "all purpose" coolant or mixing traditional ethylene glycol coolants (Prestone) with a new formulation (Dex-Cool). You can also get solid sediment in the engine from adding too much leak sealant.  The photo above shows what solid sediment looks like on the passages inside a cylinder head.

Solid deposits can block coolant passages. Even when passages are not totally blocked the deposits inhibit heat transfer, which causes localized overheating.  The most visible evidence of that is when one piston gets too hot and "grabs" the cylinder wall, leaving vertical streaks on the piston. When this happens the clue is often an engine rap and the damaged piston is discovered on teardown.

Before that level of failure the inside of the cylinder liner may show spots where the liner has deformed from uneven heating.  Here is an example:

The cylinder above is a rear one, so the coolant circulation is weakest at the back. That rusty looking mark inside the cylinder is evidence of the deformation.

That damage would ultimately lead to the need for engine overhaul, but it's not what brought this motor into out shop.  The car in question is a lovely 1980 Bentley T2.  It had 65,000 miles on the clock when the owner drove out to see us. His was a rare fuel injected variant and the car was running a bit rough. The tech at his Bentley dealer diagnosed a fuel injection problem and suggested we would be better able to address it, as we specialize in vintage engines and we are an authorized service center for the Bosch fuel injection.

Unfortunately the engine failed before we had a change to do any work. The owner was driving down the Mass Turnpike, and the car was running well when it suddenly lost power and began making a loud rap. The owner pulled over and the engine stalled. He restarted it and the rap was scary enough that he did not move the car.  A tow truck brought it to our shop.  There, a compression check revealed zero compression in the A4 cylinder. We proceeded to start taking the head off.  When we removed the intake manifold Bruce Caron's inquiring eye picked out a problem.  See if you see what's wrong in this photo:

There's no valve stem in the second port from the front!  We realized this engine had dropped a valve.  The first head to come off was on the other side, because that was where we first measured zero compression. When the head came off the cause of compression loss was obvious.

The broken valve had made its way across the intake manifold and fallen into the A4 cylinder where it punched a hole in the piston.  The piston, head, and liner are all ruined on that side.  There was similar damage on the other side. The broken valve came to rest in the B3 intake port.  This is what it looked like when Bruce pulled it out:

Why did it fail, you ask?  There may be a clue in the slight rap we heard when the owner was at our shop, before the failure.  When the injectors clog one result is an uneven pattern of fuel sprayed into the intake.  Over time that can cause carbon to build up on the valve stem and that carbon can make a valve jam in the guide. When that happens, the piston hits it, and at 3,000 rpm on the highway it only takes a moment to snap that valve and send pieces flying. 

The cylinder head shows uneven carbon deposits in the combustion chamber, which suggests all the injectors had some degree of uneven flow.  That is the main reason we replace injectors in older Bosch CIS cars (This system was used in Rolls-Royce and Bentley from 1980-1993). 

Can you prevent that?  Maybe.  On pre-catalyst cars, pouring GM Top Engine Cleaner or Sea Foam into the intake and letting it sit may dissolve carbon deposits. The surest fix is to remove the heads for service. 

These engines used thermostats with lead plugs that melted and allowed increased coolant flow if the coolant got too hot.  This engine's thermostat still has all its plugs but some are damaged which indicates the coolant was going bad, and it may have been on the edge of thermal runaway more than once.  This is one more sign that a problem was brewing in the coolant passages.  There's not much we can do for that short of overhaul.

What's it going to take to fix this engine?

- As noted above, this engine will get new injectors.  We will also send the fuel distributor out for overhaul, making sure the flow is even to all eight cylinders.  

- Both heads are ruined. The first injected heads were found on California certified 1980-81 Shadow II, Corniche, Camargue, Continental and T2 models. Those are unavailable. The more common Spirit/Spur heads are similar enough that we can adapt a pair for this engine.

- This engine will need a set of pistons and new liners. Here's a tip on those: When you clean the top face of the block - the deck - you should see the liners protrude very slightly. The degree to which the liners protrude from the engine block is determined by the depth of the counterbore, which was done at the factory.  The workshop manual calls for about .003 protrusion, but we often see liners that stand .005 or more when they come in for rebuild.  Given the number of original engines we rebuild this was an acceptable spec for the factory, even though the workshop manual suggests we correct that clearance by milling the top flange of the liner. We have never seen a factory liner that was milled in that fashion (you can tell because they are marked on top). We don't generally cut them because genuine liners are becoming scarce and we don't like to weaken them. 

These engines were designed with thick composite head gaskets. Those gaskets are thick enough to easily absorb .005-.010 differences in the liner and deck area, which was probably a result of the machine tool tolerances of the day.  That's why deck protrusion is not a big deal. The same liners and head gaskets were used on all Bentley and Rolls-Royce motors through the 2002 model year.  At that time they switched to a stronger multilayer steel gasket for better durability in the high pressure turbo engines.  

That's when they had to pay attention to deck height.  When these engines were introduced the designers wanted some liner protrusion to increase clamp force at around the combustion chamber. The gaskets were plenty thick to compress there and still seal other areas.  Steel gaskets don't compress much, so they require liners that are close to flush - 0 to 5 thousandths. Cutting the deck for this is a complex operation and one we almost never do on these older engines. 

A few years ago Bentley stopped making the old style composite replacement gaskets, but they are readily available in the aftermarket and most owners of these older engines prefer to use them.  That is the typical solution for these motors. 

Some owners will want the latest Bentley parts in their engines, and if this owner feels that way, his engine block will have to be machined to fit liners to the tighter spec required for the multi layer steel gaskets. The engine will also need to have the cylinder head studs pulled and new inserts set into the block to accept the newer high strength steel head mounting hardware.     

We will need a set of pistons. The alternatives are a set of genuine pistons (if available); a set of good used pistons (if available) or having a set of custom pistons made (if we want to wait for the foundry).

We have seen a lot of issues with corrosion inside the Cloud era blocks. Many of them are junk; most of the others require some degree of welding inside the liner bores.  T2 era blocks are a bit newer and most of them are still usable. 

Follow along as we move through the overhaul process on this engine.  

John Elder Robison

J E Robison Service of Springfield MA is an independent repairer and restorer of Rolls-Royce and Bentley motorcars in Springfield MA.  John is the technical editor for the RROC, the official club for owners of Rolls-Royce and Bentley cars in the United States.  He has been rebuilding these engines (and other engines) for 40 years.