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Mechanical Devices: The Bellcrank, Part I

by | Sep 27, 2017 | Tech Seen


An interview with Gene O’Hara
By Michelle Cousineau

Gene O’Hara has been rebuilding Classic engines and drivetrains for more than 45 years. Gene and Phil Hill constituted the mechanical department at the legendary Hill & Vaughn restoration shop in Santa Monica, California. Gene is a technical advisor for Classic Car Seen, and he continues to restore Classics to this day.

Seen on the field at the 2014 Pebble Beach Concours d’Elegance…a spectacular 1923 Rolls-Royce Silver Ghost RRCCW Salamanca Town Car. Here, Gene expresses his appreciation for the Rolls-Royce engine and the versatility of the bellcrank.


“A bellcrank is a device that turns reciprocal energy into rotational energy, and vice-versa – it will also turn rotational energy into reciprocal energy.”

Photo #1

A manual shutter control
Gene: Here’s a bellcrank that takes reciprocal energy, turns it into rotational energy and then turns it back into reciprocal energy [photo #1]. As someone inside the car pulls a knob on the dash, it pulls this lever [arrow A] towards the dashboard, or from our perspective, towards the firewall. That’s the reciprocal energy.

This moves the lever’s arm which rotates the shaft that the arm is on [arrow B]. That’s the the rotational energy. So the rotation of that arm moving will move the shaft, rotating it. It will move this arm with it [arrow C], and that will pull on that cable [arrow D], which goes all the way up to the front of the radiator and opens the shutters. Instead of having an automatic thermostat, like modern cars have to control the cooling of the engine, here you have a manual one and you are in control of it. If the engine needs to warm up, you can close the shutters a bit. If it needs to run cooler, you can open them more. It’s just like life, you have control of it all. It’s wonderful. That’s why I love these cars.

“It’s just like life, you have control of it all. It’s wonderful. That’s why I love these cars.”


1923 Rolls-Royce Silver Ghost RRCCW Salamanca Town Car
Photographed at 2014 Pebble Beach Concours. © Michelle Cousineau

Photo #2

A sophisticated system
This is the carburetor, which in itself is an unbelievable work of art [photo #2]. There’s a bellcrank here [arrow E] and a bellcrank there [arrow F]. They are joined by this rod here [arrow G], which of course is adjustable at each end. There are nuts at each end, and the rod is threaded, going into these clevises. The clevises are adjustable and basically that rod can be extended or contracted. The reason you do that is to synchronize these two bellcranks, which control the jets of the carburetor. There is a low-speed jet and a high-speed jet; the low-speed jet is for idling and the high-speed jet is for driving.

The way this is set up is incredibly brilliant, it’s controlled by the driver’s foot and hand throttle controls. This low-speed jet has a left-handed thread on it. You see those circular brass pieces [arrows H and I]? Those are adjusters for the jets, the jets themselves are down in the carburetor. That adjuster is threaded and so is one for the high-speed jet.

Basically this is a system for operating both the high-speed and the low-speed jets. The low-speed jet has a left-handed thread, which is a reverse thread – most threads are right-handed. Basically when you step on the gas — or you work the lever for the gas on the steering quadrant — and you move that rod… when you push it open, because the low-speed jet has a left-handed thread and the high-speed jet has a right-handed thread — when one of them opens, the other one closes, because they are threaded in opposite directions. So when you move these bellcranks, you are closing one and opening the other, depending on what you want the car to do.

So when you’re idling, the low-speed jet is open. Then when you step on the gas and you push that rod, it closes this jet and opens the other one. One movement adjusts both jets simultaneously.

They are designed and set so that one will open as the other one closes and vice-versa. You have to have the adjustment of this rod and these two bell cranks just right, otherwise it won’t happen. They’ll both be open at the same time or will be partially open when it shouldn’t be open at all and you have to know that. People who work on these cars without knowing them well will have a very tough time getting this set correctly.

1923 Rolls-Royce Silver Ghost RRCCW Salamanca Town Car
Photographed at 2014 Pebble Beach Concours. © Michelle Cousineau

Understanding the Rolls-Royce and early engines
There are two kinds of knowledge. One is general knowledge…you can be a general mechanic and know the general workings of most automobiles. The other one is, you can be a marque-specific person and have only knowledge that relates to a particular marque. You would have to be a marque-specific mechanic to understand these Rolls-Royce carburetors. Not only would you have to be marque-specific, but you would also have to be Silver Ghost-specific. The Phantoms have different carburetors, and different linkage.

The PIII is totally different from the other Phantoms, too. The PIII is the only one that has the V-12 mini-Merlin engine, all the others have inline engines. The PI and PII are very similar to each other. They have different cylinder heads. There’s lots of things that separate them, but the cylinder heads is really the key to understanding the difference between a Phantom I and a Phantom II.

Way back when, there was a guy by the name of Harry Ricardo. He figured out that the best design for a cylinder head was to have cross-flow induction and exhaust. In other words, the inlet would come in one side of the cylinder head and the exhaust would exit on the other side of the cylinder head, go throughout the combustion process, and exhaust out of the other side of the cylinder head. So it was just a continued flow in, combustion out. Whereas prior to Ricardo, everybody was trying all kinds of crazy things. It worked to some extent – the cars ran, but there was no real performance out of them. Ricardo really started the high-performance race right thereby inventing the cross-flow cylinder head. That was in the early ‘30s. Prior to the cross-flow cylinder head, fuel would come in, combust and go out the same side of the engine. It was very inefficient. It was always the combustion gases and the exhaust gases and the intake gases running into each other, causing congestion.

When he came up with the idea of the cross-flow cylinder head, it allowed everything to flow with a continuous motion. It came in, it combusted and it flowed out. It was brilliant for its day. Everybody started re-designing their cylinder heads.

The Phantom I had the old, non-cross-flow design. If you look at a Phantom I, you’ll see that the intake and the exhaust manifolds are on the same side of the engine. It’s a very inefficient way to develop power and it’s terrible for fuel consumption. You waste a lot of fuel and you don’t get the benefit of a lot of performance.

So Phantoms I and II are very similar to each other, but the cylinder head makes them different. The PII is a better-running car than the Phantom I, because the PII has the Ricardo-styled cylinder head. And the Phantom III has a completely different engine altogether.

Then the Phantom IVs and Phantom Vs have eight cylinders, inline and V-configuration, respectively. The inline 8 is what they call “Big Bertha,” because it was this huge, massive inline 8-cylinder engine that they used during the War to power tanks. It was a massive straight-8 engine, it was huge. It ran and drove the cars very well, but it wasn’t very sophisticated, and Rolls-Royce wanted to be more sophisticated than that. So they ran “Big Bertha” for awhile, then they went back to an inline 6. That was underpowered and that’s when they went to the V-8s in the Silver Cloud II series. The first Silver Clouds had the inline 6 and they were not very impressive. I wouldn’t say they were underpowered, but they were sluggish. Then when they went to the V-8 which is the Silver Cloud II, the second series, they came out with their V-8 for the first time. That gave the engine a lot of power.

The were a couple other models that Rolls-Royce made, but they weren’t well-known and didn’t push the brand into the forefront. The Silver Ghost really launched Rolls-Royce and brought it to the forefront of the automotive world.

Henry Royce raced one in ‘06 or ‘07, which was the first Silver Ghost. They had a big endurance race which the Rolls-Royce easily won. The world was basically shocked by how much this car outperformed all the others. That’s how Rolls-Royce began to develop their name as comfortable, smooth, powerful, reliable, better than everyone else — the best car in the world. That was when they acquired that reputation. That was the Silver Ghost, which went from ‘07 to 1926.

Everything about these engines is so incredible. All of the other parts that other engine manufacturers took for granted, these people went out of their way to ensure that everything in there was the best that was offered to the public anywhere, by anybody. The crankshafts, the connecting rods, the pistons…it goes on and on..

Photograph by Michelle Cousineau

Making Adjustments
Bellcranks are used in many other engines, but not with the proliferation that Rolls-Royce uses them. They are all over Rolls-Royce engines, and not just the Rolls-Royce engines, but the entire Rolls-Royce automobile, even the modern ones. They’re under the dash, they control the heating and air-conditioning system, they are everywhere.

They can be damaged if you’re not careful making adjustments. You can also have problems if you have adjusted them incorrectly and then have a hard time getting them back to their original position. There’s certain things you do as an educated mechanic, someone who is professionally trained, not trained by himself or by some other non-professional person.

Anytime you take anything apart that is adjustable, the first thing you want to establish is where you are with your adjustment before you even start fiddling with it. The reason being, if your adjustment doesn’t work or if it makes things worse, you need to be able to get back to your starting point. If you don’t do that, chances are it will never run as well as it did when the car was brought to you in the first place.

On counting adjustments to the bellcrank: “There’s nothing haphazard or hit-and-miss about it, it’s very scientific. It’s a perfect scientific method. When you’re taught it and you use it, you can’t go wrong.”
So when you take apart something that is adjustable and change the way it is adjusted, you count those adjustments. If you loosen the nut on that clevis and rotate that clevis to extend that rod, you want to count the turns that you are giving the clevis [image 2, page 45]. You usually count in quarter turns or half turns. You would count half, one, one-and-a-half, two, two-and-a-half turns out from where you started. Then you write that down and road test the car to see if you have improved it, made it worse or made no change at all. If it has improved and you think you can improve it more, you come back into the shop and you take the pin out of the clevis again and you start at two-and-a-half turns out, and three, three-and-a-half, four, four-and-a-half. Then you lock it down and take it out and drive it again. If you go for a road test and now it doesn’t seems a good as it was the last time, you know that somewhere between two-and-a-half turns and four-and-a-half turns is where you want to be. So you start backing it up, from four-and-a-half, to four, to maybe three-and-a-half. The you write that down, and road test. Ultimately, you get it to the ideal spot, where you know it’s running absolutely the best it can. Then you’ll start with something else. That’s how you do it. There’s nothing haphazard or hit-and-miss about it, it’s very scientific. It’s a perfect scientific method. When you’re taught it and you use it, you can’t go wrong.

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