Long-bolt engine
Encyclopedia
A long-bolt or through-bolt engine is an internal combustion piston engine where, following usual practice, the cylinder head
is held down by bolts or studs. Conventionally the cylinder head is bolted to the cylinder block
and the crankshaft
main bearing
s are in turn bolted to the crankcase
by separate bolts.The cylinder block and crankcase may either be bolted together by a third set of fasteners, or more commonly today, cast together as a monobloc. In the long-bolt engine however, a single set of long bolts is used, spanning from the cylinder head right through to the crankshaft bearing caps.
s, particularly those that combined high power and lightweight construction of aluminium alloy. These early engines used long studs from the crankcase to the cylinder heads. As most at this time used monobloc heads only one bolt would anyway be needed to join the two pieces, but a long fastener the full height of the cylinder was used so as to support the tension forces in the stud, rather than stressing and possibly distorting the cylinder walls. Curtiss
and air-cooled Renault engine
s even used an X-shaped cap above the outside of the cylinder head, with the studs bolted though that so as to distribute the forces evenly across the head. As forces were still transmitted between these studs and the main bearings by the walls of the crankcase, these early engines are not however considered as long-bolt engines.
The final developments of the aircraft piston engine were the horizontally-opposed H-engines such as the Rolls-Royce Eagle and the Napier Sabre
. These compact engines were tightly packed and, unlike the previous V engines, there was no access to the crankshaft or its bearings once the two halves of the crankcase were assembled. Accordingly long through-studs were used, passing right through the engine from one side to the other. On the Sabre, some of these studs were short and served only to clamp the crankcase halves together. The others passed between cylinder heads on each side. The entire tensile force across the engine was taken on these studs, with no tension being placed on the crankcase.
invested heavily in robot automation throughout its factories, to both improve product quality and to reduce labour costs. The new Fully Integrated Robotised Engine
was developed as part of this, to replace the previous range of small Fiat engines. Although taking the opportunity to update performance and emissions from Fiat's venerable 903cc pushrod engine, the primary goal of this new design was for easy assembly by robots. Key to this was the 'layer cake' construction of the new engine, held together by its long through bolts.
The FIRE engine has been regarded as a success, having produced efficient and reliable engines from 769 cc with growth into 1368 cc and 16 valve versions. The 1242 cc 16 valve 'Super FIRE' also used a ladder frame main bearing carrier, giving a crankcase and block more rigid than any comparable engine in its class. One drawback to the FIRE engine was a perception by mechanics servicing them that it was now difficult to perform a top-end overhaul without also requiring the bottom end to be dismantled, with re-assembly and re-torquing in strict sequence.
In the late 1980s, Rover
were aware that they needed to adopt innovative and reliable new techniques if they were to survive as a car maker and, like Fiat, to throw off their previous problems of poor build quality. One result was the K engine
, to be built by Rover's partner Powertrain Ltd
. This used a ladder main bearing in all models, giving an extremely rigid block that would later permit a very high redline speed in applications such as the MGF
, Lotus Elise
and for racing. Unlike the FIRE though, and despite the large numbers sold that served reliably for many years, this engine was tarred with Rover's past flaws and gained a reputation as unreliable. The flaws that gave rise to this reputation were caused by only some components used in only some models: particularly the head gasket
and wet liners. Regarding these problems with particular component versions as indicative of poor overall engine design was undeserved. Equally, none of these problems are inherent with the techniques of wet liners or through-bolting.
Cylinder head
In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket...
is held down by bolts or studs. Conventionally the cylinder head is bolted to the cylinder block
Cylinder block
A cylinder block is an integrated structure comprising the cylinder of a reciprocating engine and often some or all of their associated surrounding structures...
and the crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
main bearing
Main bearing
In a piston engine, the main bearings are the bearings on which the crankshaft rotates, usually plain or journal bearings.All engines have a minimum of two main bearings, one at each end of the crankshaft, and they may have as many as one more than the number of crank pins...
s are in turn bolted to the crankcase
Crankcase
In an internal combustion engine of the reciprocating type, the crankcase is the housing for the crankshaft. The enclosure forms the largest cavity in the engine and is located below the cylinder, which in a multicylinder engine are usually integrated into one or several cylinder blocks...
by separate bolts.The cylinder block and crankcase may either be bolted together by a third set of fasteners, or more commonly today, cast together as a monobloc. In the long-bolt engine however, a single set of long bolts is used, spanning from the cylinder head right through to the crankshaft bearing caps.
Aircraft
The long-bolt design began with aircraft engineAircraft engine
An aircraft engine is the component of the propulsion system for an aircraft that generates mechanical power. Aircraft engines are almost always either lightweight piston engines or gas turbines...
s, particularly those that combined high power and lightweight construction of aluminium alloy. These early engines used long studs from the crankcase to the cylinder heads. As most at this time used monobloc heads only one bolt would anyway be needed to join the two pieces, but a long fastener the full height of the cylinder was used so as to support the tension forces in the stud, rather than stressing and possibly distorting the cylinder walls. Curtiss
Curtiss OX-5
-Bibliography:* Angle, Glenn D., AEROSPHERE 1939. New York: Aircraft Publications, 1940.* Gunston, Bill, World Encyclopedia of Aero Engines. Somerset: Haynes Publishing, 1995. ISBN 1-85260-509-X...
and air-cooled Renault engine
Renault 80 hp
|-See also:-Bibliography:* Gunston, Bill. World Encyclopaedia of Aero Engines. Cambridge, England. Patrick Stephens Limited, 1989. ISBN 1-85260-163-9...
s even used an X-shaped cap above the outside of the cylinder head, with the studs bolted though that so as to distribute the forces evenly across the head. As forces were still transmitted between these studs and the main bearings by the walls of the crankcase, these early engines are not however considered as long-bolt engines.
The final developments of the aircraft piston engine were the horizontally-opposed H-engines such as the Rolls-Royce Eagle and the Napier Sabre
Napier Sabre
The Napier Sabre was a British H-24-cylinder, liquid cooled, sleeve valve, piston aero engine, designed by Major Frank Halford and built by Napier & Son during WWII...
. These compact engines were tightly packed and, unlike the previous V engines, there was no access to the crankshaft or its bearings once the two halves of the crankcase were assembled. Accordingly long through-studs were used, passing right through the engine from one side to the other. On the Sabre, some of these studs were short and served only to clamp the crankcase halves together. The others passed between cylinder heads on each side. The entire tensile force across the engine was taken on these studs, with no tension being placed on the crankcase.
Cars
In the early 1980s, FiatFiat
FIAT, an acronym for Fabbrica Italiana Automobili Torino , is an Italian automobile manufacturer, engine manufacturer, financial, and industrial group based in Turin in the Italian region of Piedmont. Fiat was founded in 1899 by a group of investors including Giovanni Agnelli...
invested heavily in robot automation throughout its factories, to both improve product quality and to reduce labour costs. The new Fully Integrated Robotised Engine
Fully Integrated Robotised Engine
The FIRE is a series of automobile engines from Fiat. It was designed by Italian design firm Rodolfo Bonetto . It is constructed by robot assembly plants to reduce costs....
was developed as part of this, to replace the previous range of small Fiat engines. Although taking the opportunity to update performance and emissions from Fiat's venerable 903cc pushrod engine, the primary goal of this new design was for easy assembly by robots. Key to this was the 'layer cake' construction of the new engine, held together by its long through bolts.
The FIRE engine has been regarded as a success, having produced efficient and reliable engines from 769 cc with growth into 1368 cc and 16 valve versions. The 1242 cc 16 valve 'Super FIRE' also used a ladder frame main bearing carrier, giving a crankcase and block more rigid than any comparable engine in its class. One drawback to the FIRE engine was a perception by mechanics servicing them that it was now difficult to perform a top-end overhaul without also requiring the bottom end to be dismantled, with re-assembly and re-torquing in strict sequence.
In the late 1980s, Rover
Rover Group
The Rover Group plc was the name given in 1986 to the British state-owned vehicle manufacturer previously known as British Leyland or BL. Owned by British Aerospace from 1988 to 1994, when it was sold to BMW, the Group was broken up in 2000 with the Rover and MG marques being acquired by the MG...
were aware that they needed to adopt innovative and reliable new techniques if they were to survive as a car maker and, like Fiat, to throw off their previous problems of poor build quality. One result was the K engine
Rover K engine
The K-Series engine is a series of engines built by Powertrain Ltd, a sister company of MG Rover. The engine was built in two forms: a straight-four cylinder, available with SOHC and DOHC, ranging from 1.1 L to 1.8 L; and the KV6 V6 variation....
, to be built by Rover's partner Powertrain Ltd
Powertrain Ltd
Powertrain Ltd was a British company based in Birmingham which designed, manufactured and marketed car engines and transmissions. It was a subsidiary of Phoenix Venture Holdings and a sister company to MG Rover...
. This used a ladder main bearing in all models, giving an extremely rigid block that would later permit a very high redline speed in applications such as the MGF
MGF
MGF may stand for:* MG F, a 1995 mid-engined, rear wheel drive roadster manufactured by the Rover Group* Machine Gun Fellatio, an Australian alternative band* Magnesium fluoride...
, Lotus Elise
Lotus Elise
The 1996 Lotus Elise weighed . Because of its relatively low weight, it was able to accelerate 0- in 5.8 seconds despite its relatively low power output of...
and for racing. Unlike the FIRE though, and despite the large numbers sold that served reliably for many years, this engine was tarred with Rover's past flaws and gained a reputation as unreliable. The flaws that gave rise to this reputation were caused by only some components used in only some models: particularly the head gasket
Head gasket
A head gasket is a gasket that sits between the engine block and cylinder head in an internal combustion engine. Its purpose is to seal the cylinders to ensure maximum compression and avoid leakage of coolant or engine oil into the cylinders; as such, it is the most critical sealing application in...
and wet liners. Regarding these problems with particular component versions as indicative of poor overall engine design was undeserved. Equally, none of these problems are inherent with the techniques of wet liners or through-bolting.
Advantages
For modern car manufacture, the long-bolt engine offers several advantages:- Simpler assembly, with fewer components, fewer fasteners to tighten, and fewer intermediate assembly steps.
- Tensile forces are taken on the studs, not the cylinder block or crankcase. In particular, the force is a pure tension, rather than a twisting force owing to any offset between the bearing cap and cylinder wall bolt. This permits the block and crankcase to be made lighter and less stiff, whilst still reducing the amount of deflection and twist exhibited in service.
- When a ladder-frame main bearing cap is used, the crankcase assembly becomes considerably stiffer.
These advantages are primarily for the initial production of engines, particularly when this is a robot-based assembly. Secondary advantages may also be gained of improved service life.
Disadvantages
There is less, if any, advantage for ongoing servicing during the life of the car. Some aspects have clear disadvantages, particularly when previous simple servicing operations are made more complicated. This is acceptable because of the greatly increased intervals between major servicing for modern cars. Many cars today will go for their entire lifetimes of over 100 thousand miles without having their engine removed or dismantled.
Specific disadvantages are:- The torque schedule for re-assembling the long studs may require both ends to be dismantled before a complete re-assembly from scratch. A usually simple task of removing the top-end for attention to the valves may now require the bottom-end to be dismantled as well, often requiring the whole engine to be removed.
- Short-cut re-torquing practices that avoid dismantling both ends may be less reliable than an engine assembled, as intended.
- The construction is 'different', thus untrusted and collects the blame for any unrelated ills that the engine may be prone to. This has been a particular issue for the Rover K engineRover K engineThe K-Series engine is a series of engines built by Powertrain Ltd, a sister company of MG Rover. The engine was built in two forms: a straight-four cylinder, available with SOHC and DOHC, ranging from 1.1 L to 1.8 L; and the KV6 V6 variation....
.
Example engines
- Rover K engineRover K engineThe K-Series engine is a series of engines built by Powertrain Ltd, a sister company of MG Rover. The engine was built in two forms: a straight-four cylinder, available with SOHC and DOHC, ranging from 1.1 L to 1.8 L; and the KV6 V6 variation....
- Fiat FIRE (Fully Integrated Robotised Engine)Fully Integrated Robotised EngineThe FIRE is a series of automobile engines from Fiat. It was designed by Italian design firm Rodolfo Bonetto . It is constructed by robot assembly plants to reduce costs....
- MercuryMercury MarineMercury Marine, founded in 1939, is a division of Brunswick of Lake Forest, Illinois, in the United States. Mercury provides engines for private, commercial and government sales. Mercury also has its own line of very successful racing engines tailored for power and speed. The company's primary...
Verado outboard motor