Why even have a head gasket?
Posted by indelico@reddit | askcarguys | View on Reddit | 66 comments
This could be an incredibly stupid question which would reveal my extensive gaps in knowledge of how a combustion engine works. But all is good on the quest for knowledge.
Why do engines need a head gasket? Why not make the engine out of one piece of metal where the compartments are insulated from eachother?
No_Mathematician3158@reddit
Rolls royce actually did this for a engine model. They weather the cast steel blocks outside with the heads and plain the heads and blocks to each other getting rid of the headgaskets.
MilesBeforeSmiles@reddit
If you ever get the chance to work on a jughead engine you'll learn why pretty quick.
VenomizerX@reddit
I mean, for builders or rebuilders or just people who work on cars in general, good luck working on a solid unified head + block.
johncartersamulet@reddit
The mercury 2 stroke straight 6 “tower of power” outboards from the 60s and 70s were like this, a single casting for the head, cylinders and half the crankcase made of aluminium. Had steel cylinder liners pressed into the casting. no gaskets other than for the water jacket.
mawzthefinn@reddit
That's the way it used to be done decades ago (so called jughead engines). They're harder to optimize combustion chamber shape and MUCH harder to machine valve seats and install valves.
Head gaskets are a pretty minimal addition for massive gain in the ability to optimize the combustion chamber design.
JustCallMeBigD@reddit
This.
Air-cooled VW engines were like this, although performance builders would sometimes use a copper ring as a crush gasket for spicier builds.
Mech_145@reddit
Aircraft piston engines are still this way
The-real-W9GFO@reddit
Which aircraft piston engine has a one piece head/cylinder? I’ve never seen one.
NoEmu5969@reddit
Lycoming and Continental engines use jugs. Name one EASA or FAA approved air cooled engine that doesn’t.
C402Pilot@reddit
Basically every single air cooled aircraft engine since WWI?
The-real-W9GFO@reddit
No, the head is separate from the cylinder; with a gasket between them.
C402Pilot@reddit
Every cylinder I've ever seen has the cylinder (effectively) permanently joined to the cylinder head with no gasket.
The-real-W9GFO@reddit
I stand corrected - sort of. The aircraft engine cylinders/heads are "effectively" one piece- but they are not machined from a single piece of metal, they are two parts that are screwed together while heated so that they function as a single piece - no gasket.
However, air-cooled VW engines are not made this way, each cylinder and head is separate, with a gasket between. Each cylinder is sandwiched between the head and the engine block held together by four long bolts.
experimentalengine@reddit
Minor correction, no gasket between on ACVWs, generally. You put some valve lapping compound on the cylinder, twist it back and forth in the head a bit, clean off the compound, and assemble without a gasket.
The-real-W9GFO@reddit
“Generally” perhaps but there are definitely both shims and gaskets that can be used between the cylinder and head on air cooled VW engines.
I think I’ve done four VW engine rebuilds. On at least one of them I used something between the cylinder and head, the reason has long been forgotten.
C402Pilot@reddit
Yeah I saw your other comment about having rebuilt aircraft engines before you deleted it. Why did you feel the need to lie about something like that?
The-real-W9GFO@reddit
Wow, way to call me out. How nice of you. I did not lie, and I think it is rude of you to make that accusation.
It's been twenty years since I've rebuilt an aircraft engine. It was a Continental if you must know. I was correct about the cylinder and head not being one piece - which was the main point the OP was making, but forgot about how those pieces were permanently joined together; without a gasket.
The-real-W9GFO@reddit
Yeah, I don't know what to say to you guys... I have rebuilt both air cooled VW engines and air cooled aircraft engines. They all have separate heads and cylinders - they are not machined out of a single piece of metal. The cylinders are sandwiched between the heads and the engine block, connected by long bolts. Maybe google it...
Mech_145@reddit
Nope
The-real-W9GFO@reddit
Which air cooled VW engine has a single piece cylinder/head? I’ve never seen one and I have rebuilt several.
JustCallMeBigD@reddit
None.
If you've rebuilt several, surely you know that the cylinders, which are not part of the block, are called jugs, thus making the head "jughead".
I_R_Enjun_Ear@reddit
There are a handful of motorsports applications that have/currently use this configuration in a modern architecture to run higher pressures. That said, their budget per engine is what allows this. I would wager the monolithic head+block alone costs more than a high power crate engine us plebs could source.
mawzthefinn@reddit
Yes, there are definitely reasons to do this (as well as separate heads with no gasket and some other form of sealing design), but all are relatively low production and tight tolerances where you give up ease of manufacture and service for max performance.
hondas_r_slow@reddit
Doing a valve job is a lot like seeing a proctologist for a root canal
Slow_Description_773@reddit
That's a very smart idea ! Let's replace a whole engine when a single valve fails !!
Environmental-Rub933@reddit
A lot of the time a head gasket replacement costs more than a brand new motor, not even used
9BALL22@reddit
Never heard of that, even if the replacement is a junkyard engine (that may also need a head gasket).
msletizer@reddit
Its very common. Heads are a huge pain to do while the engine is in the car, therefore they often have to take the engine out. If the engine is already out, it can be cheaper to buy a whole used engine than it is to pay the labor to replace the head.
A bad head gasket can mean the engine was overheated and could have warped or ripped out head bolt threads. Oil also mixes with coolant which could have wiped out bearings.
9BALL22@reddit
Thank you.
JSTootell@reddit
Friend of mine just did an engine swap instead of a head gasket. While the engine swap was more expensive, the difference in price was negligible.
The work was done at home too.
yyytobyyy@reddit
Maybe on a chinese scooter when you pay western labor prices.
Holiday-Poet-406@reddit
It's a job you can DIY if you have the time and a machine shop you can call on to skim the head. I've done 2 in my 33 years of motoring.
HobsHere@reddit
Do you work at BMW?
nerobro@reddit
Radials often have what amount to one piece cylinder and heads... It's just a few bolts to pull a cylinder off. Big diesels are often built the same way. Sometimes you can even replace a cylinder and head with the engine still running1
gravelpi@reddit
Let's not get crazy, a valve failure would only mean pulling the engine and taking almost all of it apart.
cormack_gv@reddit
You need to be able to install the pistons and valves. And even if you could do that through the cylinders, there would be no practical way to machine the cylinders and combustion chambers and valve seats out of a solid block.
MysticMarbles@reddit
Get rid of the head gasket, swap it for a butt gasket! /s
cormack_gv@reddit
I think that's called a pan gasket.
Thereelgerg@reddit
How could the components of an engine be one piece of metal while also being insulated from each other?
compu85@reddit
Detroit diesel 2 stroke engines don't have a head gasket, just fire rings.
Aromatic_Pie_9706@reddit
Apparently super high HP, alcohol dragsters use a fire ring as well. Just learned that myself recently.
Ok_Leader_7624@reddit
This whole time, I thought it was due to different engine parts warming up at different intervals and temperatures. I learnt today
Aromatic_Pie_9706@reddit
Not a bad thought, think about different materials, like cast iron block and AL heads. They grow and shrink at different rates. You can have issues where the gasket is “scrubbed” I personally don’t know much about this, but your assignment for tonight is to look it up and learn more!
Aromatic_Pie_9706@reddit
Maybe a better question is why do we still have shitty head gaskets. Capitalism?
hiker1628@reddit
One thing I thought of was a single block could be the basis for multiple engines merely by swapping heads.
Jacktheforkie@reddit
Cost, multi part assembly is easier and cheaper, head gaskets are dirt cheap when ordered in the thousands
Over_Pizza_2578@reddit
Manufacturing gets more complicated (valves dont sit parallel to the cylinder unless we are talking about a diesel) and the engine would get a whole lot longer. You cant remove pistons by pulling them towards the oil pan as the main bearings are wider than the pistons are apart from each other
Heavy_Choice_1577@reddit
hard to machine it whole and stuff all the important bits in afterwards
kick6@reddit
Everything the head is and does what have to be accomplished through a long stick all the way through the cylinder bore. It’s be like trying to fix a stomach ache by cramming things up your butt. Maybe doable, but horribly inefficient.
john464646@reddit
I think Henry Ford invented the removable head
Holiday-Poet-406@reddit
To make the head and block have a gas tight fit.
Sparky62075@reddit
The main reason is to make the engine easier to assemble and easier to repair if something goes wrong with the valves, camshaft, or cylinder heads.
There is another reason. If the engine builds too much pressure or overheats, it's cheaper and easier to fix and safer for a gasket to blow rather than having the entire engine explode.
Milnoc@reddit
As someone who's had an exhaust valve blow on him, I can confirm this. It was quite a bit of work, but being able to pull and work on just the head made the job accessible and somewhat affordable.
Khandious@reddit
An example of this would be:
If a Piston Pin fails and wrecks the block, you can replace the block for thousands cheaper than replacing the components of the head as well
A Head once assembled can have: Injectors, Plugs, Hold Down Clamps, Internal Harness, Engine Brakes, Selonoids, Valves, Valve Springs, Cam(s), Cam followers, Accumulator, Fuel tubes, Valve Cover, Gaskets, Cover Plates, Mounting Studs, Mounting Cap screws, and a Bunch of Head Bolts
kinga_forrester@reddit
Porsches and some others don’t have head gaskets.
Particular_Plum_1458@reddit
Was just going to post this, think it was the 964 which didn't originally have a gasket (I assume they thought they could machine them flat enough), but I think they had to retrofit due to leaks. In response to the OP, I imagine the machining of the lower face would be a nightmare if going down the cylinder.
jasonsong86@reddit
I thought it’s the cylinder liner? Some cars in the late 90s decided not to use steel liner but a coating on the aluminum cylinder walls and the high sulfur in the U.S. gas was eating into the coating.
jasonsong86@reddit
It’s hard to make it one piece. A lot of cars share parts between engines. Honda for example different heads can be used on the same block.
Tomytom99@reddit
Honestly this is a great question, because simple thinking would lead you to think as long as it's smooth enough of a mating surface you won't need a head gasket.
The unfortunate reality is that it's super difficult to reliably machine a surface that smooth, and things are going to want to warp as they heat up. Not as big of a deal on small single piston motors, but once you add more cylinders that head grows in size, and so does the thermal expansion and contraction. The gasket just creates a somewhat forgiving interface for the block and head to seat together, allow some expansion and contraction, and compensate for a level of unevenness in the mating surfaces.
AlaskaGreenTDI@reddit
Fun fact, Deutz and a few others have built engines with machining quality that didn’t require a hg, but if you ever tear them apart there are gaskets available so that you don’t have to meet the original spec. So like others have said, the precision required isn’t worth it.
Rare_Improvement561@reddit
The engine has to split in two in order to be assembled/repaired. That’s just how we’ve all agreed they work best and should be designed, and have spent the last almost 100 years building off of and improving upon this basic near universal design. In order for a proper seal to be made where the two halves of the engine meet; a gasket is needed.
Sometimes when a vehicle has a head gasket failure, it’s not even the gasket itself that fails. The head can warp and block can crack which will break the seal, and the head gasket will be completely intact.
Bring_back_sgi@reddit
Gaskets are there to fill in any gaps in the imperfection of manufacture of two facing parts. They are cheap and generally effective (rather blow a head gasket than a cylinder wall or piston head). IF you could manufacture the facing components of a cylinder head and block such that they were PERFECTLY FLAT AND SMOOTH and given a perfect and uniform attachment, then you could ostensibly not need a gasket. But then you would lose that advantage of absorbing potential damage caused by excessive detonation.
TPSreportmkay@reddit
It's difficult to machine the valve seats and pretty much impossible to repair. Since head gaskets are not a real issue for the OEM it's not worth it to them to figure it out.
Really high performance stuff sometimes gets away from head gaskets by having tight flatness and surface finish tolerances with o rings. Some air-cooled engines don't have head gaskets either but still have a separate head.
The most feasible way to do this would be in a 2 stroke gas engine since they don't have valves in the head.
nerobro@reddit
This is a good question.
First, you don't need a seperate head. You can, with proper tooling, build a cylinder and cylinder head, that are one peice, eliminating the need for any kind of head to cylinder seal. This was how they used to do it.
Assuming we're talking modern engines.... you need a seperate cylinder head. You need a seperate head, from the block, or cylinders because the geometries, and casting complexity, or machining complexity says you need the part to be accessible, and you can't do that working up inside a tube.
Also, the material properties that make for a good cylinder head, are not the same that make a good cylinder, or even engine block.
Formula 1 engines don't have headgaskets. There are cylinder heads for dirtbikes that don't have headgaskets. Airplane engines typically don't have headgaskets.
Formula 1 engines, have super precisely ground heads and blocks, and lots of bolts to hold them together. I suspect they grind dishing into the head to provide the clamping forces they need... But I haven't heard that confirmed.
Airplane engines have a spigot design, and the cylinder screws into the head, providing the seal.
Dirtbike engines, will use a gasket to seal the water jacket, but often just clamp the aluminum head to the steel liner inside.
So those are the negative examples. Why do most engines use headgaskets? Getting a perfect seal bewteeen surfaces is hard. So a gasket takes up the slack between the surfaces. Also, if something goes wrong, a gasket is easy to replace without needing new, major, parts. Headgaskets also allow some movement between the parts of the engine, this prevents cracking of components.
Equana@reddit
Some early engines were designed that way. The cylinders were bolted to the crankcase that carried the crankshaft. It was done because head gasket technology was not very good. The Offenhauser race engine was built that way. That 2.65 liter 4 cylinder was turboed to 1200 hp.
Why don't we do that now?
1) It is a bitch to machine the valve seats 2) It is a bitch to assemble the block the the crankcase at the same time the rods are being bolted to their caps. and 3) Head gasket technology is much better than 80 years ago.
Muttonboat@reddit
Its cheaper to put a gasket in their then it is to machine a piece of metal to where it wouldnt.
Also its cheaper to replace a gasket then it is to replace and engine that been machined to that spec if it fails.