Car Experts: What is the "Bottom End"?

I was asking the auto parts store about noise (pinging and knocking) in an older car, a 2002 Mazda with almost 120,000 miles. While discussing a fuel injector cleaner, he ultimately recommended a pressure test on the engine (having already had a recent tune-up). He said something about how the source could be top-end noise (which I assume is compression issues with low-octane) or bottom-end noise? It sounded to me like he was talking about noise from the end of the stroke. Yet, what happens at the end of a stroke? I never heard anything about the “bottom end”*, and I come from a long line of cars with problems! *In passing, he mentioned an oil additive to better coat moving parts. So, maybe the “bottom end” refers to the the crank case?

OK, to avoid assuming anything: (a) What causes top-end noise? I WAG low octane, clogged fuel injectors, and/or spark plug fouling. And, (b) What is meant by the the “bottom end” and what are the common causes of “bottom end noise”? Thanks!

here’s a quick Google result.

Another way to simplify is above the head gasket is top end, below it is bottom end. Car guys, is that about right?

I’ve had friends with diesel trucks tell me that Bottom End refers to the crank shaft and associated parts, including the rod bearings. As always, I may have been misinformed.

Knocking/pinging noises have a few causes, mostly related to filth & wear inside the combustion chambers (basic explanation).

Bottom end is the part of the engine below the head gasket. Generally, the bottom end is the beefy bits where the work is done and it’s relatively robust. Top end, or head(Go to about 1:25 in the video to see the head come off the block), is where you have intake and exhaust valves, cam shafts, sparky bits, fuel injectors–the mechanical brains of the engine. It’s much more fragile and given to wear/tear issues.

Top end noise would be like rattles & clicks caused by wear and stuff becoming misaligned over time. Think of several dozen steel tinker toys moving at really high speeds and with very tight ranges of motion. Wear happens and the movements get a little sloppy, parts start smacking into surfaces and things get noisy. Generally you’ll see some power loss as well because valves aren’t closing fully and ignition chamber pressure leaks out rather than forcing pistons down. Most times if you just get the head rebuilt you can get brand new performance without messing about with the block & pistons. Maybe you scrape the carbon off the piston faces while you have the head off, but that’d be about it.

GaryM is correct. It usually refers to the crankshaft, main bearings, and rod bearings.

When I hear a fellow mechanic mention bottom end noise, I assume they’re talking about worn crankshaft bearings.
The main bearings are what the crank rotates on, and IME, don’t give trouble as often as the big end bearings, which connect the crank to the conrods and hence are subjected to much of the reciprocating forces in the engine.
When any of these bearings wear out in a common engine, it tends to be cheaper to swap the engine out than rebuild, and it may often be cheaper to change the car (vehicle dependent, a newer, high end car could well be several times the price of an engine or a rebuild).

A (possibly) small point I will make is that while ‘pinging’ and ‘knocking’ are onomatopoeic terms used by laymen to describe noises made by their cars, they are also used by the trade to refer to preignition or detonation, a potentially damaging phenomenon which can be caused by low grade fuel, electrical gremlins, a failed sensor and probably other things I’ve forgotten. This would cause symptoms like misfiring under load and lack of power, but probably not much in the way of actual knocking sounds.

WRT your actual issue, while I’ve known plenty of people claim good results from oil additives, I’ve always regarded them as a short term measure and would certainly recommend you get the car looked at by a competent mechanic if it’s making a mechanical knocking noise.

When does it make this noise? Is it at tickover? Acceleration? High, low revs? Constantly or intermittent? Have you tried or been able to replicate the noise while listening under the bonnet? These are some questions any good mechanic will want to know the answers to, to give them a head start on diagnosing the problem.

Although I can’t say I’ve ever heard them referred to this way, but yes, the ‘bottom-end’ is the engine block (and everything inside it) and the ‘top-end’ is the cylinder head(s) (and everything inside it/them). Suffice it to say that problems in the so-called ‘bottom-end’ are much, much more serious than in the head.

When purchasing a new or remanufactured engine if it includes the head it’s called a long block, if not it’s called a short block.

You can conclude that the whole engine is quite worn out if the crank bearings are worn, so that means the crank and block (cylinder surfaces, corrosion of the conduits,etc) are worn too… Or at least you have to redo every crank bearing if one is gone…

Is it worth spending money on when you can buy a younger engine ready to go from a wreck ? Also the mechanic doesn’t want to repair a crank , as its hard to be sure that a repair is accomplished correctly first time… if you drive across the continent and discover its not correctly rebuilt… your engine is wrecked,and you want your money back on the labour and the parts…expensive parts.

And don’t confuse them with small blocks and big blocks. Totally different. :slight_smile:

Top end noise would be caused by worn or maladjusted rocker arms, or any gaps in the valvetrain. Tick tick tick tick tick tick. Solution is a valve adjustment. Everyone selling a used car will tell you that the ticking is in the top end because the fix is cheap.

Bottom end noise would be a spun bearing, essentially meaning the engine is toast and needs to be replaced (because it’s cheaper than a rebuild). Knock knock knock knock knock knock.

Pinging would be a detonation issue. Bad fuel injectors or plugs would cause a misfire, not detonation. Using the wrong octane (not likely in this case) would cause detonation, or excessive carbon buildup.

Exhaust leaks can often sound a lot like pinging or ticking.

This is my understanding. The reciprocating and rotating assemblies are usually referred to by the short hand “bottom end.” The “bottom end” would include the block and oil pan/pump, but the interesting bits are the moving parts attached to the crank.

Thanks to all for the replies.