Car mechanics - whats the "big end"?

Factual Questions
1

Anyone know what a “big end” is in a car engine? Apparently, once it goes thats it for the car.

Specifically I wondered:

  • what does it do?

  • where is it?

  • why is it so hard to fix?

thanks.

2

I’ve heard of a bottom end and a top end when discussing engines, but not the big end in this context. The bottom end being the block, crank, piston, etc. The top end being the heads, valves, intake, carb/fuel injection system. Maybe some locales call the bottom end the big end? If so, the reason the problem is serious if it is in the big/bottom end is that a complete engine rebuild is pretty much to only real solution. And, this will require completely removing the engine from the car (typically). And there can be damage down there that can make the engine not worth fixing. For example, breaking a connecting rod (connects the piston to the crank shaft) can cause catastrophic damage. Like smashing a hole in the block and beating the crank to pieces.

Probably not applicable, but I have heard the term “the big end” used around cars, but not to describe a part of the engine. It has been in reference to a drag racing track. The end of track, where the cars are at their highest speeds is usually called the big end.

3

You have a connecting rod that transfers power from the piston to the crankshaft. This con rod has a small ring that fixes to the piston and a big ring that wraps round the crank. It’s one of the first things to go if it doesn’t remain properly lubricated. I had an old motor that was at the end of its life which I’d decided to just run into the ground. Eventually, the cap come off the end of the rod, freeing it from the crank and the piston pushed the whole thing out through the side of the motor. Very impressive it was too.

4

Looking at this cross section of an engine, the connecting rod connects the piston to the crankshaft. The small end of the connecting rod attaches to the piston by way of a wrist pin; the big end wraps around a bearing journal on a throw of the crankshaft.

Or, for further reading, click here…

5

The big end is the bearing at the big end of each con rod, as mersavets said.

If it “goes” it is not by any means necessarily the end of the car. However, even if a big end lets go other than catastrophically as mersavets describes, it’s a reasonably big job to fix. You don’t necessarily have to pull the engine on all cars and in all cases, depending on what “goes” means exactly. You can possibly just work through the sump from underneath.

However, a big end going is usually a sign of bad lubrication and/or extreme wear, which means that the big end is unlikely to be the whole problem. And as mersavets has said, they often fail in spectacular fashion, and then they very often do wreck the engine entirely.

6

You sure can, and I’ve done it myself. All you need is a good engine hoist to hold it up while you remove the bottom supports, and faith that you aren’t wasting your time replacing bearings on a rod that is damaged. However, a friend of mine who is a better mechanic than I replaced his bearings, fixing two knocking rods, and drove the car for more than 30,000 more miles without incident (well, until it caught on fire :eek: ).

7

I once read a newspaper story about a guy who got his 50s something Cadillac limo to go 1.6 million miles by changing the oil every 3,000 miles and doing all the other maintenance. Remember folks, take care of your vehicle and you can drive it a long, long time. It’s a lot cheaper than payments on a new one.

8

Big end repair.
Shop that I once worked at had a 1974 Olds Toronado come in with a a rod knock. To remove the oil pan on this car ment either: pull engine, or tranny. Both big jobs. The mechanic that taught me was one of the best. He convinced the owner he could do it (repair the big end, new bearing) without pulling anything. He drained all the oil, cut a hole in the oil pan (sump for the Brits) R&R’ed the bearing, retorqued the cap, MIG welded the pan together, poured in oil. When I was told to start up the engine I whispered a prayer and twisted the key. It purred No knock, no leak. Over Twenty years ago. Car ran for three more years till the owner sold it and I lost touch.

9

Thanks for all the responses. I think Ive got the idea now.