Deep Singh

Can someone explain what it actually means when people talk about a big end?
Also what is a short motor?
What is a long motor?

andy97

iTrader: (1)

Hi I'm no mechanic, so here goes. There are big ends and little ends, the big ends are the bearing shells connecting the crank to the con( connecting )rods and the little ends are the bearings in the other end of the conrod attaching the piston.


Short motor- complete engine except with no cylinder heads
long motor -a complete engine with the cylinder heads attached
Andy

johnfelstead

long motor is the engine assembly complete with heads.
short motor is everything below the heads (engine block, pistons, crank etc.)

The big ends are the name for the bearings that are used between the crank and the connecting rods. All the force from the combustion process is passed via the piston, through the gudgen pin, through the little end bearing of the connecting rod into the connecting rod, through the big end bearing and then into the crank, which transfers this energy into rotational motion.

So the piston goes up and down in the cylinder bore, this pushes the conrod down, which follows the rotational path of the crankshaft.

From this you should be able to see there is a lot of force on the big and little end bearings, so these need to have a good oil fim between the bearing surface and the crank/gugeon pin to allow them to run freely. If the oil film breaks down or runs dry the bearings start running metal to metal, they get hot, then either sieze solid or wear out and spin, giving large clearances which reduces the oil presure. You then get a sloppy fit between the crank and conrod so the force in the piston pushing the conrod down hammers the crank at the big end, which is why you hear a thuding noise when the big ends fail.

Deep Singh

Thank you. Thats an excellent explanation. Saw you on Men and Motors btw, nice driving!

Deep Singh

More questions.
Can detonation by a transfer of power cause 'big end failure' before it actually melts a piston?
If not, does that mean all big end failures are due to some kind of oil issue,or inherent weakness in the big end?

Cheers, Deep

john banks

Yes, by a transfer of shock wave. The cylinder pressure can be 10 times higher than for normal combustion.

Detonation can typically damage the ring lands, which are the areas which form channels for the rings to sit in. It can also give pitting on the piston crowns.

Arguably, pre-ignition is more likely to catastrophically just melt a piston, this is usually far more damaging than detonation, it occurs before the spark plug fires rather than after, and so the pressure wave occurs at a really inconvenient piston position.

Deep Singh

ok, so do you think that the majority of big end failures are due to fuel issues( ie poor fuel quality/supply of fuel or filters,decat needing more fuel) causing det/preign or is it due to oil issues?

john banks

Poor fuel or oil quality or supply problems can certainly contribute.