Rod knock

[ham204stag]

Is 'rod knock' the same as 'piston slap' ?

[jbuckl]

+1

When you drain the oil strain it and look at the bits - will be just as informative as looking in the oil filter. If the big ends are knocking then they won't last that long anyway and the more you drive it the more damage you may do to the crank. It is possible when it was rebuilt that the crank was reground but not hardened. In that case it may well only last 10k miles before wearing.

You say the oil pressure rises but what to?

Is your car an auto or a manual? You can tell quite a lot about what is knocking by how it responds to engine load and engine revs - harder to analyse in an auto.

Roger

[1959terry]

Is 'rod knock' the same as 'piston slap' ?

Thanks a good question as i have never heard of "rod knock" Ive heard of knob rot but thats another story😁

[Jeff]

True rod knock results when the rod bearing gets wiped out. This happens when the hydrodynamic oil wedge is lost between the bearing surface and the connecting rod pin on the crankshaft. This could be due to one or a combination of several factors:

Insufficient oil pressure
Low oil viscosity
Insufficient oil flow in the bearing surface
Excessive oil temperature
Excessive combustion pressures
Overrevving the engine - high piston inertial forces

All of those can cause overheating or physical damage of the bearing material, which in modern engines is a lead free aluminum alloy. If the bearing material overheats it will spall and then can literally melt away, leaving a very large (in bearing terms) gap between the bearing surface and crank pin. Now the crank pin will start "slapping" the big end of the rod that encapsulates it.

True rod knock can turn into a walk home condition pretty quickly, as the now rather large bearing gap can lead to a broken connecting rod and the subsequent hole in the side of the engine block, as well as other cascading effects of a drop in oil pressure and bits of bearing material making it though to other parts of the engines

There are other noises that can be mistaken for rod knock:

Detonation
Piston slap
Piston pin ticking
Valve train ticking noises

Piston slap is nothing new to piston driven internal combustion engines and compressors. It is the secondary (sideways or perpendicular) movement of a piston against the side of a cylinder bore where the primary movement of a piston is intended to be parallel (up and down) to the cylinder bore. All piston driven internal combustion engines and compressors have a certain amount of piston slap.

Excessive piston slap occurs when the clearance between the piston and the cylinder bore is too great. The piston to cylinder bore clearance becomes too great either through wear, mismatched pistons and cylinder bores at manufacturing or, a combination of both. The audible noise associated with excessive piston slap is due to the perpendicular impact of the piston against the wall of the cylinder bore. Audible piston slap is typically loudest when the engine is first started up. The pistons then expand with heat reducing the piston to cylinder bore clearance thus, reducing the perpendicular impact of the piston against the cylinder wall and its resulting noise.

[1959terry]

Sounds very American to me

[Jeff]

And your point is?

For those not qualified, experienced or technically minded it still explains the difference.

[ham204stag]

And your point is?

For those not qualified, experienced or technically minded it still explains the difference.

Hi Jeff, I'm sure your post was totally different some time ago. Then it was funny, now it's.....pointed?

Nice explanation in your previous post, by the way. Thanks