Kevin G

Hello,

Could anybody explain what 'piston slap' is?

Am researching Subaru's at the mo with a view to possible purchase very soon :-)

Cheers

Colin Berry

Err, I think the pistons are slightly too small for the bores

JohnD

iTrader: (5)

In some engines piston slap can occur even when all components are within tolerance. In modern engines the pistons tend to have a short, solid skirt to reduce inertia, friction and cope with the higher temps. The lack of skirt below the gudgeon pin and no expansion slits often means a reletively slack cold fit. But this is not the whole story. Modern engines tend to run with a fairly high ignition advance and this promotes slap before running clearances are reached. Two examples of this :- 1) I once was the proud owner of a Nissan Sunny 1.5 (1983)which had very audible piston slap right from new, my kids thought I had bought a diesel! I tried different ignition advance and this reduced the slap a little but so did the performance.I tried removing the vacuum advance - hey presto no slap! unfortunatly the performance was crap
2) Some of the 1.8 CVH engines in the Ford Sierra Sapphire displayed piston slap. When stripped tolerances were OK so Ford produced a device which retarded the ignition when cold, reverting to normal as the engine warmed up.
As a point of interest, the handbook for the earlier Sierra Cosworth actually makes mention of possible "piston noise" when cold and states that this is normal and should be ignored. This was addressed in later Cossy engines by offsetting the gudgeon pin a very small amount towards the thrust side of the piston, a technique also used by other manufacturers (Did Subaru do this on phase 2 engines?) The amount of clearance necessary to produce noise is very small. The thing to remember is to keep off the boost until the pistons have had time to settle at working clearance.
All the above does not address the problem of slap caused by wear, distortion or damage to pistons or bore, just to say that it can be a characteristic of certain engine designs.
JohnD

Jza

John D

Nice explanation!!

Jza

Kevin Greeley

As I understand it, the pistons are slightly too large for the bores and this causes a nasty 'knocking' or rattling type noise when the engine is cold. As the temp. rises, the pistons expand to reduce the gap and the noise disappears.

Kevin G (the other one).

Doh! I meant too small.

[This message has been edited by Kevin Greeley (edited 21 June 2001).]