piston slap

Top End (cylinders pushrods etc.)
Post Reply
socalrider
Senior Member
Posts: 550
Joined: Sat Jun 14, 2008 5:22 am
Bikes: 1963 pankenstein ; 1965 flh ; 1978 shovelhead lowrider
Has thanked: 67 times
Been thanked: 128 times

piston slap

#1

Post by socalrider »

so what are the tell tale marks of piston slap? i have wear marks developing on the fore and aft surfaces of my front cylinder. prior to pulling the head off, i did a intake manifold leak test which showed that i did in fact have a leak on the front cylinder. i read in a thread in the knowledge base that these marks are caused by overheating in the cylinder which can be attributable to a intake manifold leak. i was thinking piston slap could also be the culprit because i get a light knocking sound on a cold start up that completely disappears once the engine is warmed up. what do you men think? the wear marks are not deep so im hoping i can just put the head back on without pulling the cylinder off.
gracias,
arnulfo
Cotten
Senior Member
Posts: 6937
Joined: Sun Feb 10, 2002 2:30 am
Bikes: -
Location: Central Illinois
Has thanked: 112 times
Been thanked: 310 times

Re: piston slap

#2

Post by Cotten »

Arnulfo!

"Piston slap" usually conjures up the notion of a machine that 'knocks' a bit before it is fully warmed.
This is usually attributed to excessive skirt clearance, and the aluminum pistons seem to expand to suit their need.

However,
I have pulled well-used machines apart with excessive clearances, that only show minimal vertical damage.

Scuffing and galling of the skirt are to be blamed upon other things than merely clearance, and certainly the elevated temperatures from a vacuum leak don't help.

The possible list of other culprits are endless.

It you let the gremlin out,
it should run fine.

....Cotten
Last edited by Cotten on Wed Oct 12, 2011 7:29 pm, edited 1 time in total.
jdpan
Member
Posts: 461
Joined: Tue Sep 08, 2009 2:20 am
Bikes: 1952 Custom swingarm panhead
Location: Austin Tx.
Has thanked: 1 time
Been thanked: 19 times

Re: piston slap

#3

Post by jdpan »

"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."
Post Reply

Return to “Top End / Cylinders / Pushrods / Lifters”