I had a machine shop bore out a block for some bigger pistons some time ago (30k miles). For the past few thousand miles, when cold, I think I'm getting some piston slap (hollow knocking sound, definitely from the bottom end, doesn't seem to be affected by load, goes away after a couple minutes, I'm not down on power or up on oil consumption).
Assuming my diagnosis is correct, is this going to eventually wreck the motor? Did the machinist bore to the incorrect spec? Could I have assembled it wrong? Broken it in wrong? If I have to do another block, what should I do to prevent this from re-occurring?
Thanks...
GVX19
New Reader
3/21/11 12:33 p.m.
What eng. is it.
Just off hand I would say clean the carbon off the pistons.
I don't disagree with Mr. Pruitt but what oil are you using? 5W30? Try a 10w30 at next service to see if it reduces the "rattle time". If you are using a 10w30 try a 15w40.
How does a piston slap noise differ from a bad crank or rod bearing noise?
Serious question, but not as serious; $100 if you can type that sound out.
Thanx, Dan
Engine is a 16V in a 1986 Saab 900 (it's in my profile).
Factory specifies 5W30; I'll try a heavier weight and see what that does.
@914Driver -- I was told that rod bearing noise will change under load (hold brake, let out clutch, listen), but won't if it's pistons. If that's wrong (he isn't a professional mechanic), that'd be cool, as I'd obviously prefer to replace the bearings!
What type of pistons were the new ones? What type did you take out IE: Cast, Hyperutectic, Forged?
They all grow at different rates and require different bore clearances.
The machinist might not have know what you gave him so he could have used the sloppiest fit numbers.
44
44Dwarf wrote:
What type of pistons were the new ones? What type did you take out IE: Cast, Hyperutectic, Forged?
They all grow at different rates and require different bore clearances.
The machinist might not have know what you gave him so he could have used the sloppiest fit numbers.
44
The pistons are OE cast from a newer version of the same engine. Next time I do this I'll bring the factory specs.
If the pistons are OE cast then I think we might see your issue... hopefully when they opened up the bore they measured it against the new pistons ;)
Try a 10w30 for giggles.
In reply to 914Driver:
The pistons in my E150 make a flap sound
To the OP,
The older Triton engines are renowned for piston slap. My 5.4 has been doing it for the last 75K. I usually let it warm up in the yard until it stops. It usually takes less than a minute. Running 10-40 rather than 5-30 helps unless it sits for a while. Try the thicker oils.
On the other hand I had a 78 Fairmont with the I6 that developed piston slap and it broke the skirt off one in less than 20K. Probably due to my wife never letting it warm up before matting the gas pedal. She's always been hard on cars.
Piston slap is generally a lighter sound than rod knock.
Rod knock does not generally go away.
In the old days we used to have the pistons knurled.
We also checked piston to cylinder clearance with a thickness gauge aka: feeler gauge, a long one.
You can run an engine with piston slap for a long time.
Rod knock, not so long.
Rod knock will go away... just add nitrous or boost!
