WHY CAN'T GM FIGURE OUT HOW TO REDUCE PISTON SLAP? 1

I understand that Piston Slap is not new and not isolated to any one manufacturer, but it does seem like it has reached critical mass for GM these days. With the new piston designs and the ring package being pushed almost all the way up to the crown for efficiency purposes, is the the piston to cylinder bore clearance target (tolerance) all that small or is the assmbly line being that sloppy? My guess is a combination of both. I also understand that a little bit of piston slap (cold start knock) isn't necessarily catostrophic but, loads of the new GM products have piston slap that gradually gets worse, reduced compression in one or more cylinders, accelerated oil consumption at very low mileage and apparently increased vehicle emmissions. GM has TSB's out regarding piston slap and oil consumption on their 1999-2002 3.1, 3.4, 4.8, 5.3 and 6.0 liter products. Their new oil consumption TSB says burning 1 quart of oil per 2,000 miles is "NORMAL" as long as the engine has less than 36,000 miles on it.

How much oil can you burn before you start failing tailpipe emissions tests (undertsanding they are not all the same)?

How much oil consumption do the other manufacturers typically consider "NORMAL" on 1999-2002 vehicles?

Why does GM seem to be having a bigger problem with piston slap than the other manufacturers (not that the others aren't having some as well)?

There is a load of upset GM consumers over at

http://gmpistonslap.cjb.net

 

[evelrod]

My old 95 Chrysler LHS never used a drop in 136,000miles @ 6,000 mile service intervals. My 91 Dodge/Cummins has never used a drop in 170,000 miles @ 6,000 mile service intervals. My 2001 Lincoln LS has used no oil in 28,000 miles @ 5,000 mile service intervals. My VERY old Dodge 360 didn't start to use oil until after 300,000 miles!!!

I have used several Chevy 350 V8's in the last 20 years and they were generall ok up to about 65,000 miles at which point the engines all died ( 5 1981 Chevy C-2500 trucks and one 1988 chevy C-3500)

I guess you boys have a problem, eh?

Rod

 

Hy,
A few car manufacturers (European cars)write in Service guide booklets about oil consumption.
1. Alfa Romeo - 156 - 0,5l/1000Km. oil at first 15000 km
2. VW (on '90 years) on big engines - 0,3l/1000Km. at first 20000km.
3. I have experience with ESPERO model from DAEWOO, 1,8L SOHC (lot of this have problems with oil consumption because wrong ring profile - engine designed by GM)
I have not information about GM's that You claim
Sorry for my bad english

 

[bluemist]

As an automotive machinist with 20+ years experience, i feel i can add a little insight to the 5.3, etc. piston noise problem. The piston design in this engine encompasses a short, lightweight piston to reduce friction to allow for better fuel economy, faster engine revs, etc. However, the short piston must be fitted very tightly (.001&quot or the piston will rock and slap. GM' big problem is that the pistons fitted at the factory are fitted up to .002" or more.
They have had the same problem with the 2.2 ohv, 3.4 60^ v-6 and others. Our shop has checked many engines for this problem and GM's fix is either install correctly sized and fitted piston or replace the short block--at GM's choice. If the vehicle is out of warranty, the owner is generally SOL. Lately, GM using a coated piston in the newer applications and for piston replacement. The aftermarket (Federal Mogul) now offers many pistons with a moly/graphite coated skirt for many applications. Others are sure to follow suit, if they haven't already.