HakosukaJD Posted July 11, 2006 Share Posted July 11, 2006 I've searched through and found a couple V8 piston slap threads...but nothing really on the L6. I'm reasonably sure it is piston slap as per the following symptoms. It is noticeable only at startup and while the engine is cold. I can hear it between 900 RPM to about 1500. If I'm cruising along (or accelerating) and step on the clutch, it becomes much louder as the engine RPMs fall (and sounds a great deal like a diesel engine). After about three minutes, it goes away completely. So my question is, common thing among L6's? The build is (from what I know) almost completely stock. I went to a thicker weight oil to see if I could make it go away but no such luck. Should I be overly concerned...or just give the engine more time to warm up before I drive off in the morning. Is it going to get much worse during a short period of time? Or just a nuisance? Quote Link to comment Share on other sites More sharing options...
jgkurz Posted July 11, 2006 Share Posted July 11, 2006 I had a slight piston slap problem with a mildly upgraded N/A L28 in my 78Z. It was noticable when the car was fully warmed up and idling. The casual observer probably never noticed. FWIW, I abused that engine for years and it never got worse. Quote Link to comment Share on other sites More sharing options...
rags Posted July 11, 2006 Share Posted July 11, 2006 Wow. This thread really has my attention as I have the same symptoms on my turbo 3 liter L6. It has done it since it was first rebuilt. I have built a new 3 liter shortblock but will wait until this one give up before doing the swap. So far it has been going strong for 3 years and the next track day is this coming Sunday. Joe Quote Link to comment Share on other sites More sharing options...
TBK Posted July 11, 2006 Share Posted July 11, 2006 sounds more like mains to me, any idiea if everthing was plasti guaged when installed? Quote Link to comment Share on other sites More sharing options...
Pyro Posted July 11, 2006 Share Posted July 11, 2006 All forged pistons have this problem. They are set-up loose when cold, so they can expland to the proper clearance when hot. If you have stock cast pistons then this could be a problem. This indicates engine was bored to big for cast pistons. The cold bore clearance on cast pistons is about 0.0015" and 0.0050" for forged. As the forged piston warms up, it will expand to the tighter tolerance. The cast piston doesn't change that much. As long as the noise goes away you should be OK. Continious piston slap will wear the bores and pistons out quickly. Yes, let it warm up before getting heavy on the throttle. Do you have cast or forged pistons? Quote Link to comment Share on other sites More sharing options...
TheNeedForZ Posted July 11, 2006 Share Posted July 11, 2006 If the problem is indeed piston slap, over time, the pistons could slowly "chip away" when they experience slap due to the impact between corners of pistons against bore wall. But they might not. The cheap and temporary fix is to knurl the pistons and turn down the diameter to fit the bore. Quote Link to comment Share on other sites More sharing options...
Pyro Posted July 11, 2006 Share Posted July 11, 2006 Piston coating can also add thickness. Quote Link to comment Share on other sites More sharing options...
zcarnut Posted July 11, 2006 Share Posted July 11, 2006 The stock Nissan pistons have the centerline of the piston pin offset by 1mm with respect to the piston centerline just to reduce piston slap. However, I have measured some aftermarket pistons that do not have this feature, so as a result the piston slap noise is correspondingly higher. Quote Link to comment Share on other sites More sharing options...
TheNeedForZ Posted July 11, 2006 Share Posted July 11, 2006 The stock Nissan pistons have the centerline of the piston pin offset by 1mm with respect to the piston centerline just to reduce piston slap. However, I have measured some aftermarket pistons that do not have this feature, so as a result the piston slap noise is correspondingly higher. Thanks, valuable info. Quote Link to comment Share on other sites More sharing options...
HakosukaJD Posted July 11, 2006 Author Share Posted July 11, 2006 Unfortunately, I was "told" the engine was put together from stock parts. So I really have no idea. What I do know...is that after about three minutes, the slap goes silent. If I were to take it apart to see what is really in it...I would likely rebuild it as a stroker anyway.... Thanks for the replies! Quote Link to comment Share on other sites More sharing options...
naviathan Posted July 12, 2006 Share Posted July 12, 2006 Doesn't piston slap occur if one of the pistons was put in backwards? I remember something from TonyD on this subject, but I can't find the post. Quote Link to comment Share on other sites More sharing options...
johnc Posted July 12, 2006 Share Posted July 12, 2006 If it goes away after then engine warms up, IMHO its not worth tearing the engine down to fix it. Just take it easy until the engine is up to temp. On the 3L I had that used JE forged pistons it slapped until the coolant temps reached 150. On my motorhome the 8.1L Vortec V8 slaps like crazy for about a minute after a cold startup. Quote Link to comment Share on other sites More sharing options...
rags Posted July 12, 2006 Share Posted July 12, 2006 Thanks guys. I feel much better now as i do have forged pistons and I did set it up for a pressurized motor where the piston manufacturer (Ross) recomended a fairly large piston to cylinder wall clearance. I'll just continue to let it warm up before beating on it Joe Quote Link to comment Share on other sites More sharing options...
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