[Supras] Puzzling sound-feel at rear of oil pan

[berniek at technicaldevelop.com]

When refreshing the JDM engine for my '91 with mild mods (550cc, Lex, 
etc.) I reused the stock pistons and honed out the little bit of taper 
in the cylinder walls (not much).  So the skirt and crown clearances are 
looser than stock, but supposedly are within the TSRM allowances.  Makes 
little difference anyway, because my intention is to run the engine 
fairly hard.


As a consequence, there is some piston slap, mainly on one cylinder 
toward the rear.  It is worst when starting from cold, does not appear 
at first with viscous oil, but appears worst as the engine warms up from 
dead cold without a load.  After a couple of revs, even in neutral, it 
appears to diminish, which may be because of the oil squirters opening 
and providing an additional lube cushion.


However, there is another very strange occurrence:  Listening with a 
stethoscope, there is a sound at the rear high area of the pan only, 
like pebbles hitting it from the inside.  In fact, I can feel with my 
fingers as I hold the diaphragm housing of the stethoscope.  It also is 
independent of speed.  The "pebble rate" is random, averaging every few 
seconds to a couple of times per second, and occurs at the same rate and 
intensity if the engine runs at idle or 2000 RPM.  I do not hear or feel 
that anyplace else on the pan.  There is absolutely nothing like the 
pebble sensation at the flange on the oil pan or anyplace else in the 
engine.  I know pretty well for certain that the bottom end is in good 
condition:  Although I still need to check oil pressure with a good 
gauge, the new stock transducer reads 40 PSI at 2500 RPM.  Moreover, 
there are no bottom end knocks when teasing the throttle at 2500-3000 
PRM, and no knocks when making the engine barely idle at the lowest 
speed when really hot by squeezing the new and flexible ISC hose almost 
closed with long nose pliers.


The mystifying aspect of the "random pebble" effect is that it does not 
vary with speed.  I can feel something tapping at a random rate inside 
the pan rear at both idle and 2000 RPM (convenient to do working 
alone).  If it is any help, the screen lip on oil pickup may be in 
contact with the taper in the pan at the rear since I moved the pickup 
back.  In moving the pickup back, I also had to cut away most of the oil 
pan baffle, which was not a neat job.  I also thought it might be dirt 
in the transmission clutch shaft bearing or play between the clutch gear 
and the cluster gear, but I would feel and/or hear that in more rigid 
parts of the engine or transmission, and nothing is present in that 
regard.  The area which is the head scratcher is only the rear, almost 
flat section of the oil pan. 


As always, any ideas would be appreciated.  Thanks.


    Bernie