Piston Debris

[richmatt]

A 1984 Chevy 3.8 V6 229Ci with one sad looking piston - but where have the bits gone? <br /><br />In trying to free the engine I got fed up standing on a wrecker bar attach to the harmonic balancer (about 450Lbft of torque) and took the heads off - all looked Ok except for No 1 piston which is missing a 1/3 segment of the crown and top piston ring. The piston itself has impact marks which judging by the shape and size are from the ring but the head and valves show no signs of damage whatsoever. Is it possible that the debris was blown out the exhaust?<br /><br />As the motor still didnt free up I stripped it down to the block expecting to find all sorts of debris in the sump but it was clean apart from some water and oil and emulsion. All the bearings appear to be in good condition as well. It turned out that the main bearings were binding becasue if free up as soon as I took the caps off - but I cant understand why as they appear to be good as well. <br /><br />Any ideas on where the piston bits went and why the main bearing would have got bound up so tight

 

[kenimpzoom]

Re: Piston Debris

Could you have bent the con rod or bent the crank? This may cause the binding.<br /><br />Also, were there any nicks in the cylinder wall that would cause the piston to hand up?<br /><br />As for the bits, only one place to go, thru the valves.<br /><br />Ken

 

[CCrew]

Re: Piston Debris

"Any ideas on where the piston bits went and why the main bearing would have got bound up so tight"<br /><br />I'll agree, the piston bits are probably out the valves and either into the manifold, or out the exhaust. <br /><br />As far as the main bearings, I'll lay money there's metal "dust" from the pistons embedded in them. Since the bearing clearances are small anyway, wouldn't take much to bind the crank. While the bearings may not look scored, I'd venture to say that a magnification will show metal particles that don't belong.

 

[richmatt]

Re: Piston Debris

The bore seem to have escaped with only two minor scratches each about the depth of a single sheet of thin paper, ie just enough to catch a finger nail in. <br /><br />Other than honeing, I was toying with the idea of leaving the bores alone as they are hardly worn - I know I'll have lost some compression and may burn some oil in that piston but what else can happen? <br /><br />The conrod looks straight but I havent really tested it. How would I test it anyway - side by side comparison with another rod is all I can think of at the moment.<br /><br />The crank runs free and easy under its own weight in the upturned block and 1/2 bearing shells with no obvious tight spots - maybe the next step would be to clean out the other 1/2 of the bearings, reassemble and then see if its still tight.

 

[kenimpzoom]

Re: Piston Debris

Also check your valves, valve seats and ports in the heads VERY closely. I personally would take the valves off that cylinder and check them for straigtness.<br /><br />And buy some new main and rod bearings, since you already have them off.<br /><br />I think any machine shop can check your rods for straightness. Also can check the crank too.<br /><br />Ken