crank rebuild question

hillelson1 · 02-14-2006, 07:51 AM

hi i am rebuilding a 89 3.3 turbo engine. the #4 rod bearing disintegrated after an extensive rebuild any ideas why it may have failed. also a few of the top rings on the pistons were broken in like 5 pieces and when i pulled the cylinder off the pieces just fell on the ground. i bought the motor broken this way so i have no idea what happened. i have taken the motor all apart except for the the case. i dialed the crank and it is within spec with no major damage the only damage on the bearing surface is some light scratching. can i just polish this out? and if so what should i polish the bearing surface with?

Jeff Alton · 02-14-2006, 03:45 PM

I am just guessing, but it sounds like detonation. Send the crank to knowledgeable machine shop and have them inspect it to see if it can just be polished.

Cheers

MBruns · 02-14-2006, 04:59 PM

Send it to Armando at CCR,Custom Crankshaft Rebuilders in Ca. I have his phone # at the shop but he is in the 650 area code.
Mike Bruns JBRacing.com

hillelson1 · 02-14-2006, 05:05 PM

i really don't want to pull the crank case apart if i don't need too.

andrew15 · 02-14-2006, 05:31 PM

It would probably be a good idea to tear the whole thing down - if a rod bearing went and a piston started to break rings, there is a good possiblity that there are bits of metal floating around in there.
AM

hillelson1 · 02-14-2006, 05:38 PM

putting the whole crank case through a parts washer a few times wont be enough to take the parts out?

andrew15 · 02-14-2006, 05:46 PM

It could, but it could also lodge some particles in a most unfortunate place and kill the engine dead.
AM

hillelson1 · 02-14-2006, 05:49 PM

i still would like to know if my crank is good or not if it isn't i'll prob just go with a used engine. or used bottom end

ChrisBennet · ChrisBennet's Garage

Don't even consider using the bottom end of your motor as a catch pan for whatever sort of abrasive you intend to use on your rod journals.

If a rod bearing disintegrated you need to tear the motor down completely, pull the oil gallery plugs and clean everything out. You also need to clean the oil cooler (ultrasonically).

On top of that, you need to pull the car's oil system, have the front oil cooler cleaned (ultrasonically), clean the thermostat out and disassemble the oil line fittings and clean them also. You will need to do this even if you get another motor.

Your Porsche's crank is nitride (hardened) so if you cut the crank you will be exposing softer metal. The balancing place I use charges ~$135 to crack check and polish a crank.

Repairing a crank is a rather expensive business requiring the plugs to be removed, journals welded up and cut and then the whole thing needs to be rehardened. Most Porsche guys go with a good used cranks rather than spending almost as much to have one repaired.

Expect to pay $1100 to $1300 for a used 930 crank - if you can find one.

-Chris
Edited for typos.