AAZ teardown... good news and bad news

[Vincent Waldon]

The good news... intermediate shaft bearings in good shape:




I'll let the bad news speak for itself:





All 4 pistons are exactly like that.  The head is brand new so it looks like the PO did the head but ignored the pistons.... the crank slot was very worn so I'm guessing it met the fate of most early AAZs.

1.9 pistons are spendy little creatures and these are in very good shape otherwise... ring lands are fine and there's very little scoring or buffing on the skirts.

I'm a first-cabin-all-the-way kinda guy, but in this case I'm tempted to carefully sand down the mushrooming, check em for weight balance, and reinstall with fresh rings etc.... the head of a diesel piston is very thick.

Anyone wanna talk me out of it  Seen worse !!!


Vince

[Henchman]

So I am not familiar with this problem, but now own an AAZ and am very interested in avoiding the same thing.  It looks like a valve hitting the piston, probably due to a timing issue.  Is this what you mean by a worn crank slot?  How do I avoid the same fate?

Thanks,

Ian

[lord_verminaard]

They should be fine.  I might even take them to a machine shop and have them shave a thou off the top of the piston.  That would remove any warping or distortion from the impact and while it would not "erase" the damage it should be fine.  I know you are not supposed to do that because you shorten the distance from the first ring land to the top, etc... but like you said those pistons are built pretty solid.  Think of it this way, now you can run higher boost with your lowered compression.  

Brendan

[subsonic]

Perhaps clean up around the edges of the impact area, and ceramic coat.  Perhaps a extra coat or two in the effected area to build it up.  Balance may be another issue.

[jtanguay]

whatever special coating the piston crowns have, is now missing in the spots that made contact.   problems down the road when you're running high boost fuel & high egt's  :twisted:

[Vincent Waldon]

 It looks like a valve hitting the piston, probably due to a timing issue.  Is this what you mean by a worn crank slot?  How do I avoid the same fate?

Hi Ian.

AAZ engines, particularly the earlier versions, have a widely-documented bad habit of developing a loose or "wobbling" crank pulley.

A combination of poor bolt construction and tightness, harmonic balancer design, and the new serpentine belt system conspire to wear out the slot in the crank.  If noticed, you fix the issue.  If not noticed, you drive until the belt fails and valves meet pistons... probably what happened here.

Do a search here for "AAZ crank" and you'll find tons of info on not only the fix, but also prevention.  If yours has not started its wobble it may already be cured, or you may want to take steps in that direction.


Vince

[jtanguay]

don't forget the clutched alternator pulley.  the alternator is a rotating mass that stores up energy, putting stress on the crank pulley when their speeds do not match up.  that is probably the main reason that the serp setup is more detrimental.

[bigblockchev]

I would clean em up and run them. Just remove the extruded metal from the edges of the indent. I would bet that they would outlast almost any other component in the engine. As long as the skirt clearances are good. The amount of thickness removed is negligible compared to the overall deck of the piston. If you are really worried get them dye penetrant tested by a good machine shop. Racing pistons are modified all the time with good results. Cheers Dan

[jtanguay]

i wouldn't remove too much material from the crown.  there is a heat resistant coating... remove it and beware!  just try to clean it up so no pieces get loose and do major damage.