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Thread: How to check for damage
02-22-2006 06:01 PM #1
How to check for damage
Since I have a Z8 with no miles on it, I thought I'd pass along the following information to help people determine whether their cars have sustained any shock tower deformation damage.
Let me begin by stating that the following measurements should serve as a reference point, not a definitive diagnostic technique.
I do not think measuring the gap between the edge of the hood and the fenders is a reliable way to determine if shock tower deformation has occurred because there is too much variation that is within production tolerances. There may be exceptions to this, particularly on a car with severe deformation and/or frame bending but even my unaffected Z8 has a noticeable narrowing of the gap towards the front of the car so I would be reluctant to trust this method. I would also avoid using the flat top of the shock tower as a reference point since my unaffected Z8 has a slight curvature to this area. Of course, you CAN use this information to alleviate your concerns in case your car has no damage since these small variations are perfectly normal.
Based on the pictures I've seen of cars with obvious shock tower deformation, it appears that the relatively flat plateau area on top of the shock tower is being pushed up into a dome shape. The maximum rise is taking place in the center of the tower (where the shock mount access hole is), as opposed to being a uniform deformation spread across the entire plateau, which results in a dome shaped deformation. This doming also causes the 3 strut mounting studs with nuts visible on top of the shock tower to splay outwards from the center.
On an unaffected Z8, if you lay a straight edge of a metal ruler across the upper surfaces of any pair of strut mounting nuts, you will find they lie in a parallel plane. As a result, there should be no visible air gaps between the edge of the ruler and the surfaces of the nuts. This was remarkably consistent on my car. If doming has occurred, you can expect to see gaps between the straight edge and the surfaces of the nuts due to the splaying. To take this one step further, if you remove the plastic dust cap covering the shock mount access hole, and lay your straight edge across the top surfaces of a pair of strut mounting nuts, you can measure the gap between that straight edge and the top of the raised rim of the access hole. Using a set of feeler gauges, I measured .052 inch. There is paint on that rim so variations are possible but both sides of my Z8 were identical. If doming has occurred, you may find a significant variation either from my measurement or from side to side. Of course, if the studs are splayed, your straight edge will not be lying flat across the surface of the nuts and it is possible that you could get, by sheer coincidence, a measurement similiar to mine despite having some doming so I suggest you also make a visual check by sighting across the straight edge resting on the upper surfaces of the nuts and view the upper edge of the access hole's raised rim. On my car, the two lines are parallel from any angle. If doming has occurred, you should expect to see a curvature to the raised ring or a non-parallel relationship when referenced to the straight edge.
I realize these are just rough guidelines but they come from a guaranteed unaffected Z8 and should provide a reference point from which to begin your examination. Plus, your own measurements should serve as a baseline to keep track of any changes over time. Hope this helps!
P.S. Don't forget to replace the plastic dust covers when you're finsihed, i.e, don't leave them lying under the hood, or you may end up with some new "domes" in your hood when you slam it shut!Grease Monkey
02-23-2006 10:59 AM #2
Re: How to check for damage
GM, does it make sense to attempt to measure/establish a nominal distance between the right side and left side shock mount access holes, either inside to inside or outside to outside, which could be easily measured with a narrow tape measure? I would expect some amount of normal variance during the fabrication of the frames, but if that variance isn't excessive, perhaps the distance between the holes could be used as an indicator of frame movement.
02-23-2006 12:17 PM #3
Re: How to check for damage
Because of the distance involved and the fact that your tape will be in contact with the top of the plenum chamber as it crosses over, there is a real possibility that this measurement will vary from car to car. However, for what it's worth, on my zero miles Z8, if you measure from the inside edge of the raised lip (the edge you would use to measure the inside diameter of the hole) at the point where it's closest to the centerline of the car, the distance from side to side measures 107.5 cm. If nothing else, this might another dimension to establish as a baseline on your car to see if there is any change over time.Grease Monkey
02-23-2006 01:02 PM #4
107.5 cm on mine as well
GM, you are right about the plenum chamber - I posted the question without first looking under the hood. The plenum chamber might add an additional variance between cars when making this measurement. FWIW, my 8500 mile 2002 also measures 107.5 cm (42 5/16") with the tape pulled tight across the top of the plenum chamber. This is either coincidence or pretty tight manufacturing tolerances for a large welded assembly; probably coincidence.
02-23-2006 01:19 PM #5
Re: 107.5 cm on mine as well
Very interesting. I believe the structural tolerances on the Z8 are extremely tight so perhaps your experience will be typical. The biggest variable should be the condition of the engine mounts since they could influence the height of the engine in the chassis. At one time I had the chassis of my driver Z8 checked for "squareness" and the shop stated the car's chassis was absolutely perfect and better than any other car they had ever measured. Hand built yes, sloppy no.Grease Monkey
02-24-2006 12:57 AM #6
Would it be possible to better triangulate the
proper location of each tower by taking the measurements from easily identifiable benchmarks under the hood (i.e. specific bolts, components, etc.)?
This would be much easier to do if we had the BMW drawings to work from, but since that isn't likely, maybe you would take some triangulating measurements to use as a reference. As a note of caution, these should only be viewed as indicators and should not be considered definitive.
2003 Alpina #480
02-24-2006 01:23 AM #7
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There should be a readily available spec
Examples, from the E31:
The gaps spec'd on mine are 5.5 mm &plusminus;0.5 mm! Somewhere it states max "permissible" deviation is &plusminus;1.0 mm. That's pretty tight for a coupe, to say the least.
Maybe it's still in the TIS CDROM?
<a href="http://www.fibersonde.com/e31/images/ICanWalk.wav"><img src="http://www.fibersonde.com/e31/images/ICanWalk.jpg" border=0></a>
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