Re: Valve cover leak...
Originally Posted by KrzyFast
I noticed something when I took my intake off. The intake manifold holes are not threaded all of the way.....at least the couple that I looked at. I wonder if the valve cover holes are the same. This might partially explain why so many people have broken bolt heads off......Over-tightening for sure will do it, but it is much easier to do if the hole is not threaded all of the way down.
I am not sure what you mean, as long as the screw has sufficient thread contact, normally (not in this case) at least full contact for a length that is equal to the thread diameter, it is generally sufficient. The fact that the head breaks off indicates that the thread engagement is long enough. It is better to break the screw rather than strip the thread.
Edit
OK, I see what you are saying, But why would any threads past the screw have any effect on the strength?
There is another method to cut threads in parts rather than conventional tapping, and this may explain what you are seeing.
Initially a hole is made which is larger in diameter than the hole size normally used for the tapping method. A tool which is like a polished screw (with no flutes) is then forced into the hole, turning at the correct thread pitch rate and this tool forms the correct thread form. Material that is in the wrong place is forced into the voids and a strong thread is rapidly formed. The benefit is there are no chips left to be cleaned away just some lubrication fluid.
This system works very well on ductile materials, clean and fast.
Tapping hole sizes are normally about the actual thread root diameter.
Thread formed holes are about half way between the thread root diameter and the thread outside diameter. That puts the difference in the two holes sizes about the thread depth.
Originally Posted by dedwards0323
Picked up the screws that were special ordered from local CDJ dealer. Turns out the replacement screws are zinc-plated and a standard SHCS (not Torx). Anyway, for my $0.02, I'd go to Lowe's and get the screws for $0.80 each rather than buy from either M-B or CDJ dealer.
Later,
They do not look like they are zinc plated to me. They look like they are anodized, that's if the colors of the photo are true.
Check to see if they are steel with a magnet.
There may have been a reason why the originals were aluminum.
One reason could be galvanic corrosion, and they may have had cases in some locales, maybe humid coastal areas, where the local conditions caused this corrosion. The use of aluminum screws in the aluminum allow heads would prevent this. Using steel screws which have been plated to prevent contact of the steel and aluminum alloy would also prevent the galvanic action.
With complaints of screw failures it's possible that they opted for the use of the more costly and slightly less safe plated steel screws.
Aluminum is prone to galvanic action with steel and magnesium is even worse.
Last edited by onehundred80; Sep 22, 2010 at 12:26 PM.