Lotus Elan

Fasteners

PostPost by: StressCraxx » Wed Sep 13, 2006 4:35 am

Some very good discussion on fasteners and good advice. I highly recommend Carrolls's books and the Machinery's handbook.

My concerns on commercial grade fasteners Grade 2, Grade 5, Grade 8 are on both dimensional tolerances of the grip, the length of the grip vs the threaded portion and the material of construction.

Most of the comercial fasteners you find at the auto parts stores are imported. 'KS' on the head of a Grade 5 bolt is Korean Steel. They are purchased by jobbers and distributors purely on price. Good luck if they are the tensile, yield and elongation specified. Specifications are NOT enforced. Only one way to find out. Buy some, send out samples and have tests done. I don't have a good feeling at all for the Chinese or Indian made fasteners if I can even tell where they might have been made.

Thankfully few fasteners on elans break. The original GKN stuff is actually very good. If you will notice, the grip lengths are correct for the application on the original bolts. Most of the suspension pieces are cushioned by rubber and are less susceptible to fail from shock load. Hardened steel washers are used to provide standoff for the nut against the grip. Most bolt failures are due to improper fastener tension (torque), particularly the donut bolts. If the bolt is not torqued adequately in a cyclical load application, it will fail.

Some of this discussion is on suspension bolts. Have you ever had a good close look at the brackets where your lower wishbones attach to the chassis at the rear of the car? They are fabbed from 0.049" (18 gauge) mild steel. I had one break from low stress high cycle fatigue. The failure was caused by vibration from a slightly bent wheel over a long period of time. The brackets (pickup points) were welded to the bottom chassis rail. There was good weld penetration on the bracket, but very poor penetration on the chassis becuase it was thicker material. Thats where it broke. Ever have a car with 'rear steer' behavior at 70mph?

My point is, pay attention to the condition of the entire joint.

Lotuses posess an economy of design. If one bolt ex. 5/16" is required for an assembly, one 5/16" bolt is used. Not two. Not a 3/8" bolt either.

If you buy the AN fasteners, they can be supplied with material certification from the supplier or manufacturer if you ask. AN bolts are also a better thread class of fit - class 3, the grip is a more precise fit in the hole of the joint. It is easy to find the right length of grip and the hardened steel washers hold up well. I replaced all of the SAE fine thread fasteners on my Zink Z-10 Formula Ford for about $200. Most of the bolts were less than $3.00 each. They even come with holes for lockwire for the truly anal retentive.

I did replace some of the worn GKN bolts in the Elan with AN because they fit properly and I have confidence in them.

If you desire higher strength bolts than the normal AN, you can obtain the NAS stuff, but its expensive and not always available in the size you want. They are not necessary other than the connecting rod bolts and flywheel bolts in the twincam. These specific bolts are available from the usual Lotus suppliers.

Best regards and Happy Lotusing,

Dan Wise

"If it hasn't broken yet, it probably isn't light enough" -- Colin Chapman


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StressCraxx
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