Lotus Elan

Fasteners - Correction.

PostPost by: StressCraxx » Sat Sep 16, 2006 4:43 pm

All,

Roger very kindly pointed out an erro on my recommendations for flywheel and conrod bolts for the twincam. Thank you Roger.

Dan Wise


Roger,

Thank you, I agree. I should have been more clear. It was getting
late and I suffered from some brain fade. The ARP flywheel bolts and
Cosworth "bodybound" conrod fasteners are based on NAS specs and are
ideally suited to the flywheel and conrod application. I will clarify
my point in the thread.

Hole prep on the flywheel is important too. The bolt holes in the
kent steel flywheel tend to collapse from the bolt head profile,
tension
and repeated torquing. I had to ream mine back to size.

Related to this, I discovered the conrod bolts in my well used
Formula Ford 1600 motor were standard NAS 12point capscrews with
beveled
washers. Motor had run over 40 hours with a 7000 rev limit. No
distress at all to bearings or journals. Damn lucky because there was
no physical location match of the cap to the rod with the standard
grip,
just clamping force.

Regards,
Dan

Roger Sieling <***@***.***> wrote:
Dan(off list),

IIRC, the NAS bolts you refer are the ones w/ the thinner concave
shaped heads. The other trait they have is a shorter threaded end than
AN bolts. There are several number series of NAS bolts, each meant for
a
specific application and I believe Carroll Smith warned against using
them unless you really knew what the number application was that
suited
your application. I believe a number of them are meant for shear use
rather than tensile use.

For this reason alone, I'd avoid using them, especially for flywheel
use and connecting rod use. Thats where ARP and SPS make specific
bolts
designed for those applications shine.

Roger

>> ***@***.*** 9/13/2006 12:34 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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