Index Home About Blog
From: John De Armond
X-Source: The Hotrod Mailing list
Date: May 1992
Subject: Re: Magnefluxing Rod Bolts

>Well, call me stupid!!!
>Anyone ever heard of Joe Sherman?  Call him stupid too! Of course
>his small blocks are probably all on the verge of loosing rod bolts.
>Rod bolt failure is quite uncommon. I suppose you hate cast cranks too!
>And god forbid, 2 bolt mains!
>Gee, how can I sleep tonight knowing my rod bolts are used!!

Now put a smiley on that so no one will mistake it for a flame :-)

I'm in the same boat here.  I've never ever replaced Datsun rod bolts
unless they're just worn out from being removed so many times.
Never blown a bottom end either.  Hmm, must be how I talk to 'em
as I twist their little heads down :-)


From: John De Armond
X-Source: The Hotrod Mailing list
Date: May 1992
Subject: Re: Magnefluxing Rod Bolts

>	ACK!  I was thinking that there must be something different about
>all these american engines that made the rod bolts reusable, so I was
>keeping my mouth shut, but I have seen some ugly Datsun rods that I don't
>want to duplicate with my engine!  I'm not sure about L series engines, but
>roadster rod bolts are designed to stretch when torqued!  The correct way to
>tighten the rod bolts is actually to measure how much they stretch, not to
>measure the torque!

I've not had the occasion to work on a Roadster engine, though it's on my
list of Important Things To Do.  I tighten L engine rod bolts by enlogation
too.  We have to use more precise terms than "stretch" to make sure
we're talking about the same thing.  L engine bolts are not tightened
to the strain limit.  In other words, they are not stressed past their
elastic limit.  Some other fasteners ARE deformed when tightened to
the specified.  The easy way to tell is to measure the fastener
before tightening and after being tightened and loosened again.
Carol Smith in "Engineer to win" makes a convincing argument, backed
by metallurgical evidence, that a fastener achieves its ultimate
strength when tensioned to about 85% of its yield strength.  For the
rest of this article only, I'll use the term "stretch" to mean

It is my *opinion* that except for a few applications where weight is
the overriding concern, using fasteners that that must be stretched
is at best bad engineering and at worst a patchup to cover other
mistakes.  I suspect that in the case of the Roadster engine, it is
simply an attempt to salvage an overstressed forklift engine.
It apparently worked, though as the discussion on the roadster list
has shown, getting bolts is sometimes, um, challenging, shall we say.

>Given the relative
>cost of rod bolts versus engine blocks, rods, and cranks, it seems like
>cheap insurance!

Let me make an equally valid counterargument.  Assuming we're not dealing
with stretch parts, unless you're willing to do pretty extensive NDT
including X-ray (remember magnaflux only handles surface and just
subsurface flaws), a fastener that has not been overstressed and
have been proven in service are more reliable than new, unproven parts.
This is just a corallary to the old racer's maxim, "never start a
race with a new sparkplug".  Been bitten on that one myself :-)

These tradeoffs are why we have to rely so much on qualified historical
evidence in deciding practices.  I've never heard of anyone having
L engine rod or head bolt problems (other than the very first non-ridged
head bolts) and have never seen a recommendation to replace them in
any of the buildup books.  Since I've never had any problems either,
I don't change them.  On the other hand, were I doing Roadster engines,
damn right I would based on what I've heard.  But I'd do some serious
NDT including X-ray.

>Of course, I don't talk to my bolts.  What do you say John?

Hey!  That's between me and them :-)  Got to have a few privates :-)

>Maybe I'll try that too, can't be too careful!

Highly recommended.  A little Led Zeppelin in the background helps too.
Ooops, that's to help ME.  Sorry.


Index Home About Blog