From: greg@saltydog.dpsi.com (Gregory R. Travis)
Subject: Re: be nice to your engine
Organization: Data Parallel Systems, Inc
Date: Wed, 2 Sep 1992 16:49:30 GMT

[I don't know if this was meant to be an e-mail note, but since it wasn't
I'll reply in public]

In <2SEP199211275904@ariel.lerc.nasa.gov> hanlon@ariel.lerc.nasa.gov (Casey Hanlon (SVER)) writes:

>I'll have to disagree.  Stress is a factor within the normal operating range 
>of an aircraft engine.  Things like connecting rod and main rod bearing bolts
>are routinely replaced at overhaul because they do stretch due to the loads
>they are subjected to.

Hmm, I gotta disagree with this one too Casey!  There are, basically, two
types of connecting rod and/or main bearing bolts.  The first are
stretch-type bolts which are DESIGNED to stretch during installation.
These bolts cannot be reused once they are loosened because they can
only stretch once.  Try and torque them twice and they'll break (with a
bang!).

The other is just like any other bolt and it's designed to be tightened and
loosened any number of times.  However, people often replace rod bolts of
this type at overhaul anyways because it's easy to do and it gives them
piece of mind.  I don't know anyone (doesn't mean they don't exist!) who
routinely replaces this type of bolt if it's a main-bearing bolt or, in
the case of aircraft, a case through-bolt.

I would be VERY worried of my rod/main bearing bolts were stretching
DURING regular use.  I suspect, nay I KNOW (I've seen it happen!), that
a main/rod bolt that stretched during use would result in almost immediate
engine failure.

>Cases and cyclinders crack due to stress caused by
>a combination of mechanical loading and thermal gradients.

Hey, I thought we were talking about aircraft engines, not
Continentals! :-)  Seriously, given the disposition to crack
that is enjoyed by big Continentals vs. that of Lycomings, I think
that case cracking is primarily a result of casting techniques.
Thermal gradients are another factor but a hard-running engine is not
necessarily one that has large gradients.

From what I've read and talking to both Continental and Lycoming, it's
thermal gradients, not stress, that're the primary cause of cylinder
cracking as well.

>Connecting rods
>are checked for straightness before being returned to service.  In short
>higher rpm's will increase the wear rate and higher manifold pressures will
>increase the stresses the individual components are subject to.

They are checked because it is prudent, and cheap, to do so.  On the
other hard not often found warped, twisted, or stretched.  Note that, in the
automotive world, it is considered acceptable to un-twist a connecting rod
using a lever in the big end and another in the piston pin hole.
Likewise it is considered OK to line-bore a crankcase and install oversized
bearings to correct crankcase twist.  Both rod twist and crankcase
warpage can be caused by a number of factors, including metal quality, heat,
etc.  Not just stress.

greg
--
Gregory Reed Travis				D  P  S  I
Data Parallel Systems Incorporated   greg@dpsi.com (For MX mailers only!)
Bloomington, IN			     greg@indiana.edu (For the others)
Newsgroups: rec.aviation