It is interesting to note that when our Ruby shed a little end bolt she suffered two bent conrods as a result. This
was exactly replicated by Mike’s engine when his failure occurred. On both
engines one bolt failed and two rods were bent.
When
we stripped the engine down we found that the bolt had been fretted away
presumably by movement of the gudgeon pin against the bolt. Technically this
should not have occurred as the bolt should lock the gudgeon pin and conrod
together. There was also a small amount of threaded bolt left in the conrod.
Recently I asked about the condition of the little end bolt which failed
in his engine. Again the bolt showed signs of fretting and there was a small
length of the threaded bolt left in the con-rod.
In the 12 months or so since our Ruby had the upset with the bolt I have periodically discussed
with others the likely cause without coming to any
definitive conclusion. However, the points that we have considered are thus;-
- The fretting on the bolt with it’s characteristic ‘pip’ in the
middle could only be caused by the gudgeon pin or little end of the
con-rod as these are the only object which can come in contact with the
bolt.
- The fact that there is a portion of the bolt left in the con-rod would
indicate that the failure is caused by the fracturing of the bolt rather
than the bolt not being tightened sufficiently ( i.e. working loose and
dropping out ).
- If the little end joint is correctly assembled and tightened then there
should be no movement between the little end of the rod, the gudgeon pin
or the bolt.
Having recently stripped down my spare engine which has done approximately
1500 miles I found two of the little end bolts showed signs of fretting. The
engine had been assembled with new pistons and gudgeon pins so one should be
able to discount wear in the gudgeon pin where it passes through the little end.
The pins were a snug fit in the pistons but capable of being pressed out, cold,
by hand. This appears to indicate that the pins had not seized in the pistons.
Indeed the pins and pistons were in excellent condition.
The conclusion that I would
draw is that the gudgeon pin is not only rotating in the little end but also
sliding from side to side. In doing so it is pressing up against the little end
bolt, causing it to be fretted away, the last point on the bolt which the
gudgeon pin will reach is the centre of the area of the bolt located in the
notch in the Gudgeon pin, this would leave the characteristic ‘pip’.
The Pistons and gudgeon pins have been used, along with the block, to rebuild
the engine in our Box Saloon. When I stripped down the existing engine, which
had been sleeved to standard but was showing signs of considerable wear, there
was no sign of any fretting on any of the little end bolts. When I combined the
Box engine's con-rods to my spare engine's pistons and little ends I found that
the gudgeon pin had to be pressed into the little end rather than just pushed in
by hand. Upon reflection, perhaps this is how it should be!
Is this the answer to the problem? Am I, are we, assembling engines where
there is too much play between the con-rod and gudgeon pin in that the little
end bolt cannot tighten sufficiently to prevent movement?
When I reassembled the engine I made sure that I clamped the gudgeon pin,
using jigs, in the vice whilst I tightened the little end and then made sure
that there was no discernable movement. Hopefully this will ensure that the
little ends remain intact. I must admit I have never used this method before and
therefore never really checked how tight my little ends were.
So I think the conclusions are, not only ensure that you use new bolts and
locking washers (or tabs) every time but also check that the gudgeon pin is a
tight fit in the con-rod and that the assembled piston does not allow movement
between the gudgeon pin and con rod.
