Little End Bolt Failure; Second Thoughts

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.

bolt1

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.

little 2 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.

This article, written by Malcolm Watts, originally appeared in Seven Focus Nov 2002 pp22-24.

Gudgeon Pins and Little-End Bearings from The Practical Motorist, August 17, 1934

Little End Bolt Failure

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