Wrist Pin Area Failure

My 3.4L XK engine suffered a failure I have not seen before and I thought I would see whether anyone else has had the same problem.
Basically, the wrist pin on one cylinder moved out of position and scored the cylinder wall to a depth of about 0.05". See pictures below.
The mechanism appeared to be that the wrist,gudgeon, pin had been oscillating violently and impacting the circlips. This resulted in fracturing of the local area of the piston over some period of time in what appears to be fatigue. This happened on both circlip areas until enough material had been removed on one side so that its circlip was released, allowing the wrist pin to move out and rub against the cylinder wall machining a groove therein.
The other side of the cylinder also had scoring as the pieces of fractured aluminum had been squeezed between the piston and cylinder.
One half of the released circlip ended up on the other end as you can see in one photo.
jammed between the wrist pin and the remaining circlip.
The wrist pin bore showed some fretting in one area.
The evidence clearly indicates violent hammering by the wrist pin and does not indicate a faulty circlip as the cause.
The other five cylinders had no similar damage at all.
The engine was rebuilt about 30 years ago and had about 30K miles I would guess. It seemed to run quite well although it did have some odd noises and it was a bit rough at higher revs.
As soon as smoke started to appear out of the exhaust, I drove gently for 50 miles to get it home and investigate.
Upon stripping I found some examples of poor workmanship in tab and wire locking and one upper timing chain tensioner had lost its rubber so that the metal bracket was being abraded by the chain. One connecting rod was seriously heavier than the others although this was not on the failed cylinder.
The only explanation that I have found is in Don Hamills book on XK engines in which he states that crankshaft endfloat greater than 0.010" can lead to wrist pin area problems.

An XK engine that has excessive crankshaft endfloat will have problems which won’t be confined to the thrust washers. Too much forwards and backwards movement of the crankshaft when the engine is running results in the crankshaft ‘hammering’ the thrust washer surface (metal to metal contact) and a reduction in lubrication of the thrust washer surfaces. The oil is not under full engine pressure at this point anyway because thrust washers have grooves in their faces. The excessive fore and aft movement of the crankshaft also gets transmitted to the connecting rods and, if the endfloat reaches 0.010in/0.254mm, can lead to gudgeon (piston) pin circlip failure (the gudgeon pin can ‘hammer’ the circlips out of the gudgeon pin bore in extreme cases). Nothing

Hammill, Des. How To Power Tune Jaguar XK 3.4, 3.8 & 4.2 Litre Engines (SpeedPro series) . Veloce Publishing Ltd. Kindle Edition.

Unfortunately I did not measure the end float when I stripped the engine since I was unaware of this explanation. However when I rebuilt with new thrust washers, the end float was 0.011" so it’s likely that was what I had before (the crank was fine). I elected to fit just one 0.004 oversize thrust washer to get to near the upper tolerable limit of 0.006".

It is really hard to understand why only one cylinder was affected and also how connecting rod movements cause wrist pin movements. It may have been because the con rod was bent or the little end seized so transmitting enough force to the wrist pin. The rod was not examined before being overhauled for the above mentioned reason.
Anyway the engine is rebuilt now and has done 1000 miles - quite and smooth!

So if anyone is still with me, are there any other experiences and opinions?


1 Like

The two main culprits likely to cause that, are worn gudgeon pin bushes, and/or worn circlip grooves.

I’ve only ever seen that failure once in 4 decades.

1 Like

Same here. A racing engine in back in the late 1970’s. Switched to Spirolox after that.

I did not want to change to a different clip style since a) there are thousands of these engines out there without this problem and b) it is my judgement that the clips were not the cause of the failure.

Digging a bit deeper, I found that there is a right and wrong way to install these clips. One side has a sharp edge and the other side is slightly rounded. The sharp side must always go to the outside i.e. away from the wrist pin.

1 Like

I used Teflon buttons on most of my race engines, but most certainly Spirolox, if not.

I clicked on your icon and see your car is a '67 Mark 2, so I’ll assume it is this car’s engine. I looked in the parts catalogue and I see there were no pistons offered with 4 rings, so yours appear to be aftermarket. And you said it was rebuilt 30k miles before this.
The sharp edge on the circlip (aka snap ring) is caused by the manufacturing process, where they are stamped out of a sheet or strip which is fed into a punch press die set. Any stamped item will show this edge, a flat washer for example.
The groove in the piston is supposed to be deep enough that the circlip will not work its way out. In theory, the rounded edge would have a slightly easier time working its way out than the sharp edge, but really only if the depth of the groove was marginal, i.e. poor machine work on the part of the piston manufacturer.
There should be some clearance between the rod and the piston, allowing a bit of fore/aft movement. The wall is pretty thin there around the circlip groove, so perhaps that’s what Des Hammill means by hammering it out.
So ultimately incorrect installation of the circlip is a possibility, either wrong side out in a shallow groove, or not properly seated in the groove.
Did you replace all the pistons? Or at least check them for poor circlips?

You are correct that my car is a Mark 2 1967.
The piston set fitted in 1995 or so was AE as they were so marked inside.
I have fitted all new MAHLE pistons.
The bore damage was such that the block needed to have a liner fitted to the bad cylinder.
The bores were all at 30 thou over. So I had one liner fitted and all were bored to 40 thou over to keep everything balance. These new pistons have 4 rings as well.