Stripping the M20 engine to get a new big end fitted I found the bore deeply scored by the gudgeon pin. I will get a new liner fitted but why did the gudgeon pin clips not do their job?
I fitted the NOS standard piston about 5000 miles ago whe building the engine and had the bore lined and machined to suit.
The pin retaining clips are the thin wire C clips.
I had the head off to fit a new gasket about 300 miles ago (as you may recall in a car park at the Welsh seaside)and didn't notice evidence of scoring then. Perhaps there was minor scoring I didn't notice but now the tramlines are deep enough that the top of the piston is oily and the head and piston covered in black carbon from burnt oil.
The deep scoring is on the nearside (left) of ther barrel and when I pulled the barrel off I found the circlip was not fully engaged in its groove and was worn where it had been rubbing. Oddly the offside (right) side of the barrel is showing light scoring (tramlines)but the circlip is still in place!
Does anyone have any suggestions what might have happened? I could do with understanding the likely root cause to make sure it doesn't happen again.
I would say that if you fitted a new circlip and located it properly in its groove, it would be almost impossible for it to come out. They're awkward to get out at the best of times when you want them out! There is no real sideways force on the pin, so to my mind, it could only be that the groove isn't machined to the correct depth (unlikely) or most probably, you didn't get it fully clicked in......You did ask! Ron
email (option): ronpier@talk21.com
The piston you had fitted was the grooves for the clip designed for a wire clip or a flat one with ears. Fit either in the wrong grooves and it will not seat right.
Ron, your final theory was my first thought. I've never had one come out before and have struggled to get them out when I want to. It
Ian, the clips were the simple round wire clips and it is possible they are not seating well. I assu,ed all would be well as they came with the piston which was original BSA.
It seems odd that its take several thousand miles to come adrift. Its is odd also that I seem to have light score marks on the other side as if the clip had been out and gone back into its groove!
The piston os still OK so I think I'll get a selection of clip designs and see to having the grooves checked/machined to fit a more substantial or rather, better fitting clip.
The grooves on the wire clip are semi circular and the flat clips are square shaped. the only thing that I can think of is that the grooves were machined too shallow. This happened on my Velocette with the same result.
The clips being pushed out and the pin eating the liner is not uncommon. It is not specific to a M20 as I have seen it on other bikes. It is almost always caused by the connecting rod being bent or the small end bushing not being bored perfectly in line with the big end bearing. Even bent a tiny bit will cause pin to move in the piston and it will give a little tap to the clips on each stoke of the piston. After a few hundred or thousands of miles the clips will pop out. Even being bent so little you can not see it by your eye is enough to pop the clips.
Put straight edges on the top of the crankcases and make sure the piston pin is perfectly in line with it.
To be clearer, let's say the rod is bent a little bit so the pin is tilting down towards the drive side of the engine. At the bottom of the stroke the pin will move towards the drive side of the piston and touch that clip. At the top of the stroke it will touch the other clip.
Thanks Bruce that is very helpful information. I'll get the engineers to measure things up when I take it to get the bottom end done.
I now have the engine apart and perhaps I have found another possible cause of the problem.
I did not have the big end done when I restored the bike, taking advice from my engineer that while it felt a bit rough it would probably be good for a few thousand miles.
Now I have the barrel off I can feel a bit of roughness in the bearing but no up and down play (it wasn’t knocking at all). However the con rod will rock from side to side (not just lateral clearance). With the gudgeon/wrist pin in the small end the rock gives about about 3mm side play. As I think it through I am not yet convinced this is the cause as the extent of play is constant throughout the stoke as of course the big end to small end length is constant so I would assume the forces acting to cause the pin to move also be constant i.e. steady.
Running a big end of unknown history is always a gamble that may get you thousands of miles of use or it may not...Any roughness is a bad sign for the big end bearing though..It should feel completely smooth when rotated..
Regarding lack of up and down play, that in itself is not a guarantee of a serviceable big end. As the BSA big end is a double row bearing the surfaces can be worn/failing on one side but not the other. The 'good side' will ensure there is no vertical play present but it isn't a good bearing overall....I've seen this condition on more than one occasion...
Some 'tilt' of the con rod will always be present in a big end that has done some miles as there will be some wear present and the length of the con rod magnifies any movement...There are no tolerances for this aspect and an assessment is usually based purely on the experience of looking at a number of used cranks/big ends...Unfortunately many people haven't had that opportunity....If the bearing has no vertical play and feels smooth some 'tilt' certainly doesn't mean there isn't any life in it....
Personally, any feeling of 'roughness' would set the alarm bells ringing for me and I'd want to take it apart for a better look...Ian
email (option): ian@wright52.plus.com
Ian, thanks, I appreciate your expert opinion and experience. I hadn;t though of the differnec bewteen twin roller big ends and the solid ones I am more used to. Hopefully in a couple of months it will be all be back together like new with the big you supplied.