2CV Piston Slap
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Thread: 2CV Piston Slap

  1. #1
    1000+ Posts pottsy's Avatar
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    Default 2CV Piston Slap

    The Alleged Vehicle (1986 2CV6 Special) has a rather disturbing "knock" at times. It appears under load, but not consistently, and it's not apparent on light throttle.

    After puzzling over this for a while I took it over to Griesy the Guru to apply his seasoned ear to the noise. We went for a drive and the consensus seems to be "probably piston slap".

    Now I realise that the only sure way to confirm this is going to be to remove heads and barrels and do some checking and/or replacing. I don't want to do this just yet, so I thought I'd ask the questions below.

    What I seek here is some discussion on ways (if any) of temporarily (or permanently even) ameliorating the issue.

    I should point out at this stage that the timing appears spot on, the rocker clearances are set as near to spot on I could get them, it's running a 123 ignition and 98 octane fuel. I've dismantled and cleaned the carby, and expanded the main jet out to 1.1mm (Big Bore!) as recommended to reduce the flat spot under normal acceleration.

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    The engine seems to idle smoothly, pull like a train (for 602cc anyway - it's all relative) and keep pace with city traffic effortlessly.

    I've yet to cobble together an oil pressure fitting to measure that, but it's on the list.

    My thoughts are to possibly try a heavier oil? Perhaps a Nulon additive? Perhaps one of those "engine honey" types of additive?

    Maybe retard the timing a few degrees?

    If it was a big end, or possibly even a dodgy gudgeon, I'd expect it to be there particularly on light throttle and also reflected in the oil pressure readings (see above).

    My understanding of piston slap is that it's the bottom skirt of the piston being allowed to "ring" against the side of the bore by excessive piston to cylinder clearance. (Allowing the piston to "rock" in the bore) Obviously this will happen under load, but not necessarily under light throttle. (Incidentally it only seems to be on the LH cylinder, but that may be my aging hearing misleading me of course!)

    Anyone care to expand on this, or correct me? (Not the hearing, clever dick, the description of slap! )

    I figure I might as well consult the Brains Trust rather than make a wrong decision and stuff the beast up more.

    So anyone's thoughts will be read and valued. As mentioned before, I realise that eventually I'm going to have to put it on the bench and do some forensic dismantling. Just not quite yet.

    Cheers, Pottsy
    Buvito Ergo Sum!

    The Fleet:

    2018 C3 Shine ("Oscar" Mrs P's)
    1974 D Special Manual Sedan ("Moby Dick")
    2006 C5 HDI Wagon Auto ("Mephistopheles")
    1982 2CV6 ("Gaston") On the road! (Woohoo!)
    1957 Slough 2CV ("Alphonse") Waiting in the wings
    1975 GS Pallas ("GiSelle") Also waiting in the wings
    And for Fun, 1968 Mini Deluxe ("The R & D'luxe" 1078cc, Grin Factor 100!)

  2. #2
    Fellow Frogger!
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    In my experience piston slap is at it worst when the motor is cold & diminishes, or disappears when hot. A more hollow sound than big end or gudgeon knock. Big end noise is deep & worst under load when hot & the oil has thinned. Also heard on first few revolutions when starting if general wear is affecting oil pressure. Gudgeons are higher pitched & also worse when hot & under load. A broken ring can give a noisy tappet type sound if it has broken through the top piston land. Carbon build up on a piston can also mimic big end noise if it is hitting part of the cylinder head . Heavier oil can reduce big end noise for awhile, unless the big ends are ball type, but seldom reduces slap or gudgeons appreciably.

    Food for thought.

    Richard

  3. #3
    Real cars have hydraulics DoubleChevron's Avatar
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    Isn't it like a GS but simpler.... Wouldn't it be about 10minutes work to whip it apart and check. It shouldn't even have head gaskets. Whip a few bolts out, drop the head/barrels and post some piccies for us of course

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    You have, of course, eleminated sagged engine mounts, out of position exhaust, drive shafts et al, the noise is definately inside the motor?

    Try a mechanic's stethescope on it, right after a cold start. Plan the route to get the probe end onto the cylinder barrels and heads before starting up! Because the shaft is horizontal the top or bottom of the barrel will be the bit making the most noise if it is piston slap.

  5. #5
    1000+ Posts pottsy's Avatar
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    Hmm. Hadn't actually considered engine mounts. The exhaust seems to be in the right spot and the only bit it touches now and then is the LH tyre under enthusiastic cornering.

    If it was the engine mounts, I'd expect it to happen fairly consistently. That being said, it does seem to be on one side only. Definitely worth a check.

    Drive shafts are certainly not pristine, but the noise happens when the motor is revved in neutral, so I reckon they're out.

    I should have mentioned it happens cold or hot. The car was certainly well warmed once I arrived at Griesy's place, about 10kM away as the crow flaps.

    I've yet to try the old stethoscope trick. Had a couple of other car related issues to deal with first.

    Thanks Bruce, food for thought indeed.

    Cheers, Pottsy
    Buvito Ergo Sum!

    The Fleet:

    2018 C3 Shine ("Oscar" Mrs P's)
    1974 D Special Manual Sedan ("Moby Dick")
    2006 C5 HDI Wagon Auto ("Mephistopheles")
    1982 2CV6 ("Gaston") On the road! (Woohoo!)
    1957 Slough 2CV ("Alphonse") Waiting in the wings
    1975 GS Pallas ("GiSelle") Also waiting in the wings
    And for Fun, 1968 Mini Deluxe ("The R & D'luxe" 1078cc, Grin Factor 100!)

  6. #6
    JBN
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    Geez mate. The car was designed to drive over a ploughed field with a crate of eggs without breaking any. Your expectations are well beyond what it was designed for. Either get some eggs, find a ploughed field OR drive the bloody stupid thing so it shuts up and stops complaining. Show it who is boss.

    As soon as its warmed up, floor the throttle and leave it in that position for the rest of the journey. Twiddle the wheel a bit to dodge the other cars. If they are not responding, twiddle the wheel violently, the car will appear to fall over and all other road users will stay right away from you.

    Don't worry about the bottom end. If its stuffed, you need a new crankshaft with conrods (the two are inseparable after manufacture).

    The pistons, rings and barrels are not very expensive. I've had 3 Deuches and as many new barrel rebuilds. The bonus is you get to replace the pushrod tube seals and have a clean non oily motor for a while (before you trash it). The engine on a 2CV is a consumable.

    There are no head gaskets, its a tapered fit. There a 3 studs holding each cylinder head, torsioned (if thats not too strong a word) at 18 ft lbs (a bit over hand tight).

    I've had a piston ring break on two separate occasions. You'll know it when you have done it. A 50% loss of power is fairly dramatic. Its amazing how many red lights you go through, because if you stop, its a long walk home.

    Seriously, if you undo the exhaust at the goose neck a bit, and let the widows flap open, the car makes so much noise that you can't hear what the problem is, so it kinda puts your mind at rest. If you combine this with a stuffed 2nd/3rd synchro shaft that nicks the gear changes and brake pads or discs that need replacing, I can tell you that piston slap is the last thing you will be worried about.

    Just flog the bugger to show it who is in charge. If the speeding fines get you down, drive with the handbrake pulled on a couple of notches.

    John

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    You haven't dented the sump recently have you?
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  8. #8
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    Quote Originally Posted by Exfrogger View Post
    You haven't dented the sump recently have you?
    The sump isn't a bolt-on sheet metal piece. It's much like a VW flat four- split case halves that incorporate the sump. A dent here would most likely be an actual break in the aluminum.
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  9. #9
    1000+ Posts pottsy's Avatar
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    John, you're either someone with serious antagonistic issues towards mechanical contrivances or just a funny bloke. Can't make up my mind which.

    I take your point that the car is an agricultural implement, but like all such devices, they're much more satisfying to a diehard tinkerer like me good self when they're working properly.

    That being said, I can tolerate all manner of odd noises as long as, and this is the important bit, I know what they are!

    Many years ago when driving my first car, a 1956 Austin A30, I would worry over noises as they appeared. Once I could identify them, a bit of rubber or string, or even a judicious tap with a hammer, would provide a temporary solution and the problem could be relegated to the background. That car was driven daily for a couple of months on three cylinders because I lacked the hooch and the time to replace the blown big end bearing. Pulled the plug lead and thrashed it unmercifully, somewhat like your advice.

    Since this car will ultimately be used by the mate and meself to go bush, reliability is of the essence. If that reliability is to be threatened by a piston skirt wanting to do a Garbo ("I want to be alone!") then we won't be happy campers.

    Great post though. I imagine that once I become a true 2CV aficianado I'll fully understand the approach. Alphonse the '57 is still waiting patiently for me to start on his resurrection, so lessons learned on The Alleged Vehicle will not be wasted.

    Oh, and Ex. Denting the sump would be fairly terminal for the reason mentioned by the warmed up sparky. In any case, if the sump was a tin one, and dented so as to impact with the twirly bits, I'd expect the subsequent noise to be reasonably constant and not necessarily load related. (Not to mention disturbing!)

    Thanks for the feedback gentleblokes. More news as I get the time to find it.

    Cheers, Pottsy
    Buvito Ergo Sum!

    The Fleet:

    2018 C3 Shine ("Oscar" Mrs P's)
    1974 D Special Manual Sedan ("Moby Dick")
    2006 C5 HDI Wagon Auto ("Mephistopheles")
    1982 2CV6 ("Gaston") On the road! (Woohoo!)
    1957 Slough 2CV ("Alphonse") Waiting in the wings
    1975 GS Pallas ("GiSelle") Also waiting in the wings
    And for Fun, 1968 Mini Deluxe ("The R & D'luxe" 1078cc, Grin Factor 100!)

  10. #10
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    Hi Pottsy,

    As ridiculous as JBN's post are he is so right! I think the combination of your tinkering and learning/understanding the orchestra of noises accompanied with JBN's advise is the best way to drive a 2cv.
    Gricey is indeed the guru, the amount of times he has got me out of the deep stuff, what a legend. My engine was rebuilt by him in about 2001, I remember being concerned/disappointed about the knocking sound coming from one cylinder, even passes by would say 'ohhh bit of play in the big end there' but I have learn't to live with it and 12yrs later it is still getting the tits rev'd off it everywhere I go, including a speeding ticket @ 116km/h fully raid laden.
    I agree with Shane and think that dropping the head and inspecting the piston and barrel is the best piece of mind approach for outback reliability.

    Harley

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    JBN
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    My ear is fine tuned to the sounds of different bits of 2CVs either falling off or misbehaving.

    A few days ago, I ran over a bit of light metal as I was turning a corner. I hadn't seen it and first became aware of the noise it made. By the time I had completed turning the corner, the mind had analysed the sound, tried to find a match, came up with a blank so I completely disregarded it.

    Yesterday I went around the same corner and hit the same piece of metal. The sound was dismissed immediately and I commented to myself that the local council were being a bit slack in keeping the roads clean.

    Citroen - drive them like you hate them.
    At the worst, you will join the great unwashed in hating them, even though they have probably never driven one.

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    Fellow Frogger! Bruce Llewellyn's Avatar
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    Quote Originally Posted by JBN View Post
    My ear is fine tuned to the sounds of different bits of 2CVs either falling off or misbehaving.

    Citroen - drive them like you hate them.
    At the worst, you will join the great unwashed in hating them, even though they have probably never driven one.

    John
    That's fiats mate...

  13. #13
    1000+ Posts pottsy's Avatar
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    News Has Come to Hand!

    This morning, with nothing better to do but fiddle, I decided to re-check the timing. When I'd installed the 123ignition, I made careful note on the flywheel of the relevant points. TDC, firing and nominal maximum advance.

    What I'd apparently forgotten was which side of the visible gap between the starter mounting and the gearbox "horn" was the reference point!

    Imagine my surprise to find I'd set the bloody thing out by about 2 or 3 teeth! Certainly enough advance to cause all sorts of knocking noises.

    After meticulously resetting and rechecking, I can now announce that the slap (or knock) is a thing of the past! WooHoo!

    Mrs P and I just went for a cruise over through the hills to Warrandyte Pub for lunch. The little beast revs its tits off, and doesn't make those deep seated banging noises under the heaviest load.

    I call that a win, and castigate myself for not checking the timing more carefully earlier.

    As for tuning out noises, isn't it amazing the processing power we have between our ears? Sound recognition is only one of the many tricks the Meat Computer has up its metaphorical sleeve.

    Thanks for all the responses chaps. I certainly plan to drive it enthusiastically, but hate it? Never!

    And Harley, yes, ultimately it'll end up on the bench, but for now, it's fun in the sun time! (All I need now is sunshine to coincide with time off)

    Cheers, Pottsy
    Buvito Ergo Sum!

    The Fleet:

    2018 C3 Shine ("Oscar" Mrs P's)
    1974 D Special Manual Sedan ("Moby Dick")
    2006 C5 HDI Wagon Auto ("Mephistopheles")
    1982 2CV6 ("Gaston") On the road! (Woohoo!)
    1957 Slough 2CV ("Alphonse") Waiting in the wings
    1975 GS Pallas ("GiSelle") Also waiting in the wings
    And for Fun, 1968 Mini Deluxe ("The R & D'luxe" 1078cc, Grin Factor 100!)

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    Well if you set the timing static, you won't any of those problems

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    1000+ Posts pottsy's Avatar
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    Quote Originally Posted by 2CViking View Post
    Well if you set the timing static, you won't any of those problems
    Hi Peter, yes, that's where it's embarrassing. I set the timing up originally with the little green LED on the 123ignition using the rod through the hole method. Then I proceeded to mark the flywheel and housing for future reference.

    Somewhere along the line I mucked up the markings and later on I set things to the markings and not the rod in the hole.

    A dumb mistake, and one I won't make again, but that's how we learn about these weird little beasts I reckon.

    Cheers,Pottsy
    Buvito Ergo Sum!

    The Fleet:

    2018 C3 Shine ("Oscar" Mrs P's)
    1974 D Special Manual Sedan ("Moby Dick")
    2006 C5 HDI Wagon Auto ("Mephistopheles")
    1982 2CV6 ("Gaston") On the road! (Woohoo!)
    1957 Slough 2CV ("Alphonse") Waiting in the wings
    1975 GS Pallas ("GiSelle") Also waiting in the wings
    And for Fun, 1968 Mini Deluxe ("The R & D'luxe" 1078cc, Grin Factor 100!)

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    Quote Originally Posted by pottsy View Post
    Hi Peter, yes, that's where it's embarrassing. I set the timing up originally with the little green LED on the 123ignition using the rod through the hole method. Then I proceeded to mark the flywheel and housing for future reference.

    Somewhere along the line I mucked up the markings and later on I set things to the markings and not the rod in the hole.

    A dumb mistake, and one I won't make again, but that's how we learn about these weird little beasts I reckon.

    Cheers,Pottsy
    You have my respect, good man.

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    Good on ya Pottsy.

    A positive outcome without hours under the bonnet is always a good thing.

    Im sure your not the first (or the last) person to have done the same thing

    Cheers

    Ian

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