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Thread: Broken piston

  1. #1
    Senior Member tzrider's Avatar
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    Broken piston

    Some pictures of the piston I took from my ex-German SR500 48T from 1990 with 25.000km on the clock.
    The engine had not much compressions and the bike sometimes wouldn't start, sometimes started at every kick.
    Almost seemed like an electrical problem, started OK, you take a ride, stopped the engine and then suddenly it
    wouldn't kick-start at all. If push started however it would run immediately.
    Didn't use any oil, no smoky exhaust, no rattling. After taking out the piston I found that several large pieces
    had broken of between the two compression rings and between the compression ring and the oil scraper ring.
    One piece was even as long as 5cm. Who knows how long it has been like that. From the beginning ??

    The rest of the engine was like new, cam and rockers: you couldn't even see the engine had been running, like new.







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    Last edited by tzrider; 01-09-2016 at 05:21 AM.

  2. #2
    Very unusual by my experience...new one to go in again!

    I have a German SR500 engine in one of my bikes.

    By the way, can you please place your location into your details...it's required for this Forum. Thanks.

    SR
    ...'Any unnecessary items on the bike, that it doesn't really need, shouldn't be there!'. (Café racers creed). SR..

  3. #3
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    More than likely some impurities in the cast/forged piston has created a crack and caused it to break at its weakest point,you have been very unlucky BUT look it another way....you've got straight on the case before the inevitable happening,it would have caused a siezure if left and no doubt damaged the bore and who knows what else.I work in the engineering trade and I've seen it before in ali diecastings,I've also seen it in solid billet as well !.My advice-go for a wiseco replacement piston rather than stock.


    Andy

  4. #4
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    could be you are seeing the result of uneven stress on the piston, check conrod for straightnessl are there any signs of detonation on the piston crown? What does the little end look like? Could very well be a metal defect but I would look very hard for the cause of the failure before I put the motor back together..

  5. #5
    Senior Member MadMax's Avatar
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    Time to drop the sump strainer out just in case any bits worked their way down there.

    By all means go for a Wiseco as Andy suggests (a little more compression means a little more woomph) but do a search in the old forum on "Wiseco clearance" or "4 point seizure" before going any further.

    I've only ever seen that happen in a 2 stroke, then again, any major 4 stroke disasters I've had involved "exhaust valve - piston interface"

  6. #6
    Senior Member tzrider's Avatar
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    The engine has been out of the frame and the filter in the sump and the net filter have both
    been thoroughly cleaned. I also washed out the vertical tube with some petrol.

    However, there was almost nothing to be found in both filters. The piston showed no sign of detonation.
    I haven't checked how the piston is lubricated from below but I assume this is splash lubrication and not 2 jets.
    In case of jets spraying the piston it could be that one of the jets was blocked and the pistons doesn't receive
    lubrication at that side.

    The previous (German) owner had put the bike aside for at least 4 years.
    Could it been a rusted liner could be the cause ??

    @Andy: I have now installed a forged Wiseco with 1:10 compression ratio and first 0.25mm overbore.
    I used the clearance as adviced (on the Kedo box): 0,089mm
    Last edited by tzrider; 01-09-2016 at 10:45 AM.

  7. #7
    Senior Member tzrider's Avatar
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    Picts of both pistons



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  8. #8
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    Similar Experience

    Reading this thread today brings back memories. Here is an image of the piston from my 500F Roadster Project. Unlike you, I can't give a history of performance over any period of time. I can only attest to one kick start @ the PO's house when I bought it and a one kick start when I got it home and rode it around the block. In fact, it ran so well that removal of the cylinder was just a precautionary measure (the valve stems were mushroomed & I had the engine out/head removed to correct the valve problem).

    I replaced mine with a Wiseco (new design) 10 to 1 to correct clearance & it is a great bike now.

    Henry
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  9. #9
    Senior Member tzrider's Avatar
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    That does look exactly like my piston problem. So it seems there must be some bad pistons out there ...
    Mine did also run well, was a little weak in performance and I could kick it straight through the compression point
    without pulling the lever. The low compression was the reason I took it apart.
    Last edited by tzrider; 01-09-2016 at 01:22 PM.

  10. #10
    Yes, strange, I've personally not seen that before...were there marks in the cylinder where the rings had been rusted to the bore? You'd think that you could also fracture the cast iron ring though...?

    Interesting, both failures.

    SR ��
    ...'Any unnecessary items on the bike, that it doesn't really need, shouldn't be there!'. (Café racers creed). SR..

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