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Honda CB250 K4 -72 racer mods

This is a discussion on Honda CB250 K4 -72 racer mods within the Vintage Motorcycle Racing forums, part of the Caferacer.net Forums category; Back from race. Third after a Suzuki T20 and a Greeves. The Honda came in heavily breathing blue smoke from the catch tank. Last race ...

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Thread: Honda CB250 K4 -72 racer mods

  1. #21
    Senior Member Mike 40M's Avatar
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    Back from race. Third after a Suzuki T20 and a Greeves. The Honda came in heavily breathing blue smoke from the catch tank. Last race for the season, so plenty of time stripping the engine to see what has happened. Best fourstroke anyhow.
    Better luck than than for a guy in a supermono class. Engine exploded at the finish line making a big oil spill. He fell off after the finish line and thus got his second place.
    Tanshanomi likes this.

  2. #22
    Senior Member Mike 40M's Avatar
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    Took out the engine. Since I have some other bikes to fix, it is now attended by a fellow racer, who has a lot of experience of tuning and preparing CB twins.

  3. #23
    Senior Member Mike 40M's Avatar
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    Got a X-mas package. Engine back from tuner. No big problems, he changed pistons and cylinder, rest of the engine seemed OK. Never seen piston and barrel worn this way. Left cylinder has 4 deep indentions at TDC and BDC, each side. Too deep for overboring so he got another cylinder from eBay. Top piston ring in several pieces, second ring tight fit but in one piece. Oil ring quite OK. Piston pins OK. Piston crown has some signs of excessive heat. Looks as piston has melted over and under the piston pin, both sides. Right hand piston had both rings in pieces. Any suggestions why this has happened?
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  5. #24
    Senior Member TrialsRider's Avatar
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    Poor fit of the wrist pin followed by catastrophic wrist pin spring clip failure?

  6. #25
    Senior Member Cyorg's Avatar
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    Hard to diagnose things from photos, but there are a few reasons why a circlip comes adrift. Could have been the wrong clips were installed, wrong clip diameter, wrong wire diameter so the clip doesn't sit all the way home in the groove, clip was damaged during installation or the rod is out of whack. A bent or damaged rod, or a small end that isn't running true to the big end can cause the wrist pin to move back and forth hitting against the clip. Eventually the clip and or groove will fail. The clip will erode the piston as shown in your photo and the pin will run up against the cylinder wall.
    "Non urinat in ventum"

  7. #26
    Senior Member Mike 40M's Avatar
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    Sorry, was not able to take better pictures. One of the circlips is intact. I was not told about the other one. The wristpin has not damaged the cylinder wall. When I restored a 1930 Harley 45" back in -63, a wrist pin had made a long straight cut in a barrel. In this case the dents in the cylinder is about 4mm wide and a about 1mm deep. The four dents differ from 2mm to 5mm in length. No damage worth mentioning between endpoints of stroke. If the wristpin had come loose it would have made two long cuts for all of the stroke, eventually progressing to a single big cut. The wristpin is not damaged in any way, except some discoulouring in the middle, indicating excessive temperature.
    You can see the intact circlip on the photo.
    If a circlip was to blame it should first make damage on one side and then went through the wristpin and made its work on the other side. And only at both end of stroke.
    I think it must have been the engine gremlins.
    Last edited by Mike 40M; 12-19-2018 at 01:21 AM.

  8. #27
    Senior Member Cyorg's Avatar
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    I didn't realize that was the clip still in there, I thought it was light reflecting off the end of the pin. It is odd.... the only time I have seen erosion like that on a piston, it has been from either a circlip or a piece of ring. You stated that the oil ring was ok and the second ring was intact. Pieces of the top ring wouldn't likely get in there without some sort of obvious trail past the lands and the two lower rings. Have a look at the top piston ring land and see if it shows any witness marks (above the pin area) from a end piece of the compression ring being snapped off during installation. How does the circlip groove look on the other side? Are there any odd marks on the inside of the pin compared to the pin from other cylinder? Something was bouncing around in there.
    Last edited by Cyorg; 12-19-2018 at 08:23 AM.
    "Non urinat in ventum"

  9. #28
    Senior Member TrialsRider's Avatar
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    Then I'm sticking with tight fit on the wrist pin, the steel got hot enough to melt piston,
    unless you have reason to believe there was foreign material in there.

  10. #29
    Senior Member Mike 40M's Avatar
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    On the piston, wrist pin is tight fit, but I can push it in with my thumb. No more marks found. A fractured circlip, one part stayed, one part slipped through the wristpin might be what could have happened.
    I'm still curious about the four distinctive pits in the cylinder.

  11. #30
    Senior Member Cyorg's Avatar
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    Quote Originally Posted by Mike 40M View Post
    On the piston, wrist pin is tight fit, but I can push it in with my thumb. No more marks found. A fractured circlip, one part stayed, one part slipped through the wristpin might be what could have happened.
    I'm still curious about the four distinctive pits in the cylinder.
    Again hard to tell from photos, but I assume the four pits line up with where the pin changes direction at the top and bottom of the stroke. If that is the case....When the rod is moving up or down, the debris will more or less stay up against the opposing side of the pin or pin boss, but it is still likely to leave witness marks (scoring) in the liner between the pits. When the rod changes direction, that's when the shrapnel starts to create the pits. Those eroded troughs in the top and bottom of the pin boss will force the debris out against the wall. That why you get more wear top and bottom. That area on the piston is not melted, it was eroded/peened by whatever was bouncing around in there.
    Didn't the engine builder have an opinion? He took it apart, so had the best kick at the cat.
    "Non urinat in ventum"

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