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Thread: Boring and Honing Service

  1. #11
    Diamond geezer 500 Club
    Join Date
    Dec 2006
    Location
    Borehamwood, Herts
    Posts
    1,304
    Hello,

    I must say it never ceases to amaze me how ingenious people can be. A customer brought a 1978 Suzuki PE 250 barrel to me the other day, along with the piston. He's been having a lot of trouble starting the engine from cold and after doing all sorts, including replacing the carb with a brand new item, he decided that the compression was at fault. He wasn't far wrong either. The PE is a reed valve induction engine, so it relies on having a good vacuum in the crankcase when cold in order to suck the fuel air mixture in past the reeds. A worn bore and/or worn or broken rings, will reduce the compression above the piston, and the vacuum below it, so that less fuel can get past the reeds, particularly when cold.

    Here's a photo of the piston he brought with him.

    Piston holes1.jpg

    See the two little holes in the area where the piston has previously seized? Some bright spark decided to drill those after the engine had seized and been freed off, so that some of the petroil mixture would be forced through them and into the previously seized area every time the piston went down the bore.

    Clever? Yes! Good practice? No!

    Thank goodness the holes were as small as they were, or the piston could have cracked around them, and broken into pieces, which wouldn't have done it any good at all.

    The standard bore size on these engines was 67mm, with a stroke of 70mm, so it was undersquare. That's understandable, as the PE was designed as a mudplugger trail bike, with lots of low down torque and only 28hp. This particular one had been bored out to 67.6mm. Looking up the normal oversizes for this engine I couldn't find a 67.6mm oversize, the nearest was 67.5mm. However, this one had definitely been bored to 67.6mm, not 67.5mm. I measured it half a dozen times to confirm this, and the boring cutter marks are still in the cylinder wall at this size, so I know it's not worn the extra 0.1mm since being rebored. Furthermore, the cutter marks I just mentioned were in a sort of herring bone pattern, as though the cutter was blunt and chattering all the way down the bore. If I can I'll try to take some photos of it and put them here, as I've never seen cutter marks that bad before (and I saw a few back in the day!), you can even feel them with your finger nail.

    The piston shown above, was 67.3mm at the bottom of the skirt, which is where pistons are normally measured for clearance purposes. So the gap between piston skirt and the cylinder was over 0.3mm. 0.3mm is 0.012" but the actual clearance on this one was 0.013", and is more than three times the recommended gap for most engines, which is normally around 0.004", or 0.1mm.

    No wonder he couldn't get the thing to start! There must have been so much blow by at the rings and skirt when the engine was cold that there'd hardly be any vacuum for sucking fuel into the engine when cold. As the engine warms up the piston expands in the bore and takes up much of the clearance, so the compression increases at the top and the vacuum increases at the bottom, thus sucking in the correct amount of fuel mixture every revolution, and the weak mixture problem goes away.

    We've managed to find a 68mm piston in the US, so he's ordered it. It wasn't cheap, I think the total cost, including postage from the States works out at around 130 or so. When it arrives I'll see about boring and honing the barrel out to match it. I'll let you know how it all works out, and whether his bike starts and runs correctly after the piston clearances have been returned to factory spec.
    XL's forever! Well, one of them anyway. It's all I can afford, not to mention the Raven and the Mini-Max. Oh, and I almost forgot the Spectrum as well :-)

  2. #12
    Thank goodness I've never been that clever!

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