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  1. #11
    Co-Pilot Arielarts's Avatar
    Join Date
    Feb 2011
    Thanked 85 Times in 77 Posts
    Hi Bob,

    Never thought of expansion of the port bridge. Obvious when you think about it - much hotter than the surrounding metalwork. You might have thought that it would put a point load on the rings it's designed to protect, each time they traverse.

  2. #12
    Co-Pilot BobH's Avatar
    Join Date
    Jan 2010
    Borehamwood, Herts
    Thanked 18 Times in 15 Posts

    That's probably why the top hole is .30" from the bottom of the lower ring. The lower hole is then .375" below the upper one. The interesting bit is trying to line up the holes with the actual bridge. In the instructions it says to put the piston on the rod, then drop the barrel over the top into its normally fitted position. After that, turn the engine till the piston goes up toward top dead centre. With the piston now visible through the exhaust port, they say to draw a line on the side of the piston on each side of the bridge by fiddling your fingers in through the offset exhaust port with a pencil! Now the barrel has to be taken off again and the piston removed from the rod. The holes can now be drilled between the two vertical lines on the piston side.

    In my case, I didn't have the engine to give me the location of the piston in the bore, so instead I had to use the existing piston as a guide. I didn't have any trouble though, as I was able to put the old gudgeon pin in the old piston, then lay the piston on its side and put the pin through the central hole on my pillar drill table. It just so happened that it was a sliding fit, and allowed me to accurately measure the position of the holes from one side of the old piston on the exhaust side. I was then able to mark the hole positions on a small metal T square and then transfer the positions to the new piston when it was put into the same orientation and position as the old piston.

    Ingenious, but I've never come across this before with any 2 stroke engine I've ever owned or machined, so I wonder if it's just a peculiarity of that particular make and model? Who knows? I'll let you know if I come across this kind of thing in the future, but unless I get to bore other 1978 Suzuki PE 250's I don't think I will somehow.

    I also spent some time chamfering the upper and lower edges of all the ports to help prevent the rings from catching on a port edge.

    Lastly, I 'relieved' the face of the bridge by just under .002" using my riffler files, but I may take that out to .003" tomorrow, as I don't want the newly bored engine seizing when the customer starts it up.

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