Author Topic: Pistons  (Read 516 times)

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Offline AlanJohn

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Pistons
« on: June 30, 2018, 06:32:21 AM »
Does anyone know what would happen if the 350 Pistons were installed with the ex at the back and not the front?
Would the bike still run ?
Thanks.

Offline Bridgestoneboy

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Re: Pistons
« Reply #1 on: June 30, 2018, 09:08:34 AM »
Majority of the time no, the bike will not run. The pistons should be installed with ex facing the exhaust side of the cylinder. This mainly has to do with the design of the piston skirts, and how and when the piston allows the fuel and air mixture to flow through the transfer ports to the top of the cylinder in time for combustion. I do think though that the installation of these pistons is even more crucial when you have a type of motorcycle where the carburetors are mounted on the back side of the cylinders, rather than mounted to the cases. Pistons in these types of bikes will have a very different piston skirts for the intake and exhaust side, because the fuel and air flow differently in this type of motor. Hopefully you haven't done anything that will be too hard to correct, and best of luck.

Offline AlanJohn

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Re: Pistons
« Reply #2 on: June 30, 2018, 09:33:08 AM »
I've got a bike just been rebuilt and it just keeps kicking back and won't run and l cant think why even on easy start spray just kicks back l don't know if l've installed the Pistons wrong it was a possible answer It's a weird  one.
I've just taken the heads off and there fine so that's not the problem.
Thanks.
« Last Edit: June 30, 2018, 10:03:27 AM by AlanJohn »

Offline BRT-GTR

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Re: Pistons
« Reply #3 on: June 30, 2018, 10:03:13 AM »
              Alan,
                      Just examined a piston to see what would be affected.
        There would be no effect on the timing of the exhaust port and rear boost port because the piston skirt is not cut away. The side transfer port cutouts are offset toward the front of the piston by 3mm so there would be some slight blanking of the transfer ports. I conclude, from the port timing point of view, the engine should run quite happily and you may not even notice any difference in performance.
         HOWEVER, and that's in capitals for a reason, the ends of both rings are only approx 4mm beyond the vertical rear edge of the transfer port cutouts. With the pistons reversed and allowing for the edge bevel on the cylinder ports THERE IS A DISTINCT DANGER THE RING ENDS COULD SPRING OUT INTO THE TRANFER PORTS AND WRECK BOTH PISTONS AND BARRELS. Can't guarantee this would happen but it will be a close thing.
        If you had the bike the wrong way round when the pistons went in  ::), turn those babies the right way round. Potential damage and cost doesn't bear thinking about.
                                    Brian.

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Offline AlanJohn

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Re: Pistons
« Reply #4 on: June 30, 2018, 10:07:40 AM »
Thanks Brian I've checked and there fine l thought they would be l can't understand why it won't start it just keeps kicking back its got to be a timing issue.
Any ideas please.
Alan.

Offline BRT-GTR

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Re: Pistons
« Reply #5 on: June 30, 2018, 10:20:13 AM »
    Known kick-back causes, Wrong plug firing, coil or points connections reversed somewhere.
      Disc valves 180 degs out, port should be open with its piston at TDC, view through carb.
                Brian.
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Offline AlanJohn

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Re: Pistons
« Reply #6 on: June 30, 2018, 11:24:17 AM »
Thanks l'll sort it and let you know.
Alan.

Offline coxy

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Re: Pistons
« Reply #7 on: July 01, 2018, 04:24:50 PM »
there have been posts like this before also had the same sounding problem and mine was by cheeping out on a crank seal
ill put my money on the crank seal or 180 degrees out

Online OldSwartout

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Re: Pistons
« Reply #8 on: July 01, 2018, 07:32:15 PM »
Not closely related to this problem, but speaking of pistons in backwards - in the 80's, my then teenage daughter put new pistons and rings in her 650 Yamaha twin and the break-in consisted of a round trip from Indy to Chicago. I cautioned her about taking it easy and breaking it in gently. She came back complaining about the rattling the engine was making. I suspected she ran the sh** out of it and scored a piston.  We pulled it apart, the pistons were in good shape, but backwards. Reversing them to the correct orientation quieted the engine.  The piston pin offset caused a lot of noise when they were in backwards.
Karl Swartout
Mooresville, IN
BS175 Roadracer
BS350 GTR

Offline AlanJohn

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Re: Pistons
« Reply #9 on: July 02, 2018, 03:20:42 AM »
there have been posts like this before also had the same sounding problem and mine was by cheeping out on a crank seal
ill put my money on the crank seal or 180 degrees out
It was a timing issue its now running

 


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