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Old 04-18-2010, 05:02 AM   #16
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not sure i put a new piston and sleeve in straight away
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Old 04-18-2010, 05:19 AM   #17
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I find it unbelievable that engine manufacturers are still useing this groove in the back plate, ive seen some manufacturers giving warnings about this potential problem on their web sites.For them to remove this potential problem they simply remove/ recast back plates with this lump of material not their so it cant get hooked in side the piston like they use to do with just a flat on the top of the back plate like they use to do. Regarding this would increase case volume your not removing any where enough material to make any noticable differerence. Check out how much material was removed when they removed material from the back plate with the 2 areas that were removed to increase transfer channels to which they have gone away from now claiming round is better. Im not suggesting this groove was done in such a way as to break pistons in this way but how many new piston and liners have they sold because of this happening. I know its down to owners to make sure the piston is well above bdc but its very easy for inexperienced people to not do this, Its also quite easy to do this with experience. You forget at times, Put a flat on = no more broken pistons. Martin.
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Old 04-18-2010, 05:52 AM   #18
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Is this grove in all engines or only the ones in this thread? I am new to the hobby. I have a STS 28 and pulled it apart when new to check it over as I had been advised by a few people. I didn't chip the piston, but this engine may not have this grove or I got lucky and had the piston at or near TDC.
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Old 04-18-2010, 06:05 AM   #19
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It varies on different make of engines as to whether they use a groove or a flat on the back plate. Some engines use shorter conrods and higher wrist pins in the piston to shorten the motors and change the geometry of the motors ie to change rod angles, rod to stroke ratio etc. Which at bdc pulls the piston lower into the bottom of the case so they need a deeper groove or flat for the pist to miss the back plate.
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Old 04-18-2010, 06:05 AM   #20
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i fyou rinse the internals of the motor with fuel and grind down the rough edge os the skirt it should be fine.
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Old 04-18-2010, 07:26 AM   #21
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Wow this sucks! Thanks for the thread though. I've pulled several of my engines apart and didn't even think about the piston catching on the backplate. I know I have some that don't have the notch but instead have the indentation so that they can't catch. Seems pretty basic now but hadn't considered it before. As for running the engine with the chipped piston I would think that the assembly is balanced and now the piston is going to be lighter. With the high rpm that they turn I'd worry about it destroying itself. I'd consult a reputeable builder for advice or just take your chances but be willing to grenade the engine. (worst case scenario right!) Sorry for your luck but thanks for sharing.
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Old 04-18-2010, 08:22 AM   #22
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Originally Posted by voodoopapa View Post
I can verify that the motor was bdc so its my bad

OUCH! Sorry to hear that bud! Well at least now you know, kind of an expensive way to learn. Bump-start engines' backplates are different than engine with pull or roto starts...best of luck from this point on!
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Old 04-18-2010, 08:42 AM   #23
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I've got a friend who had this happen to him. Since it was an old motor he decided to run it until it blew up. He's been running it for about six months like that and it's still going. It sounds funny though.

I've got no idea if he just got lucky or not but you can try to run them. The gamble is of course that the piston shatters or rod breaks, then the engine is basically a write off.
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Old 04-18-2010, 08:53 AM   #24
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The engine will probably still run as stated but you will have massive sub piston induction. This is wear the piston goes above the bottom of the exhaust port. Some aero engines are made with sub piston induction which allows the motor to breath better but this is only intended for use with a silencer on the motor. Tuned pipes and sub piston induction does not go well together and should not have sub piston induction. Martin.
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