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 Post subject: SR500 Piston compression
PostPosted: Sun Feb 06, 2011 9:04 pm 
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I just recently bought an sr500 with a blown engine and realized it was bored out to a 90mm piston.

I'm debating on buying a 9.1 or a 11.1 I want more power, yet I don't want to sacrifice reliability what do you guys think?

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Photo0181.jpg by foundation982, on Flickr

The piston I pulled out


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PostPosted: Mon Feb 07, 2011 1:21 am 
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Joined: Mon Feb 23, 2009 9:15 am
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Location: Germany Stuttgart, where car was born
Hi
this piston was pinged to death.
In my opinion the max bore for me is 88,25.
I can get the 88,25 kit for a good price, add 3 poin stress relieve.
The compression is stock.
With a hiha-cam 10° late , 60mm Venturi and stock downpipe with slightly modified BSM short classic or dunstall muffler it ran 178km/h without loosing reliability.
Regards
Rei97


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PostPosted: Mon Feb 07, 2011 9:38 am 
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Thanks Rel97, But I'm not looking to go overboard on the engine (at least not yet) just wondering if I should buy the stock compression piston or the 11.1


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PostPosted: Mon Feb 07, 2011 5:37 pm 
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Hi :)
I'm pretty much an amateur myself, but I was very enlightened in the two threads that I am currently following:

Where are you in the world? I'm talking 535 tuning in my thread about GW Racing Parts in Germany. They have 30 years experience with doing engine work. They only deal with companies though.
http://www.sr500forum.com/forum/viewtopic.php?f=9&t=4478

And
http://www.sr500forum.com/forum/viewtopic.php?f=9&t=4675

If your piston was pinged to death chances are your cylinder is pretty worn too ?? Might call for some serious inspection of tolerances ?

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PostPosted: Mon Feb 07, 2011 10:05 pm 
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I'm in the states. But my cylinder is almost perfect shape. my dads a mechanic and his friend owns a machine shop so I'm sure we can fix it, just a matter of getting exhaust and intake valves and a piston and she should be good to go. I'm planning on buying a wiseco 90mm but they have the 2 my dads telling me not to get the 11.1 because it may not be as reliable and may vibrate more. on the other hand some power sounds fun but I'm thinking of using this for a commuting bike and don't want a lot of vibration


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PostPosted: Mon Feb 07, 2011 10:25 pm 
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Foundation982 wrote:
I'm in the states. ...


Please, see this post: Click Here

This is an international forum. It will help you and us a lot.

G Man

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PostPosted: Tue Feb 08, 2011 3:35 am 
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Foundation982 wrote:
my dads a mechanic and his friend owns a machine shop so I'm sure we can fix it, just a matter of getting exhaust and intake valves and a piston and she should be good to go. I'm planning on buying a wiseco 90mm but they have the 2 my dads telling me not to get the 11.1 because it may not be as reliable and may vibrate more.


You know what, those were exactly my thoughts before I started getting in to the complexity of tuning a relatively small engine as a SR 500 !
My dad's a "mechanic" too and he's always advicing me to go for durability rather than extreme power.
If I understood it correctly high compression gives you more power and more low end torque, but it also developes more heat. It may bring hammering vibrations to the crank, eventually wearing it down, calling for the need of a lighter crank.... However that's not necessary with lighter compression. Only means you're giving up a little power over durability.

Has your head and cam been worked on?

Personally I'm going for

1) 535cc (90mm) bore 9:1 or 10:1
2) Porting head and grinding valves and seats / maybe new rocker arms
3) Possibly changing to bronze valve guides
4) 1 stage (low end torque) cam
5) Balanced (original) crank
6) Stock OR TM 36 carb

This would give me roughly 20 % more power meassured on the rear wheel.

Even if your head and cam has been worked on or not I would go for the 9:1 (which is standard) compression for the best durability. Possibly your piston was worn because of the high compression and heat ??
Remember your double oil line for better oil flow to the head and top !

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PostPosted: Tue Feb 08, 2011 9:36 am 
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thanks! you've been the most help I'm going to go with the 9.1 piston. I'm not sure if the heads been worked on but the cam and rockers seem be in good shape. But my dad is a "legit" mechanic hes worked on John deere tractors for about 45 years now and probably has rebuilt thousands of engines.

I'm ordering new valves their bent up but not too bad (enough that they need to be replaced)

and mine only has 1 oil line to the head that may be why the valve smacked it?

wheres a thread on modifying the head for 2?


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PostPosted: Tue Feb 08, 2011 11:05 am 
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Location: southeastern Iowa 80 Yam XS1100, 78 Yam SR500E, 87 H-D XLH 1100
the pinging (detonation) from low octane fuel/out of time is what destroyed that piston and valves if you don't have a reliable source of high octane (+96) fuel the 9:1 is the wise choice ... oil line thread viewtopic.php?f=9&t=4689

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PostPosted: Tue Feb 08, 2011 12:57 pm 
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Remember....Octane rating numbers vary on how they are calculated, depending on where you are in the world.

http://en.wikipedia.org/wiki/Octane_rating

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