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Enduro Engine - 2 stroke Cylinder, Piston, Tranny, Bearings, Clutch, Pipes & Silencers, etc.


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Old 02-12-2009, 10:27 PM
Treesmacker Treesmacker is offline
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Default Cyl/Head o-rings why so small

My question is why are the cylinder o-rings on the GasGas undersized compared to the o-ring groove?? They need to be stretched alot to fit into the groove, resulting in a smaller cross section. Are they undersized for thermal expansion?? http://catalog.precisionassoc.com/pr...lication-data?

I'm a mold designer and spec out o-ring grooves all the time.
When we size a o-ring groove we match either the outside or inside of the o-ring to the groove, depending on if it has internal or external pressure. The groove is designed so that the o-ring protrudes above the surface but is wide
enough to accomadate the o-rings cross sectional area.
Bob in MN
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Old 02-13-2009, 12:25 AM
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What size bike do you have? You could have got 200 O-rings. I find using some grease helps keep them in place.
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Old 02-13-2009, 08:10 AM
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I'm wondering the same thing. The o-rings I have put in my 250 fit well and only one was a hair small. It only took a small amount of massaging to get it to fit.
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Old 02-13-2009, 08:26 AM
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They shouldn't have to be stretched excessively. Just a light stretch through your fingers with a light coating of grease. Drop in the groove and run your finger around a few times, they will then stay put with no issues. Drop the head on square and rotate slightly back and forth, it should feel smooth. I agree with the above post, if its more trouble than that you may have the wrong o-rings, it has happened to me.

o-ring specs/grooves have a lot to do with the material, pressure, temp, and if its a static or dynamic seal. I'm no expert but have some experience in both medical and industrial applications. Published guidelines are available. The GG head /cyl design has never caused a problem if assembled correctly. I have seen o-rings pinched causing a leak. Thats why I rotate the head slightly on install, if an o-ring jumped a groove you will feel it.
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Old 02-13-2009, 11:20 AM
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When I did my 250 awhile back I was afraid I was going to rip the o rings in half. I tried putting them in hot water and everthing else finally stretching them seemingly to the point of tearing.
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Old 02-13-2009, 12:49 PM
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That is just not right. Likely you got o-rings for a 200.
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Old 02-13-2009, 07:25 PM
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I had the same problem the first time I got an outer o-ring in the mail, it was way too small. The next o-rings I got in another order were just a little small but fit with minor stretching and stayed in place. I believe the first one sent to me was for a 200 and not a 250.
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Old 02-13-2009, 10:02 PM
Treesmacker Treesmacker is offline
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Well the first try I did have the wrong o-rings now the inner is correct, still a little small but the outter is way small again and is much smalled than the ones I took out.
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Old 02-18-2009, 12:54 PM
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Sometimes the dealer gets sent the wrong parts. Back a few years, I was buying 250 head o-rings and the dealers whole stock was wrong, under the correct part#. Not his fault, you expect people to pack things correctly.

If you can't get the right part just order from a supplier, the size is in the parts manual.
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Old 02-20-2009, 12:09 AM
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I received the corect o-rings and they were pretty easy to install.
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