laser1 Posted September 19, 2011 Report Share Posted September 19, 2011 Ive been having an ongoing problem with the s3 head leaking on my bike. I switched the head on my 09 250 pro to a s3 star head with a high compression insert for more power. I like the power, but dont like that the head develops leaks, very quickly, on a regular basis. The leaks are small, but enough to cause poor throttle response. When I check the plug, it looks gritty and the threads have a wet silver slurry on them. I pull the head apart and the inner oring is usually shot - melted and hard in spots. (not always the same spot) With really bad leaks, I also get coolant steam out the back of the exhaust, but thats not typical. One thing I noticed is that the inner oring fit on the s3 with the std OEM GG oring is a very, very tight fit. Much more so than the std OEM head. The problem is that the inner oring diameter on the s3 head is aprox 2.5mm larger (yes that is in MM!) and causes the gasket to stretch very tight. I think too tight and thins the oring too much. Anyone else see this type of issue on there s3 heads? Im about ready to switch back to the OEM head but figured I'd throw it out there in case im missing somthing. I have not changed the upper oring (up near the spark plug)as it doesnt appear to be leaking, but it may be affecting how the lower orings seal. Anyone with any good idea's on how to fix this? I think a better fitting inner oring would be a good start, but havent found one yet. Anyone have a source for a correct fitting oring? idea's from the mechanical engineers in the forum? FWIW: I torque the head very slowly and uniformly and retorque after a few heat cycles. Quote Link to comment Share on other sites More sharing options...
richt Posted September 19, 2011 Report Share Posted September 19, 2011 The inner O'ring groove should be the same diameter as your original head. I am suspecting the insert you have got is for the 280cc which the O'ring groove is 3.5mm larger in diameter. (the cylinder bore of the 250 is 72.5mm and the 280 is 76.0mm) Check your insert it should be marked 250A and coloured red. I hope you have not got a 280A. The outer cylinder head is the same for 250 280 & 300. The way you say you have tightened the head up is good and should'nt be a concern. Quote Link to comment Share on other sites More sharing options...
laser1 Posted September 19, 2011 Author Report Share Posted September 19, 2011 (edited) Thanks - I wish it was that easy - its a red insert and labeled 250A. Edited September 19, 2011 by laser1 Quote Link to comment Share on other sites More sharing options...
laser1 Posted September 21, 2011 Author Report Share Posted September 21, 2011 The correct oring to use on this S3 head appears to be a 81mm x1.78mm A non std size. Can find 81mm in 2.0cs, but will over fill the groove. Looks like I will have to have a custom one made. Thanks s3. This completes the trifecta for me. Have now had problems with every s3 "upgrade" ive ever bought. (risers,pegs,head...) nice! not a fan. Quote Link to comment Share on other sites More sharing options...
richt Posted September 22, 2011 Report Share Posted September 22, 2011 I can't see why you need a different size O-ring as it should just be the size to seal on the top edge of the cylinder, I went the other way and have fitted a low comp insert in mine and used the standard o-rings for a 250. all is fine. Something is not right and you should not have to source a non standard part to make it fit! and with any future maintenance you will need more of these. I still suspect you have a 280 insert and a standard 280 o-ring should fit it. maybe its an odd one that could be marked incorrectly though you could expect there to be many more if a full batch were affected. Quote Link to comment Share on other sites More sharing options...
laser1 Posted September 23, 2011 Author Report Share Posted September 23, 2011 I thought the same thing - but the 280 oring is too big and cant be used. I called the shop I bought it from and they checked another 250A insert and it is the same as mine - maybe the batch got messed up or this (HC only) was designed this way - dont know. All I know is the std oring is a very very tight fit and tends to be short lived. I have a few different sizes coming in tomorrow to try out. should work. I have my fingers crossed anyway. Thanks for the replies. Quote Link to comment Share on other sites More sharing options...
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