value seat dropped

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jonduc74
Cucciolo - the Lil Pup
Posts: 19
Joined: Tue Apr 24, 2007 4:55 am
Location: New Zealand

value seat dropped

Post by jonduc74 » Mon May 26, 2008 3:23 am

Hi, I have a problem with my rear cylinder exhaust value seat has just dropped. My bike is a 1974 750 GT and I have heard this was a major problem with the 860 motors.
Does anyone have any information on how to fix and prevent this form happening again?

Lumpy
SD900 Darmah
Posts: 327
Joined: Sun Mar 25, 2007 5:49 am

valve seat

Post by Lumpy » Tue May 27, 2008 2:24 pm

I rebuilt my GTS 860 engine and found a crack in the alluminium head at the back of the valve seat recess. Had this been left it would have possibly dropped the valve seat. The only reason it was found was because the inserts were removed to be replaced. I`m told that different places, (try Vee Two or Road and race in OZ) will reweld the heads and replace the seats. One theory I heard about the cracks occuring on the GTS is that the actual head castings are the same as the Desmo head and were not designed to withstand the hammering of valve sprung heads. How true this is I don`t know. I was fortunate to have access to some Desmo heads which happily sit on the old girl to this day.

User avatar
jonduc74
Cucciolo - the Lil Pup
Posts: 19
Joined: Tue Apr 24, 2007 4:55 am
Location: New Zealand

Dropped Value seat

Post by jonduc74 » Sun Jul 27, 2008 1:26 am

Hi having a cracked head is not correct, as I have had a front value seat drop as well, there were no crakes found.
It seems to be a heat problem with the heads and seats. I would still like to find out more information on this problem and how to fix.
John

wdietz186
Cagiva Alazzura
Posts: 703
Joined: Sun Sep 10, 2006 5:40 pm

Post by wdietz186 » Mon Jul 28, 2008 8:32 pm

If you look at the casting thickness around the exhaust seats you will notice how little meat is at the short radius of the exhaust port. I was told the bore the seat fits in expands unevenly and if the interference fit isn't tight enough the seats will work loose if the engine runs too hot. I think thats why the valve seat material is bronze so it expands with the aluminum and stays in place better. It takes careful machining and fitting to install them securely. I was told the automotive seat boring tools wern't accurate enough and a Bridgeport should be used. All of this was years ago and i'm sure the newer Serdi equipment does a better job.

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