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#642123 - 02/28/16 2:00 pm valve seat width?  
Joined: Aug 2012
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kootbiker Offline
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kootbiker  Offline
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British Columbia Canada
I am cutting the valve seat width on a Triumph 1973 T100R. The manual spec is 3/32" or 2.4mm which I think is too wide. I have always cut seats on other motors at 1/16" or 1.25mm. What width is recommended?

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#642132 - 02/28/16 2:50 pm Re: valve seat width? [Re: kootbiker]  
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triton thrasher Online content
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triton thrasher  Online Content
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scotland
I wouldn't go below spec on the exhaust side. What would you gain?


Amateur Loctite enthusiast.
#642154 - 02/28/16 4:51 pm Re: valve seat width? [Re: kootbiker]  
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John Healy Online content
John Healy  Online Content



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The width of the seat has a lot to do with cooling the valve. Having a wide exhaust seat is one way of toughening up your engine to the ravages of detonation. More heat is exchanged through the seat than through the guide! The narrower the seat the less heat is transferred. With today's fuel an .080" to .090" wide seat isn't the exception on an air cooled street engine.


#642174 - 02/28/16 6:47 pm Re: valve seat width? [Re: John Healy]  
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kootbiker Offline
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kootbiker  Offline
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British Columbia Canada
.080" to .909" seems like quite a variation?

#642176 - 02/28/16 7:01 pm Re: valve seat width? [Re: kootbiker]  
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triton thrasher Online content
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triton thrasher  Online Content
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Originally Posted By kootbiker
.080" to .909" seems like quite a variation?


Seems like a typographical error.


Amateur Loctite enthusiast.
#642196 - 02/28/16 9:38 pm Re: valve seat width? [Re: John Healy]  
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kootbiker Offline
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Originally Posted By John Healy
The width of the seat has a lot to do with cooling the valve. Having a wide exhaust seat is one way of toughening up your engine to the ravages of detonation. More heat is exchanged through the seat than through the guide! The narrower the seat the less heat is transferred. With today's fuel an .080" to .909" wide seat isn't the exception on an air cooled street engine.
The machined valve face is only 2mm wide so cutting the seat any wider than the valve face doesn't seem logic. Also too wide of a seat would increase carbon build up around the valve to seat contact area.

#642240 - 02/29/16 11:04 am Re: valve seat width? [Re: kootbiker]  
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John Healy Online content
John Healy  Online Content



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OK Kibblewhite 70-2904 exhaust valves for Triumph 650-750 twins have a solid .115" wide seat area and 2mm is within in a cat's whisker of .080".

Quote:
Also too wide of a seat would increase carbon build up around the valve to seat contact area.


Fair enough when this is applied to a modern combustion chamber or there is readily available fuel with appropriate octane, but Triumph hemi-combustion chambers are anything, but modern.

With today's gasoline running in a hemi-combustion chamber detonation, and the resultant increase in heat, is an ever present challenge to an engine builder. Because the force behind detonation the protective layer (boundary layer) gets penetrated allowing the hot gases to contact engine parts directly. The exhaust valve is no exception and the added cooling of a wider seat helps protect the engine from further detonation or catastrophic pre-ignition. During episodes of pre-ignition so much heat is absorbed into the engine that normally exits the exhaust that the exhaust gas temperature actually drops significantly.

Yes, it upsets the gas flow a bit and lowers spring seat pressure, but on an every day street engine it would not be noticeable.

Toughening an engine against detonation, especially a hemi, is more than one thing. We already retard the ignition, slow the advance curve, run a slightly rich mixture, and install barrel shims to lower compression all from original optimum settings. Running a slightly wider exhaust valve seat is just one more tool in our war chest to keep these engines running.

We are not preparing a F1 engine, but something some one can use with today's fuel at highway speeds.


#642256 - 02/29/16 11:57 am Re: valve seat width? [Re: John Healy]  
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kootbiker Offline
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British Columbia Canada
Originally Posted By John Healy
OK Kibblewhite 70-2904 exhaust valves for Triumph 650-750 twins have a solid .115" wide seat area and 2mm is within in a cat's whisker of .080".

Quote:
Also too wide of a seat would increase carbon build up around the valve to seat contact area.


Fair enough when this is applied to a modern combustion chamber or there is readily available fuel with appropriate octane, but Triumph hemi-combustion chambers are anything, but modern.

With today's gasoline running in a hemi-combustion chamber detonation, and the resultant increase in heat, is an ever present challenge to an engine builder. Because the force behind detonation the protective layer (boundary layer) gets penetrated allowing the hot gases to contact engine parts directly. The exhaust valve is no exception and the added cooling of a wider seat helps protect the engine from further detonation or catastrophic pre-ignition. During episodes of pre-ignition so much heat is absorbed into the engine that normally exits the exhaust that the exhaust gas temperature actually drops significantly.

Yes, it upsets the gas flow a bit and lowers spring seat pressure, but on an every day street engine it would not be noticeable.

Toughening an engine against detonation, especially a hemi, is more than one thing. We already retard the ignition, slow the advance curve, run a slightly rich mixture, and install barrel shims to lower compression all from original optimum settings. Running a slightly wider exhaust valve seat is just one more tool in our war chest to keep these engines running.

We are not preparing a F1 engine, but something some one can use with today's fuel at highway speeds.
Just re-measured Kibblewhite 70-4291 exhaust valve face width .080" max.

#642273 - 02/29/16 1:53 pm Re: valve seat width? [Re: kootbiker]  
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John Healy Online content
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Joined: Mar 2005
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Boston, Massachusetts
Yes the Kibblewhite 70-6854 replacement exhaust valve for the 500 does have a .080" face, but what are you trying to tell me?

Triumph is saying .095", for the average street rider I recommended something more like .080" and you want to do it at .060".

You have my opinion, and some reasoning why I hold this opinion, so....


#642322 - 02/29/16 6:42 pm Re: valve seat width? [Re: John Healy]  
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kootbiker Offline
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Thanks for your opinion, greatly appreciated, that's why I come to this forum.


Moderated by  John Healy 


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