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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #1  
Old 2 Weeks Ago
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davixlrtt davixlrtt is offline
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Default new seats or bigger valves 900

Hello friends, the last time I fitted the heads to my engine I noticed the seats were sunk a little to the point where the valve stems were borderline for maximum protrusion and a bit sloppy in the the guides too, I did shim the springs to correct the coil bind at the time. It’s not my daily rider as it has no lights of any kind so it probably only sees about 5000 miles a year, but I do run her pretty hard. So my question is do I get the seats replaced or go for bigger valves, I’ll obviously do the guides at the same time, the motor is stock except for PB+ cams as the faithful P’s we’re pitting on the lobs, XLR style pipes and currently a Keihin butterfly.
Thanks.......Davi.
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i would go with the bigger valves.they're made for that very reason.more machine time more money for new seats.and you wind up with stock size valves again.
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Hi Bruce, Two things are sort of troubling me about each option, will the bigger valves hurt flow low down, like will It feel less torquey at the bottom end? And regarding seats, even if done properly just how well are they going to stay in place? I’ve seen seats come out of aluminium heads several times.
.......Davi.
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900 Sportsters have cast iron heads, as in IRONHEAD SPORTSTER. With a 900, get new seats and stay with the OEM size valves. FYI - when unleaded fuel came out around 1975, it was common to replace the valve seats and valve guides since there was no lead in the gas to help lubricate the valves.
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Replace the valves with the smallest oversize head that will restore the seats.
You're not limited to the common "+1/16", etc." size choices: your machinist can make that into a +.040" if that's what it takes.
New seats do not transfer heat to the head casting as well as the original, use only when there is no choice.
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It takes at least a 1/16" increase just to get a good seat that's any higher than the recessed one.

.040" ? Ain't happening on an IH.

I've been putting seats in for a long, long time and I can tell you the heat transfer issue is THEORETICAL , not a viable issue on a properly installed seat with .003" fit.

Theoretical is the polite way of saying mental masturbation.

P.S. if you have PB's you already don't have any low end. Stay as small as possible with your valve sizes.
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.040" ? Ain't happening on an IH.

Did it 40 years ago.
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^^ by reducing the o.d. of a +.062 valve?
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.020" per side?

How do you get even a .030" wide seat out of that when the valve's sunk level with the chamber ?

Curious minds want to know.
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Still waiting to learn how to put 6 gallons in a 5 gallon bucket.
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