Anyone machine a stem steerer clamp ?

The clamp gap looks very tight....stupid question, but are the bolts undone?

If not, like others say, try wedging the stem open a touch . I find plastic construction wedges very good for this type of thing or a wooden wedge.....saves the paint / aluminium.
 
I agree with the other comments, the top of the steerer looks like it could be flared slightly? The star nut fitting can do that depending on the way it's pushed in.
 
I'd take a file to the edges of the steerer first. If it has been cut down, often the top flares if a tube cutter is used, or rough edges if sawn. Some stems are more forgiving than others in this regard, I've had some stems that are super tight over a steerer vs some that just slip on over the same steerer.

Personally I would not want to remove any material from a stem.
 
I know it's a month late and you've probably already sorted this but nobody who has replied has sussed that the fork is a Cannondale Headshok.

Looking at the bikes in your sig you know about Cannondales but that stem would appear to be a 1.5" diameter stem if it doesn't fit the Headshok steerer and you think it needs opened out by 1mm to fit.

Headshok steerers are nominaly 1.562", actually 1 9/16ths inch, Americans don't do metric, not 1.5". That 0.062" or 1/16th of an inch, makes all the difference. So 39.68mm minus 38.1mm is 1.58mm, a bit more than the 1mm you thought. Looking at the Thomson stem I'd be a bit worried about removing 0.75mm from inside of stem where the M & S of Thomson is, That's it's thinnest part and also where it's under greatest stress, the rest of stem is thick enough though. Think my calculations are correct but I'm old so may have made a mistake so happy to be corrected.
 
I know it's a month late and you've probably already sorted this but nobody who has replied has sussed that the fork is a Cannondale Headshok.

Looking at the bikes in your sig you know about Cannondales but that stem would appear to be a 1.5" diameter stem if it doesn't fit the Headshok steerer and you think it needs opened out by 1mm to fit.

Headshok steerers are nominaly 1.562", actually 1 9/16ths inch, Americans don't do metric, not 1.5". That 0.062" or 1/16th of an inch, makes all the difference. So 39.68mm minus 38.1mm is 1.58mm, a bit more than the 1mm you thought. Looking at the Thomson stem I'd be a bit worried about removing 0.75mm from inside of stem where the M & S of Thomson is, That's it's thinnest part and also where it's under greatest stress, the rest of stem is thick enough though. Think my calculations are correct but I'm old so may have made a mistake so happy to be corrected.
I was going down the road of gently wedging it open and possibly taking microns off the steerer
 
I know you may be set on using the Thomson stem but you'd be stressing the stem in wrong direction by wedging it open and it wouldn't grip steerer with whole inner surface as stem is different curvature to steerer. Removing metal off top of steerer evenly would be difficult without taking it apart as if you got even one or 2 bits of steel swarf in needle bearings of Headshok it would be effed.

One of my Cannondales, a Team Blue F800 with a 100mm Lefty, has a Hope Headshok stem as Hope made the 1.56" type for a while. No idea if they still do though. Got mine off ebay. The Cannondale stem with the 4 bolt handlebar clamp doesn't look too bad, my Super V with a Lefty had one.

Having said the above I'm almost at the point of doing something along same lines to a Marin. It's a 1998 Wolf Ridge that I've rebuilt with a 140mm Fox fork and the longer swingarm off another similar Marin to give 120mm at rear. It needs a dropper post since my modern geometry bike that I bought last year has one and now I'm used to using one on steep stuff. The seat-tube I.D. is 30mm and closest dropper is 30.9mm, there are 27.2mm droppers but they are not long enough for bike. So I'm thinking I can ream out 0.45mm from inside seat-tube to make one fit, someone please stop me......
 
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