Smaller Shims

[chrislb]

Well thought i'd jump in and have a bash at doing my own valve clearances, but all was going really well, only did rear cylinder for today, and well its really weird, of the shims that need replacing, i need to replace them with smaller ones, which i find odd, for example outlet valve on cylinder 3, valve clearance, was .20mm, shim is 2.33mm, and following haynes manual (and one off here), in haynes they use a mid number between .21mm and .27mm which is .24mm, and the new shim has to be 2.29mm? I did .20mm - .24mm + 2.33mm which is 2.29mm. Have i done something wrong here? Any help would be great as i'm hoping to start front cylinder tomorrow

edit, NC30 forgot to add

[monkeywithaplan]

you will find the valves will stretch faster than there is wear between the cams, rocker arms and shims, hence the space always closes up. if you find it the other way round it often indicates the valve is not seating properly due to carbon deposit buildup caused by very bad running

[chrislb]

Okay cool so i assume i'm doing it right! So because it some of them need smaller shims i take it this is to put the valve clearances back into tolerance then? Just about to do front cylinders in a bit, thought i'd ask just in case i thought i'd done something wrong!! lol

[fumb]

you will find the valves will stretch faster than there is wear between the cams, rocker arms and shims, hence the space always closes up. if you find it the other way round it often indicates the valve is not seating properly due to carbon deposit buildup caused by very bad running

Is that right? Valves stretching, i've never heard of that before.

Wouldn't all bikes get quieter with wear rather than louder then?

[viper_biker]

you will find the valves will stretch faster than there is wear between the cams, rocker arms and shims, hence the space always closes up. if you find it the other way round it often indicates the valve is not seating properly due to carbon deposit buildup caused by very bad running

Is that right? Valves stretching, i've never heard of that before.

Wouldn't all bikes get quieter with wear rather than louder then?

RC30's suffered from valve stretch. It's actually recommended that if you own an RC30 to change them to stronger one.

[zimm]

valve's stretch and valve seats recede into the head, so clearances close up..

its been like that on all shimmed engines i've worked on.

[fumb]

So... at what rate do valves stretch compared to shim wear? If only Honda got their sums right and made them both equal. :)

Seriously though.. if valve stretch was common on NC30's then they would become quieter. From what little i know and heard they don't.