Valve shims

-Poor performance especially compared to a correctly adjusted motor.
-Hard starting, especially once warm.
-Poor fuel economy.
-premature wear of valve seats and valves, especially if too tight.

 

Blue....if the shim is too thick the valve is open too long, or too much. If the shim is too small, the valve doesn't open enough.

As the valve recesses into the head over a period of time, the clearance gets smaller, hence the need for a thinner shim. If a thinner shim is not installed, eventually the clearance gets so small that a feeler guage can't even get in to measure. If the clearance STILL is not corrected, then the shim rides on the cam lobe all the way around and never closes completely. Now you have a loss of compression with all attendant problems.

if a shim is put in that is way too THICK, then the valve opens too soon, or doesn't close all the way, or OPENS TOO FAR and the valve strikes the piston, it bends, and then there's all THOSE attendant problems.

Dave

 

FORGIVEN - did you re-radius the lower edge of the shims??

No I did not, just sanded the top surface. To me the shim is quite small, has a simple job to do and sanding it down will not cause it to break in half IMO. Any slight crown will wear away in time, no big deal. If I turn out to be wrong I will let you all know. Bike ran fine all summer such as it was....cold and wet a lot....so not a lot of riding.

 

You're missing the point: If that shim comes OUT of the bucket, for whatever reason, at say, 6000 rpm it will do an unimaginable amount of damage inside the top end. I wouldn't worry about breaking in half as much as I would a spit shim.

If you turn out to be wrong, it'll explode on you.

I like my bikes too much to chance it for $8 a shim.

 

You are supposed to "surface" the bottom of the shim, if you are inclined to "deck" a shim at all, but please keep us posted if you have an issue.

The cam-side surface (top) is to remain hardened. Or just spend $8.00

 

If you DO resurface the bottom, make sure that you get the size stamped on it in some way that it won't come off or you (or someone) in the future will have no clue what it was.

Please don't push those into the shim pool when you pull them, though.

Dave Fox

 

If you know someone with a machine shop, just surface grind them then lapp 'em just use a micrometer to measure.

 

If someone were to run an XJ with excess clearances (the umpteen thousands of unwashed, unshaven XJ owners) by four thousanths, you would get marginally less duration and lift which would marginally IMPROVE idle quality and even measured compression, as the intake would close earlier in the compression stroke, at the expense of peak Horsepower.

The XJ runs sixty degrees of overlap and is one heck of a hot-rod.
If a 350 Chevy were built to the same level, it would make 625 Horsepower (with mufflers)

 

What's a muffler ?

 

a muffler is a thing that goes on the exhaust pipes that keeps people from considering you a ass

 

When the valve lash is within specs a spit shim is imposible unless it breaks apart, all the shims have the same depth where they cover the top of the valve thus all are locked in with the same tolerances on each valve. Thus changing the total height of a shim will not affect at all the possibility of it being spit out. I am not worried about that at all.
And no I would never put a modified shim into the shim pool. I do have a couple that are original in size, just cant find them now.