Rotating Shims?

[92ShoOff]

Hey everyone, I have been learning about this whole valve lash adjustment and I have a question for you that I could not find an answer to via the "search" feature.

If my shims have not been rotated yet and the clearance on the cam side of the shim is out of spec will simply rotating the shim and placing it in the same bucket put it back in spec? For example, say I have a 2350 shim and it's worn down on one side. From the factory that 2350 shim put the gap within spec. Will flipping over the shim simply put it back in spec? Or will I have to swap out that shim with the correct shim given when using the Valve Clearance Worksheet?

Another example: Say intake valve "A" has a gap of .013"/.3302mm. After plugging that gap into the worksheet along with the current 2400 shim size it claims that I need a new 2525 shim. Do I not want to simply flip the shim over and expect good results on that valve because of cam lobe wear? Is cam lobe wear the reason why it recommends a thicker shim now in that bucket?

Probably a stupid question, I'm sorry... but I guess you can only rotate shims when using them in whatever bucket requires that size after the Worksheet results. Also, does it look likes these shims can be used for our application? Click Here

Much thanks,
Andrew Grayson

 

[revhardSHO]

Yes... flipping the shim will generally get you back into clearence, or at least back to the original height of the shim. Just be sure to measure it once its flipped.

 

[SHObill]

Andy, call me & I'll answer all your questions.

 

[gmorrell]

Having done valve adjustments on many SHO motors, and possessing a valve shim inventory of just over 1000 shims, I can say with pretty high certainty that flipping over the shim will not get the clearance into specification. I collected data on every engine I adjusted, put it all in a database and did some statistical analysis on the data set. This is where SHONut's Gary Morrell shim set came from; its a bell curve distribution of shims based on what real engines measure initially, and then actually need to come into spec, rather then just 6 shims of each size like you got from the other vendors. In my database, there are even adjustments to the curve center based on the mileage of the engine at the initial adjustment.

In 99.99% of the cases, it's not the shim that wears, it's the cam lobe, and some basic knowledge of metallurgy is all one needs to realize this.

Valve shims are a type of tool steel, and are low mass, so it's fairly easy to achieve extreme surface hardness: Rockwell C62 or a few points higher. I have many Yamaha shims that have visibly damaged surfaces, yet they still measure to spec thickness. If you find a damaged shim in your engine, carefully mic the lobes on that camshaft, and I'll wager that the lobe with the damaged shim has lost more material then its neighbors.

OEM camshafts these days are typically chilled gray cast Iron with very small amounts of added Nickle, Silicon, and Molybdenum to improve strength and wear properties, and a few percent Chromium to control the chill depth. The best surface hardness to expect with this process is about Rockwell C57 to a depth of 5 to 7 mm for a casting the size of the SHO sticks.

In summary, for those of you who are still awake; its the cam that wears, not the shim, so flipping the shim isn't going to help.