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New valve shims scratched cam lobes, what to do now?

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I am hobbyist building an Mazda MX-5 turbo engine targeting 300-400 whp. To achieve the correct valve clearance on the exhaust cam I bought some 27mm shims from ebay here.

After installing five of these aftermarket shims together with 3 OEM ones, I noticed that the five cam lobes that are using the aftermarket shims had been scratched. Not on the top of the lobe, just on the part that runs against the edge of the shims. Picture on the worst scratches here.

I did remove and inspect the shims. I could not see or feel any fault with them but found some metal shavings on one of them so I buffed the sharp top edges with a 240 grit paper and reassembled. But after setting the timing I believe that the new shims are still making new scratches on the cam lobes.

I am using stiffer valve springs than OEM, but since the lobes using used OEM shims are fine I don't think that is the problem.

This is my first engine build and I would appreciate some help to decide what to do about this. Should I try to polish the shim edges with finer grit or is the problem something else? Perhaps the new shims will break in by themselves against the cam lobes? Do I need to find another exhaust cam?


It sounds like you've done a great job diagnosing this already. OEM shims are the safer bet, but if you want to chance it with the ebay shims, further polishing them may help. When working by eye, small but critical differences in surface finish can be hard to detect. The hardness of the material may also be the cause, and not something you'd see by eye.

At this point since the cam lobes are scratched, it may be too late to save them even if you switch to OEM shims now, depending on the severity of the scratches. Sometimes replacing one bit at a time cannot correct what's already been done and you have to start over with all new parts. Hopefully you've caught this in time to avoid that scenario, and some new OEM shims will get you on your way.

Best of luck with the project and please let us know how it goes.

I am confused. On my 2006 - 2013 MX-5 / MZR LF engines, you replace the tappets to change the thickness, there are no shims.

What year / displacement is this engine?

It is a Mazda BP 1.8 liter VVT engine, made between 2001 and 2005.

I did polish two shims today with quite good result, so I think I will try to polish all 5 new shims and the scratched lobes. Wet-sanding with 800 + 1500 and then Abralon 2000 + 4000 using a Mirka. The soft Abralon takes off the sharp edge around the shim. To my knowledge new OEM shims are not available from Mazda.

Picture of a polished shim and a new untreated ebay shim

This picture shows the worst cam lobe before and after polishing.

With the camshaft and shims polished I reassembled and turned the engine by hand. Now I can't see any new scratches.

I will check again after starting the engine and if there is a problem I will use another camshaft and Toyota shims from https://www.toyotapartsdeal.com/ with help from this excellent crossreference created by TheCarPassionChannel.

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