M20 Rattle No More

Ever since my engine build was done I’ve had a rattle from the top end. I pinned the location to inside the cylinder head around cylinders 3-5. When my Dad first heard it, he asked if it was a Diesel.

It was a metallic rattle under 3,000 RPM but would smooth out over that mark and sounded great. The engine pulled fine and with good power so I wasn’t concerned.

It was not rod knock. It was not valve-to-piston contact. I had settled into the idea that it was piston slap.

I couldn’t track down the source. The first tech that looked at the car advised me that I should look for a new engine. He said I didn’t have too long. But he couldn’t tell me what the noise was. Mr. Optimistic!

I asked the engine builder that did the machining (but I did the assembly) and he agreed that it was not rod knock. I felt assured that the engine was not about to fall to pieces.

The engine ran like this for three years and about 3,500 miles. But while tracking down another noise (loose water pump pulley) I found my rattle:

M20 rocker arm rattle

This is the oil distribution pipe that runs across the top of the cylinder head. It drips oil onto the rocker arms. Note that the pipe is contacting the rocker arms! If I rock the arm it mimics the rattle sound!!!

I removed the pipe and looked at it closely. Where the rockers were hitting the coating had worn away but no cracks or breaks. I reinstalled the pipe and very gently pulled it up and off the rockers.

The engine is now smoother than ever. It’s an amazing change. All this time with such a dumb and simple fix…. I missed it and two other techs who looked at it.

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