Valve/piston interference - expert opinion needed

[pauldridge]

I wound up with a "barn find" 280SL, that has had little exercise over the past couple of years.  My first mechanical issue upon taking possession was a very loose timing chain.  With help from friends, we managed to get the chain replaced using guidance from the forum.  We checked and double checked the cam timing to TDC, and I'm confident we got it right.

Afterwards, I started getting a knocking sound from the engine, under load... sounded all the world like a bad rod bearing.  Yanked the engine out, and we found that 4 out of 6 pistons had light indications of intake valve imprint on the face.  I had my very experienced neighborhood BMW/Mercedes mechanic come over and take a look.  He says this was caused by improper valve timing.

I'm now in the process of replacing rod bearings and rings, etc.  However, I'm having this nagging feeling that there was nothing wrong with the valve timing, and wondering if perhaps the head (or block) had been previously shaved to the point that there was inadequate clearance for the valves? 

The intake part of the head which mates to the cylinder is flush with the remainder of the head... should there be some indentation to accommodate the valve?

I know I read that there is a limit measurement vis-a-vis piston top at TDC in relationship to the block face, but I can't find it now in the BBB.

Any guidance will be greatly appreciated?

[al_lieffring]

When setting the valve timing, the mark on the cam sprocket must be set to the ATDC side, there is no clearance for the intake valves when the timing is set BTDC and they will hit the pistons, Moving the chain on the sprocket moves the timing in increments of 18deg at the crankshaft, most likely the cylinder head has been resurfaced and is affecting the timing, The head being thinner will cause the timing to be slower, and so will wear on the crankshaft sprocket. Even if the timing mark comes closer to lining up in the BTDC position, It must be set to the AFTER TDC side, and if necessary use an offset key to advance the timing.

After assembling the cam sprocket, leave the rocker arms out and rotate the motor (using the crank shaft pulley bolt) to the right until the marks on the cam tower and the timing washer line up. The marks on the crank shaft MUST be on the ATDC side, if it is more than 18deg after, advance the cam timing one tooth forward, Most of these cars the cam timing will line up at about 4-8 deg ATDC and will run just fine set there.

Al

[JimVillers]

Paul .... There is a specification on the distance that the intake valve must be below the flat of the head.  For the 230SL, it is 0.4mm.  If the head has been shaved, the machine shop might not have also set the intake valve low enough into the head.