PeachParts Mercedes-Benz Forum - View Single Post

tcane · #5 03-20-2002, 10:42 PM

Mark:

The M-B engine manual has a section discussing refacing/grinding valves - both intake and exhaust - for your turbo 300D (see page 05-280). New intake valves have a width (I called this the valve margin, M-B calls it spec "h" for the distance from the edge of the valve that contacts the valve seat to the face of the valve) of 2.54 mm and the exhaust width of 2.49 mm. When both are refaced the minimum width is 2.00 mm. Your spec for the recess allowed for the valves is for new valves contacting new seats of a maximum 1.00 mm. If M-B allows the valves to be refaced to 2.00 mm with a loss of about .5mm from the width, then a recess of 1.2 to 1.3 mm for your exhaust valves does not seem to be excessive - especially since a refacing job on the valves also means M-B anticipated the seats would also be ground to clean them up for the refaced valves, then a lap job to finish the contact surfaces between the valve and seat - meaning a slightly greater recession of the valve into the head. I would think your head is OK to use with the dimensions you gave. As a comparison the non-turbo 300D has a maximum recess allowed of 1.5 mm for new valves and new seats, but no spec for refaced valves or ground seats - strange to not have a spec for this, but the M-B manuals have other inconsistencies as well.

I would check to see if you can get the correct valve to rocker arm to camshaft clearance. On the bench assemble the head (valves, springs, rotocaps, lock nuts, cap nuts, rocker arms, cam bearings/towers, camshaft, etc., etc.) and check to see if you can adjust the valve clearances for a cylinder or two since the valves will be closer to the cam than before. This would be easiest for cylinders one and two, but you can do all five cylinders if you want. See what I'm driving at?

Keep a close look on valve clearances and adjusting them more frequently will ensure they do not get too loose which means the cam lobe will let the valve strike the valve seat a bit hard. Living in Boston means it gets colder than other parts of the US and M-B has a different valve clearance spec when it is colder which you need to consider (at -4 F. add .0019 in.) if you should get this cold - let's hope not. The valve rotocaps will help a lot with ensuring even wear to the contact surfaces of the valves and seats.

You're right, any compression loss would be insignificant and should not be a worry - in my opinion.

Good Luck!
Tom