| Blowby is not caused by o-rings, it is caused by worn piston rings. The M103 has a tight crankcase so removing the oil filler cap while the engine is running should cause the idle to change and air sucked *into* the oil filler opening, not coming out.
Still, unless the engine has 250k miles, hasn't had regular oil changes, or has been severely overheated in the past the bottom end should be fine. You should check for a clogged or kinked breather hose, the smaller ell-shaped one that goes between the rocker cover and the intake manifold. That alone can cause big issues.
When I swapped the 135k mile 300E drivetrain into my 260E I wondered about doing bearing and rings at that time. Every Benz-familiar machine shop I talked to said don't do it. Totally unnecessary. So I did the head only and have not been sorry. At about 196k now with no worries or oil consumption or whatever.
So don't mess with the bottom end unless you have reason to believe it's toast. Oh, of course I did a compression test on the 300 engine before pulling the head. Numbers looked good and even so I took my chances. I won. |