Looks like a nut I had a hell of a time trying to remove several years ago.

Tango Whatever it is it is part of the thermostat. Wether it is a return flow blocking valvbe or not, it has a spring that connects to the thermost underneath. So whatever it is, it works with the thermostat. Now that weve all agreed, what does the return flow blocking valve do and why does it do it? Tango, you seem to know what your talking about here

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1981 300SD 512k OM603

I a not taking sides but here take at the pic on page 4 in the manual:
http://www.pauldrayton.com/uploadfiles/merc/Service/W123/w123CD2/Program/Engine/617/18-005.pdf

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84 300D, 82 Volvo 244Gl Diesel

Could it have been really bad if i didnt replace that broken piece?

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1981 300SD 512k OM603

No, it isn't. The purpose of the return flow locking valve is to prevent the oil from the filter housing from draining back into the engine when the system is not under pressure (ie: when the engine is not running.) That function is not related in any way to the function of the thermostat. Please note that the thermostat functions when the engine is operating; the return flow locking valve serves its purpose when the engine is not operating.

It is possible that the broken part could allow the valve to become misaligned in the bore and stick in the open position. That would allow oil from the "dirty" side of the filter to drain back into the engine.

I get you, thats the black plastic piece has a spring behind it. When there is sufficient pressure the piece pushes back, allowing oil into the space behind it.

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1981 300SD 512k OM603

I put the housing on the engine today and it sucks. My brand new oil cooler lines dont line up, my turbocharger oil line is too long and I need to cut and reatap a bolt thats too long for it. Not to mention that one passage Im worried about. So instead of buying 100 dollars in new cooler lines, getting another oil supply line and then cutting and tapping a bolt to fit, and then worry about the housing ruining my engine a year later Im just going to buy some brass fittings and do a mechanical to electrical sender conversion. I just need someone with an sd to look and see if there is room for the 300d housing. It seems to protrude an inch or two further backwards. Im curious if there is clearance left behind it for a bigger housing. Thanks anyone who looks

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1981 300SD 512k OM603

Ok and do I need to change the mouting arms on the engine? I hear you have to and others say i dont.

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1981 300SD 512k OM603