I've got a few droplets of diesel fuel in my recovery tank in my 286,000mi 1968 300 sdl engine (#14 head). I also have oil dripping down the side of cylinder head at #1,2, and 3 piston. I want to put a head gasket sealer - like Barrs- for a short time before I pull the head. Can I do this on this engine?

Barrs is for coolant leaks, you have a oil leak. I would not recommend using it. The diesel you have in the coolant is probably lube oil. See exactly where oil is coming out of the engine. It could be leaking out of exhaust manifold or timing chain tensioner. If leaking at the head gasket consider tightening the head bolts 45 degrees. It may help, I do not think tightening them will hurt anything. My car leaked oil into the coolant for over 47,000 miles, when oil started leaking into number one cylinder at 347,000 miles,the head gasket was changed. I should have considered tightening the head bolts.

In you post you have 1968 300SDL.
I looked up 1989 300SDL listed on your profile and that only diesel shown for that year is a 190D?
So something is wrong.

Some of the newer Diesels have A Fuel Heater. If yours has a Fuel Heater and it is heated by the Coolant that would explain the Diesel in the Coolant Overflow tank.

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84 300D, 82 Volvo 244Gl Diesel

I'm guessing he just switched 1986 to 1968. The 300SDL was only made in 86 and 87.

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Serenity-86' 300SDL 482,000 miles (on a "14" head!)

Angle torque the head bolts 45 degrees??? DUH! Why didn't I think of trying that? My head gasket is leaking just a teensy bit of oil into the coolant and I was getting all spooled up for a gasket replacement without ever even considering tightening the bloomin' bolts... you're a genius!

Jay.

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On the road, currently:
'83 300D (German spec.) 168k mi. - - Wolfgang (tucked snugly away for the winter!)
'87 300D 375k mi. - finally went over to "The Dark Side"
'87 300TD 225k mi.
Cannondale 66cm CAAD5 w/Campy/Jamis Aurora
Hujsak Custom Fixie/Landshark Track Shark Custom/Ahearne Custom 29'er

Oops, it is a 1986 30 sdl. Thanks, retorquing the head bolts is a good idea. Something I don't get is if it is oil and not diesel it would be black droplets, I have only oily looking droplets.

The lube oil I had in my coolant was tan colored. Maybe the black soot drops out when in the presence of antifreeze. Most can be removed from the expansion tank with a paper towel. The fuel heater tubes can be bypassed. They run with the heater coolant pipe, outboard of the oil filter.