I ended up buying one of those sump plug kits from SCA. It has a good range of square plugs in it.
By doing so, I've confirmed that the tank filter plug size is 10mm square.
Unfortunately, the SOB wouldn't budge. Even with the rattlegun.
@carnut222 - Glad you've had no dramas with the D-Jet fuel pump. Do you use a filter on the low pressure side between the tank and pump?
Y'see, I've finally pulled the K-Jet fuel pump from my ex-@egads '74 145 (christened Chloe) which had died, and I found a lot of crud in the front end of the pump. Some pictures from the pump autopsy follow.
Yes; a lot of crud!
Chloe was getting fuel surge with about 1/4 of a tank when I took her home on an unreg vehicle permit a few years ago. While I have her in the air and doing the fuel pump, I figured this may be the best time to also deal with the tank filter on this car, as it may have been the cause of that issue. I hope so, because otherwise it may be a problem with the hard line inside the tank, between the tank filter and the tank outlet.
The 74 140s share the same type of fuel tank as the 75/76 240s. They didn't have a lift pump, but instead had the tank supply connection located on the LH rear side of the tank. For YM77, Mother Volvo did away with in-tank filters with these plugs, added the lift pumps to the tank level sender, and instead put a sock on the end of the lift pump intake. See Cleanflametrap's copy of the Green Book here. The 74 140s have what they call 'Type 1' without the lift pump, and the main K-Jet pump is mounted on a bracket at the LH front corner of the tank via a pair of 1228599 rubber mounts.
The picture below shows one of the pump bracket mounting holes on the fuel tank - this is the area where the pump mounts in late 140s / early 240s with Type 1 main pumps.
Below is the main pump and accumulator without the 1228599 rubber cushion mounts fitted. I'm going to temporarily mount the pump via a urethane bush and a 240 door flange from my odd bits bin and a long bolt, until my iPD order arrives.
(I gotta tell ya, the remnants of those main pump rubber mounts are truly a PITA to remove once they perish!)
I'll have to have a closer look at the YM78 parts car I have to see if it has a tank drain plug - it may not, as IIRC the later 240 models didn't have one either.