Thoughts and recommendations on e320 engine swap (W124 wagon)
 

The head gasket in my '94 E320 wagon gave up recently. The original engine has 260K miles, and while it hadn't overheated it did get very close on two occasions, and the oil pressure had dropped very low. Also, the transmission has become slow to engage reverse on occasion.

I picked up a supposedly 112K mile M104, and again supposedly 26K mile transmission last weekend. Both were already pulled before I acquired them, so there was not possibility to check their condition prior to purchase. Fortunately the price on each was reasonable, and I'm hoping for the best.

My question for the community is given the M104's traditional head gasket woes, and with an unknown history on this engine would you replace the gasket prior to installation into the car? Anything else that would be best to address now? Also, while the transmission looks great, and the fluid appears to be good what would you do while it's out of the car? I'm certainly planning on replacing the rear main on the engine and the input seal on the transmission, but anything else that I really should consider?

Both the engine and transmission were sold to me as being from a '96 model year w210 E320, but everything I researched seemed to indicate that these should go into my '94 W124 chassis fine.

the ecus, harnesses, etc are different. it will drop into the 103's place but it won't plug in.

regarding the gasket, look for oil leaking at the front and rear of the head where it meets the block. if 26k is correct, you shouldn't see it. i'm used to seeing it closer to 100k. good luck, chuck.

Only replace the rear main and trans seal if they are leaking. Not leaking is the best you can hope for and you already have that. The trans seal would have been done on the rebuild 26K ago. Replacing it may make a leak.

About the head gasket, if it is dry now it probably has been replaced. So no I would not change it.

Are the oil pans the same? Look closely to see if there are any clearance problems.

Do consider new motor mounts, trans mounts, flex disks and center support bearing unless they are excellent. Look at the water pump closely for leaks too and change it if necessary.

Worn steering linkage would be rather accessible to replace while the trans and motor are out.

My 93 300E 3.2 has just over 360K and ran incredibly well but the head gasket is leaking and at 360K I probably will not do it.

'93-'95 W-124s came with M104 engines
 

Both the engine and transmission appear to be leak free. Neither look to have been cleaned after removal from what I can see, but both look great with no obvious signs of gasket failure.

Good suggestions on the mounts, and driveline..
 

The transmission is supposedly a 26K mile original, not rebuilt. It looks fantastic. Thanks for the tip on the seals. They do look good and if disturbing them will just open a can of worms then I'll leave them be, join the two parts and give it a go.

Honestly worst case is that I have to pull it all out again.

Thanks for your suggestions.