What do you think happened?

Don't trade a Merc.

Hoots Mon standard issues with a WIZZER.
Wizz Pop Bang.
engine goes Pop Transmission goes Bang.
[engine went pop mmkay. has to be a wizzer]

I’m thinking, lines from trans to radiator not hooked up? Dumped all trans fluid out of cooling port?

Someone nicked the trans cooler with something while fitting the new engine in?.

I'm guessing he sold the car, motor went bang. Instead of refunding the guy he decided to replace the motor. Mechanic put new motor in and trans leaked oil. Mechanic said not my problem, pay me for the motor and my labour then I'll look at the trans and tell you how much more it's going to cost you.
He is now loosing money on the deal and wants a easy out.

That does sound like what the question could be about Different.

So the easy answer is the mechanic stuffed it up.

the leak was "massive". pipe, or took the convertor off with the motor. and tried to put it back in the same way !

Subaru and those little flat bolts. haha.

Oh, are we still waiting?

what went 'pop' exactly. did it bend valves,or through a rod out of bed? how old, history of car? make would be helpfull

Yes by the sounds of things the mechanic did do something wrong but without knowing the facts or even a little bit of information everything is only guess work.

It's a bit like saying. My motor leaks oil. Where is it coming from

From MY prospective this is a case of helping out or put simply I have no personal skin in this one.
A person purchased a mid 2000's car with moderate k's almost but not quite 3 years before their engine got hot. It had developed a crack in the top radiator tank and the red "light o doom" came on.
Towed to a garage local to the overheating happening it was established that the cracked rad leaking coolant had caused the head gasket to blow. It was covered under a good MBI. It was checked by the MBI company assessor (who saw no indication of oil leaks) so they agreed that the best /most prudent solution was a replacement motor.The motor was a good Low Km example supplied by the MBI company.
The engine swap and radiator repair was carried out. Whilst having its test drive the transmission then dumped a large quantity of "oil" down the driveway and down the road whilst on a test drive.
Where its gotten really messy is that the Garage is saying "wasn't me mistuh" The MBI has expired during the time its taken to be fixed so the MBI company has said its not covered.
The vehicle was checked by the MBI company assessor and the tranz fluid leak is definitely "internal" -ie not any external lines etc so appears to be coming from the joint between the tranz and engine.
The MBI company is saying they are unwilling to pay for the repair as clearly the workshop made a mistake and it wasn't leaking before the engine swap was carried out
The garage is saying its pre existing condition. They refuse to strip motor/box from the vehicle to establish what's happened unless the owner or MBI pays for the work already carried out and refuse to release the vehicle to another workshop independent of them and the MBI.
The owner has the money to pay the excess for the MBI but don't feel its fair for them to pay for one excess and a complete removal /refit/repair of the oil leak. OR pay two excesses if its established to be caused by the blown head gasket.
To be clear -all servicing was up to date with invoices to show less than 5000km previous the vehicle had had its regular service carried out. The pop happened in the middle of a long drive so the vehicle was a long way from home for its owner.
My advice to the owner was to agree to pay for the strip down.
I figured that either the seal was pinched/torn fitting the motor OR was so cooked by the overheating it had gone "crispy critter" and tore when the vehicle was driven. I perhaps wrongly thought that the gearbox shaft seal was routinely replaced when an engine gets cooked. My thinking being that if its crispy critter then its claimable under the MBI. if its just torn then the garage should cover the extra labour and shaft seal.

yep the garage was at fault by what you have written. Best bet is to get a interpretant person to remove the transmission and work out what went wrong and work it out from there who pays what. Until you find the problem the buck will just get past around.

When we take an engine out we disconnect the torque convertor and push the converter back a tiny amount, a safe amount it's designed to accommodate, and we leave it on the trans. There's no way the seal should get damaged.
Seals don't overheat or go bad that quick,without the trans being cooked well and truly. An overheated motor can cause a bit of overheating of the trans but not that much. If it's been hot enough to cook that seal in the bellhousing, it would have cooked everything and the trans would show it.

If it was clean and oil-tight before the work, and there are witness, it should be easy to take the garage to MTA. Haha. Or sue them.

The MTA though is there to protect it's members. Not the public. It's carefully worded, but it says members shouldn't support claims or do anything to discredit other members. Another MTA member is supposed to refer anyone complaining to the MTA's lawyers.

It's a huge scam. A club to cover arses.

ok red light hmmm why do i get the feeling that's a Toyota,

This is what they do now.

https://autofile.co.nz/mta-calls-for-big-spend-on-vehicle-maintenance-

Lol. And if you search on their site for "MTA pledge to the public".

You get.

"*£%@!
It looks like we don’t have anything like that on our site, try searching for something else? ".

They used to have their (deceptivly worded) pledge to the public in all their branches hanging on the wall. They decided that wasn't a good idea years ago.

Or a first. LOL

Now asking them about it just makes them swear.

Beat me to it.
To cook gearbox parts, you first need to cook the gearbox.
Seeing as the engine was cooked, it would have needed to be steaming the auto cooler pipes in the radiator for a very long time to cook the gearbox oil & damage an oil seal to that extent.

I have seen a few engines cooked so badly that they basically turn blue inside & fall apart when dismantled but the auto gearbox has always been ok.

In saying that . . . weird stuff happens sometimes

Unless the trans had been serviced recently and had the bands adjusted wrong or something, and fitting the new motor jerking the seal around was enough to bugger it.

Did you catch the bit where them garage wont release the vehicle until they are paid for the work carried out and the insurer refuses to pay for the work.

seemingly I wasn’t too far off the mark!