Can anyone help with this. I have just got an 1978 XJ12 series 2 which should have a Borg Warner box but looking at the car on the ramp it looks as though someone has fitted a GM box. The car has the original engine so my question is what are the issues of fitting the later gearbox and would i not be better finding a Borg Warner to put back in?

The car needs renovation and is not running.

Cheers

 

I have managed to get to the gearbox number and it matches the car and engine to the car but is definitely a GM box and not the BW box as stated on the chassis number. is this common?

 

Pretty sure the BW on the body or chassis number has nothing to do with the transmission.
I thought that too until other members corrected me.

 

The GM TH400 was introduced mid-1977 or mid-1978. I always forget !

In the days when Jaguar automatic transmissions were exclusively Borg Warner, the "BW" in the body number signified "Borg Warner automatic transmission". But...the BW designation was used to signify an automatic transmission even on cars with the GM transmission.

Old habits die hard, I guess

Cheers
DD

 

Doug is correct.

If the engine number matches, then itis a TH400 engine.

The crankshaft between the BW12 and the TH400 is different in the convertor spigot hole. A BW12 can be fitted to a TH400 crankshaft with a spacer (I know this from hands on), but a TH400 will NOT fit a BW12 crankshaft, no matter how "special" you are.