420G transmission

I need some advice please! If someone can please id the current transmission of the 420G for me please, and tell me whether it is worth restoring it or ? I am not concerned about originality should this be and issue.

Kind regards

Looks like the BW 8, same as on the 420. (also used on many other models including Volvos iirc). Many internal parts are interchangeable. It should have a serial number stamped on it - that would help with a definite ID.

Thanks! Parts availability an issue?

That I can’t say. It’s been 30 years since I opened an automatic transmission. The BW 8 and 12 were in wide use for a long time, so there is a good chance that someone is still making wear parts.

Your best bet is to id what you have conclusively, then find a local transmission rebuilder and ask them about parts. You’ll probably end up going to him for parts anyways if you do the rebuild yourself.

Much obliged! I will then first wait for additional comments.

Take a pic of the other side. iirc one model had an external modulator valve, the other didn’t.

Does this help? Thanks in advance.

Think about a GM 700R4 overdrive transmission. You will have to get the bell housing converter piece from someone like John’s in Texas but it will make a much better car than the current slush box. I am considering this and then putting an XJ-6 type floor shifter in the car to avoid the hodge podgery of all of the current linkages. Some “slight” adjustments will have to be made to the trans tunnel.


That is a BW8,see the num 8 on the box.

BW8 has the same rebuild kit as a BW12 and Ford FMX (factory fitted to Mustangs and even Big Block)

The iron case is 9-7/8" long same as FMX

if your car is RHD, an T7004 will not fit, as per John himself, not sure on LHD.

the trans tunnel in these cars is ridiculously tight for such a big car.

a 4speed from XJ40 can be adapted

there will be a serial number stamped on that oval plate…I have several BW8

1 Like

Would you agree that the DG250 is woefully inadequate for the size of these big Jaguars? Not sure of the durability of the BW8 or 12’s for this car. Could the XJ40 transmission work and be easily repaired if necessary?. Would allowances have to be made for losing the automatic kick down which I feel would just be a matter of pulling the shifter down when extra power is needed. A tight fit is indeed the problem and some handy work with a plasma cutter might be in order and then subsequent reconstruction of a console to handle the added girth.

My DG250 leaks like a sieve and I am unable to drive the MK10 at the moment. I really do NOT like the shift patter with reverse to the right of low. I made the mistake once of pulling the shifter down to what I thought was low only to find myself in reverse at about 25MPH. What a sound that made!!!

Andrew might see this as a “fun” project!!


one solution would be to acquire a wrecked 4.2MKX or 420G, and replace the
trans, bellhousing, steering column with all shifter parts, (and entire front end with steer box as a bonus)

fitting a BW12 from an XJ is also possible

The XJ40 have a ZF trans, and is an expensive job if done professionally, has been done by a homie, but I dont know the details

the BW8 and 12 are by no means weak, they have Iron cases, big gears, and came out behind millions of Fords as an FMX, the BW12 and FMX have vacuum modulation kickdown (8 has a cable)

The V8 Mustangs had ~320 ftlb of torque, and even Factory fitted behind big block Fords

Not every “hard” internal part is absolutely identical
The various overhaul kits for all 3 trans are generic

There is no question in my mind the BW8 or 12 is strong enough for a MKX

BW12 also fitted to S1 XJ6 and XJ12, and is a known upgrade for MKX, as it has better kickdown, is more common to obtain 2nd hand, and is marginally stronger…for instance, despite appearing absolutely identical, the input shaft OD on a BW12 is .020" greater than a BW8

It also seems an A340 trans, especially from a Volvo or Toyota can be be fitted, but its a big project

from what I have heard, once the DG250 needs an overhaul, that is difficult to get parts, experienced rebuilders, and to stop them leaking

one thing I did find out is the front drum on these trans is a limiting factor, as they suffer internal bore wear from the iron rings, and cannot be saved if worn, and although I do not know 100% for sure, suspect this is one part that is unique to each “variant”

The interesting feature of the BW model 8 and 12 is that they have four mounting ears that permit the gearbox to be removed separately from the bell housing and torque converter. This as a very American design feature and why the Brits never adopted this on their manual gearboxes is surprising as it would make changing a clutch much less of a task. Model 12s were used all the way up through the early XJS cars before adoption of the GM Turbo Hydramatic 400, circa 1980.

in the MKX, its very difficult, to almost impossible to remove trans in vehicle without engine & trans coming out, which is no fun either.

However it IS possible to R&R by taking it apart from the rear, piece by piece, reverse to install.

a previous lister did it this way when he inserted a V12 into a MKX

Thank you all for the comments. I guess that, unless I am able to find and fit a manual gearbox, rebuilding the BW8 would be the easier option!

It’s more work to change over, but an all synch MOD in the Mark X models is sweet (and rare).

Totally agree, but I need to be able to source all the bits necessary for the change to MOD first. Not sure it is going to be that easy to find locally, but will give it a try before I start the rebuild

Ask Micah on this list about a conversion to a MOD.


very rare now, even so a rusted out MOD MKX/420G or S1 XJ6 is the most likely bet, or specialist Jag wrecker, but they will probably charge more

I fitted a 5speed Supra box to a V12, or a JT5 may fit