New End Ratio for 1954 XK120 FHC

Some update from my side. Guy Broad told me that for the 4HA, at 3.31 (to lower ratios) you need to change the carrier as well (it will fit in the housing though).

I am now in doubt, as the advice is not to go below 3.31 (but that is still too high for my tastes) and rather go the 5-speed route, with minimal modification to the cross member. Fact is, that 2800 at a real 100Km/h is too much.

Time to sleep on it, I guess.

No doubt. I was just painting with a broad brush there, not drawing the fine lines. I used to own an early steel XK 120, circa 1950 (670660, IIRC?) , which was ENV, and I currently own 672027, 4/4/52 dispatch, which was a 2HA when I got it.

If the Moss 'box is in decent shape, a 2.88 would be the way to go, IMHO.

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He’s got me there. I’m only aware of two carriers for the Dana/Spicer 44, 3.73 and down and 3.92 and up. The Dana/Spicer factory catalog (1968-1978) shows the same part number carrier, 706615X, for everything below 3.73, all the way down to 2.72.

I agree with Mike about the Dana axle break,
(but I cant say for sure about the Salisbury)

I fancy you are in Europe ?

Someone in the classifieds, he is from France was advertising a 3.058 Track loc from an E-type

Power lock diff 3.058 - Classifieds - Jag-lovers Forums

I think that would be a 3.07 ?

anyway, maybe Mike can answer,
but can he just transfer the carrier, crown wheel and pinion and have a 3.07 track-lock ?

…and he is just down the road, not like Ozstralia, where I send parts 3000km

3.07 (open) diffs were stock in Australian spec XJ6, so are fairly common and cheap here

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IF (big IF) If the carrier uses the same axle size and spline count and the splines on the pinion are the same, then it’s a painless swap. If the axle splines are not correct, then you would need change the side gears in the differential to match the XK axle splines. It used to be that you could simply purchase a front yoke that matched a later fine-spline pinion to the early style companion flange u-joint, but they have been NLA for many years, and NOS or used are hard to come by. It is usually easier to modify the driveshaft.

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Isnt the gent going to face this issue if he changes to a late 2.88 which would be a Dana out of an XJS or similar ?

I dont think they made early Dana as low as 2.88 ? ( but I could be wrong and often am)

I agree that a custom driveshaft mod should be expected

The Jeep guys do a lot of mods to driveshafts for Dana 44, parts are available

Yes, they were a little vague about that, perhaps using up leftovers.
image

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Honestly, I have zero experience with the XJS differentials, so I do not have any idea what might be required to swap those around. I pulled up the Dana catalog (screenshot above) and scrolled down until I found the section for 1960’s gear and carrier sets under 3.73. Low and behold, they cataloged a 2.72 ratio for 68-78 Ford trucks. The specified carrier for that ratio was the same as everything else under 3.73. So I would say, IF you could actually find a Dana/Spicer 2.72 gear set, it should fit the 3.73 Salisbury 4HA carrier. The Dana part will likely have larger 7/16" ring gear bolts, which is a very simple fix.

I have driven, and owned, xk120s (53-54) with 3.77, 3.54, 3.27…and much preferred the 3.27, mainly for the lower rpm at cruise speed, and not being interested in stop light drag races. There are many simple web site calculators for Rpm-diff ratio-tire diam (or radius). Nick

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3.27 is an ENV ratio (I have one fitted to my 120). Nearest Salisbury was 3.31 or 3.07. There was also a 2.88. These are probably for 4HA rather than the 2HA.

Many thanks to all the very useful replies.

I think that in the summer months I am going to take the thing appart and see for myself whether a 2.88 is a direct fit.

My bother owns a car restoration company with all machining equipment, so I guess -just guess- that that is an option.

The 2.88 must work in a XK, the car will really smoke the tires (6.40") in 1st and 2nd when you push it with the 3.57 and I really like the Moss, which additionally will have parts available long-term (can I say that of a Supra gearbox?).

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probably.

I personally fitted a Supra box to my V12 XJ, and it went like a scalded cat for many years.

I recently sold the entire car, and it was in huge demand.

The new owner is using the complete running gear for a Ferrari replica

Just my personal opinion, but I do not like too tall a diff as it directly robs you of torque and acceleration.

Perhaps your roads are legal for speeds over 110kmph

Here in the Netherlands most of my travelling time is at 100-130Km/h.

I will use the car already in summer (I hope/think) through Belgium and France, more at 130. 4000k is ridiculous taking into account the torque.

4sp and a tall diff can only a problem in 1st gear, as the 2.88 would give the equivalent of a Supra Overdrive (0.82) in respect to the original 3.57. The intermediate gears, with the weight and torque of an XK can be no problem at all (mine seems to be in pretty good shape, which is not my merit).

So my inclination is indeed a 2.88, even 2.72, but what I will not find out until I tear the thing appart, is whether it’s a direct carrier and pinion swap (and that seems to be messy enough) or whether it will need more modification.

I have an XJ-SC I believe with the 2.88, so at least from the outside I can check.

Now of course the thing is that when talking to Mike (my brother) he has swapped boxes (Moss to BW or Getrag- I recall) in 8 hours including the minor welding and using the bell housing. Keep the Moss in a nice boxand go the simple route has also advantages, hence my doubt. The prices for full kits here are about 3000€ at least.

Best,

Ll.

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I don’t think I’d want to go any higher than a 3.07 on a standard XK120. If the engine is enlarged to say, a 4.2 litre, or is somehow producing huge torque, then maybe a 2.88 or 2.93… With the 3.27 my 120 is doing 70mph at about 2800rpm on Michelin X 165 x 16 tyres.

Don’t forget to calculate revs/mph with tire diameter of the tire you will select. They vary from about 27.6 in many current tires, to 28.2 which was the approximate original Dunlop 600-16)…then use the rev calculators on the web. small differences…but noticeable. Nick

Thanks, yes, on the Excelsior 6.4 the theoretical diameter is precisely 700 mm, that would be 27.6". Traction radius is roughly 685-690mm, at the pressure I drive which is high (2.8 bar). GPS controlled speed and 123ignition-tune app controlled rpm at 62mph is exactly 2850 rpm at 62 mph with the 3.57 drive. The minor difference between theory and practice is the traction radius vs. the theoretical radius.

I note then that a 3.07 gives 2450 at 62, 2.88 gives 2300 and so on.

Indeed, a new FD seems to be my inclination, rather than a 5 speed, as it brings the rpm to the levels I like.

I like this web rpm calculator: Transmission Ratio RPM Calculator | Spicer Parts.

indicates with a 27.6 tire diam: at 60mph a 2.88 is 2105rpm , a 3.07 is 2245rpm.
Most roads here, even in the countryside will encourage road rage if you are below 60mph.
I do not like to run 65mph at 2800 rpm with the 3.54.
The 3.07 gives 2430rpm at 65 (calculated). BETTER…
The 2.88 at 65mph is 2280…HAPPY CRUISE…AND ALLOWS A HAPPY 70-75 as well. and allows nice use of 3rd gear.
I feel that first gear with the 3.54 is almost unusable, the shift to 2nd is so soon…with the slow Moss gear box the cars behind damn near rear end you at the shift point.
Nick

Yes, the slightly higher rpm in my message are due to the use of the real traction diameter (compression of the tire at the traction point).

I agree that for my tastes, 2.88 will be the way to go and have already asked a second hand parts dealer in the area to find me an XJ-S or XJ6 diff, it will be with carrier as I really do not think the crown and wheel can simply be transposed.

shame you are not in USA

1972 Dodge Mopar Charger rear end ring and pinion gears carrier parts 2:88 ratio | eBay

When I recently had a 3.54 Jeep Dana 44 apart, I checked to see if the parts interchanged with a Salisbury 3.54 that unfortunately had water ingress, it appeared to me the R&P from the Dana fitted into the Jag pumpkin on the Salisbury carrier, and that the required different pinion yokes were available.

I didnt fully check axle/carrier spline counts

Be interesting to see how it goes with your conversion

I have a couple of XJ 3.07 open diffs, but it hadnt really occurred to me they may be useful to XK owners

With 4wd applications, almost everyone wants to go the other way, most often to accommodate larger tyres for more ground clearance

My 420G auto has 3.54