MK IV 2.5 litre Duralamin con rod replacement

Hello, I am new member and this is my first post. My late father left his 1948 MK iv (in bits) to my sister ,so I am assisting with it’s resurrection. Many times over the years my Dad told me the Duralamin con- rods were crap and the steel rods from the earlier version of the engine were superior. A kind gentleman today informed me that the original rods would have to go , otherwise an engine failure would be more than likely at some point. I am just seeking assistance to determine the most effective method of rectifying this issue, please. Any assistance would be appreciated please, this beautiful car hasn’t been on the road since about 1971 and there are too few left to have this one immobile.

Your father was correct. The old duralium rods are prone to breaking.
There are several options, some easier than others . There were some engines made with 2 1/2 litre steel rods. people have been searching for these Holy Grails for many years. Mostly with little success [ it’s a seller market]

Or… you can use 1 1/2 litre conrods . These however are slightly offset front to rear and require a bit of fiddling to ensure they have enough clearance insde the piston. Then there’s the problem that 1 1/2 litres had only 4 conrods , and if getting 2 sets [ to use 6 of] you need to make sure tey are a balanced weight . Not as easy as it sounds.

Or [ my preferred option] use a 3 1/2 litre crank shaft and steel con rods from most of the XK engines [ not XJ though]
This give a 3 litre capacity more torque and s bullet proof.of course this mans finding a 3 1.2 litre crank… although new modern design ones are available. Not as expensive as it might seem as you don’t have csts like … finding your crank is under size , bent, cracked , out of balance and the usual needing to be ground and balanced etc. They also take Chev 350 mains and so will out last the owner. and can use post war copper lead big end bearings .
I and others ahve used them and found a big improvement

Or … you can get a set of Carrillo rods made . but then you don’t get the benefit ofthe better performance, and they won’t be cheap.

But apart from that you will still need to do a few other mods… twin timing chain set up from 3 1/2 mK IV? V, stronger flywheel bolts, and assume every part of the engine is stuffed till you measure and prove it otherwise.

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Thanks very much, this information is most helpful and enlightening. Would you be willing to share with me, a contact from whom I might source an upgraded crank and suitable rods, or somebody with expertise in these engines . I’ve built a few engines, but don’t know much about these. The 3.0 litre capacity sounds good and certainly affordable but how difficult is it to source other items such as the twin timing chain set up from the 3 1/2 .? Thanks again.
P.S ,I am a resident of Adelaide, Australia.


Cranky’s in melbourne { 03 09894 0000 [ Brett or Mick] make the cranks,
there is a lead time

For rods , look for early XK ish , NOT XJ 6 as they lack the oil feed
and are heavier.
Crankys can supply the Chev mains, and you would need the have the
block line bored to fit them

Alan Gibbins in UK sells twin row sprockets etc
Attached pic of one of the cranks , you will need to check that none
of the casting lugsinside the block touch it … and file out the
scallops on the bottom of the bores to clear the greater stroke , Not
hard or rocket science.


Thank you, I will look into these options. I appreciate your help. Cheers.

With best regards,


Hello again, is there a specific piston to be used with this crank and rod combination please, I assume I would be unable to use the originals.

With best regards,



The steel rods from a 3.4 or 3.8 engine are best NOT XJ6, Look for a
matched set, and if you have a choice pick the lightest. Check the
bearings haven’t turned in them [ big end] and no sign of heat effect on
the big end.

The original will be excellent to melt down and make saucepans.


Come on Ed.The old rods will be premium scrap metal to ship to China and contribute to the build of a new Great Wall SUV.

Hey Anthony,

I’ve got a set of rods I just took out of a 3.4. (I wanted the crank) PM me if interested…

Now Roger, you know that Duralimin is too good for a Great Wall… I believe they are crafted from a precise blend of mulched egg cartons and faith.


and you] need new pistons in any case

Because the 3 1/2 crank has 4mm longer throws the pistons will need to
be 2mm lower in the crown [check this on assembly as it depends to an
extent whether the deck has been faced and by ow much It may be worth
while getting pistons with a small crown on the top to increase the CR [
like 8:1 XK pistons have after all whe n they were new the octane
rating was about 63.

JP pistons in Adelaide will/ can make them


Thank you, I assumed this would be the case, I will see what I can come up with. Your help is most appreciated.

With best regards,


Hi Anthony,. I can vouch for Crankshaft Rebuilders too (Crankys) they rebuilt my MkV’s 3.5 and put in the later XK rods, JP pistons and ended up using 350 Chev bearings in the bottom end. I’m very happy with the result. (I don’t think the scrutineers will notice).

Thanks very much, it appears to be an effective solution. The only scrutineer ever to look over this particular car(if it ever is in one piece again), will be me , if the engine is revived, it will be done just the once , so I am grateful for the advice and guidance re. this particular matter. More important is the issue of the restoration of the car, it has been apart for over 40 years. My main focus at this point is the toolkit, Roger Payne has been a Godsend with this, I am making good progress here. I am hoping to have it mostly completed and accurate for a 1948 MK IV 2 1/2, by this year’s end. The two short box spanners are proving to be most elusive, however, I haven’t given up hope just yet. I will post an image of it once I get closer to completion and look forward to feedback. I’d just like to add, this forum is the first I have ever participated in and to date has been a friendly, helpful and rewarding experience , so my thanks to you all. Cheers.

Just the engineer’s curiosity in me, but I would like to see a picture of these 2.5 duralumin rods and the bearings that were used with them. The 2.5 is almost unknown in my part of the world, so I am unlikely to come across one here.

There were quite a few different varieties of Dural rod starting with the most elegant and ending with the chunky.

Here is a steel rod and an elegant Dural rod side by side.


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That’s great. IIRC the Barrie Price’ book “Rise of Jaguar” had some photos of con rod variations / development.


Pekka T. - 647194

A lip seal conversion, front and rear is worthwhile.
Photo shows 1/2" adapter plate with a Mercedes rear seal.
Requires a sleeve pressed on the return scroll.
Do Crankys offer a plain rear journal with lip seal?

So I see the chunky one has an offset oil tube.
What about the bearing shell used with this one?
Did it have an offset oil passage hole to match?