Major oops in MK V 2.5 rebuild

I seem to have made the most basic screw up in the engine rebuilders manual. When I removed the pistons and rods, I didn’t make a notation of where the #1 piston lives, next to the water pump or the firewall! Anybody know on a 2.5?

According to the Jaguar Service Manual number 1 is at the rear.


Peter, Thanks! I’ve read through the engine section twice but still haven’t found that. Which page is it on?

Ok, I just found it on page B.58.
I guess my reading skills need to be revamped!

Thanks Peter!!!

So according to my service manual, the oil tubes on my 2.5 are supposed to be on the front side of the connecting rod in relation to the engine. The pistons which are +.02 have Front stamped on the top but is on the opposite side from the oil tube. Not sure why the rods were put on wrong but I’m changing it to what the service manual says. The odometer shows 70k miles and I have no way of telling how many rebuilds it’s had. I did notice that any where I removed an exterior part, there was a light green paint there with no other color showing under that. From info I’ve gotten on this site, I’m assuming the engine color should be black, even on the 2.5L engine.
We’ll see what new discoveries I find tomorrow!

The oil tubes, and the associated holes in the bearing shells, should line up with the oil holes in the crankshaft. If the holes in the shells don’t line up, you have my permission to open them up a bit with a round file, dressing up your filings around the edge afterwards of course.

It is my theory, unfortunately unproveable at this late date, that failure to have these holes lined up for various reasons such as backwards installation and wrong shells during a refurbishment back in their day, caused oil starvation at the wrist pins and is what led to failures of aluminum connecting rods and a commonly held prejudice against them.

My '38 engine also had pale green paint on it, as well as pale yellow, both applied with a brush, but after careful examination I determined that there was medium grey under that, so that’s what I went with.

Rob, that’s just a very pretty engine!
I don’t know if you know Walt from Vintage Jag Works but it is his opinion that as long as at least half of the oil hole in the bearing shell is open, “all is good”. He thought that the wristpins would still get enough oil and this would increase oil pressure a bit. I’m not really sure how I feel about this but he builds a lot of Jag engines and knows the MK V pretty well. He also thought that since I’ve been having so much trouble sourcing 2.5 parts, I might think about swapping it out for a 3.5. To be honest, I’d rather have a 3.5 but this chassis came with this 2.5 and I tend to try and keep things original.

Hi Wayne, a lot of Mark V (and Mark IV) cars have had engine replacements over the years. If your engine block number does not match what is on the car ID plate, maybe you can check if the ID plate number was for a 3 1/2. If the engine block and plate numbers differ, you might not feel so guilty about switching to a 3 1/2 and if the plate number was for a 3 1/2 then the urge might overpower you. Do you know what is the number on the block and what is on the ID plate?

Answer to my question is on another thread here already. Your car build date was in 1949 somewhere from summer to year’s end, it looks like your engine number on the plate is an “H” series which is 2 1/2 and your car is right hand drive. Oh well, but here are some thoughts on comparison.

2 1/2 rated 136 lbs. ft. max at 2200 rpm, 3 1/2 rated 184 lbs. ft. max at 2000 rpm.

Rear axle ratio for 2 1/2 is 4.55:1 and for 3 1/2 is 4.27:1

This may be the time to consider what acceleration and typical top driving speed and engine rpm you wish to run. If thinking of changing to a 3 1/2 will you be thinking of changing rear axle also?

I’m not really sure what I’m going to do. I’d certainly like to have a comfortable run down the highways and am thinking the 3.5 might do that a bit easier. On another thread here, the subject of overdrives came up again so that is something I’m going to keep an eye on.
I have a couple prewar LaSalles and they just aren’t up for highway driving. I’d like the MK V to fill in on my longer rides. Plus, availability of parts certainly make the 3.5 a bit more attractive. Not even sure where to look for a 3.5 anyway.

I’m leery of that statement. What you need at the wrist pins is not necessarily high pressure but rather a high flow rate. Half a hole overlap seems to me like a flow restriction. The aluminum con rod failures I have heard of were associated with high speed driving on the super highways. I would oval out the holes with a file or drill bit.

Hi Wayne, you should consider what kind of highways and speeds you wish to do and then include your thinking about engine rpm to achieve that condition.

In Southern California where I live, normal highway speed is 80 mph in moderate traffic. The Mark V is not comfortable for me above 65 mph on the level for engine load and safety feeling. Modern cars have better brakes and don’t leave as much room in front of me as desired frequently on the highways. And remember the Mark V is dangerous in any crash compared to modern cars. Danger increases with speed. In heavy highway traffic, the car also does not have enough braking for the darting traffic we have at heavy traffic levels. And it is not fun to drive in heavy traffic, I’d rather have a modern automatic and cooled air and music.

On the street, my Ford Model A is fine in heavy street traffic, but I don’t take it over 45 mph.

My suspicion is the 2 1/2 Mark V might be comfortable up to about 55 mph with a 4.55 rear end. The 4.55 helps make up for the lower torque compared to the 3 1/2 and will have slightly quicker times to shift points and lower top speed without wanting to blow up the engine. Others with experience can clarify, I’ve only owned 3 1/2 Mark Vs.

I had a Willys utility wagon with 5.38 rear and another one with 4.27, similar L6-226 engines and T90 trannys in both. The 5.38 was great for pulling tree stumps, but on the road, from a stop you would have to shift from first to second before you even thought the clutch was fully engaged and top speed allowed napping by the driver.

Boy you guys are tough!
Rob, yes it’s better to have volume of oil rather then pressure of oil on the bearings. Just thinking out loud about the things said bu an older fellow who has been building these engines for a long time. I figure he has a bit more insight then I. I’ve rebuilt a lot of engines over the years but mostly were newer diesel mains and genny’s on yachts. I hate carburetors, especially what the English engineers considered carburetors, so I stuck to compression and injectors. Don’t think I ever built one with restricted oil passages, at least on purpose. I was just repeating what had been said to me. The car had 30lbs of oil pressure after warmed up and that’s pretty good for a 71 year old engine.
I would like to be able to run 60-65 in the right lane comfortably. My 1930 LaSalle will do 60 on the highway but it’s stressing me if not the engine. Add to that, 90 year old wooden wheels and I get a bit nervous. And talk about not having breaks. Yikes!!
The MK V braked quite well last summer. I drove it every day with no stopping problems but I guess we all drive super defensively in our fun cars. I’ve since rebuilt the entire brake system and hope to see even better stopping response. I’m just wondering if the 2.5 is a bit under powered for American highways and if an overdrive or a 4 speed tranny from another model would help it be more comfortable. Better to do it now since I have it down to its skeleton.

Im pretty adventurous, wrt driving old cars at speed.

Artillery wheels… would always be forefront in my mind!!!

Hello Wayne, I don’t think the MkV is too different from the old SS Jaguar and my 2½ litre is perfectly happy running at 60 mph at which the revs are just under 3000 rpm. The key things are to make sure that everything is balanced, engine, transmission and wheels and tyres. You also want your engine to maintain at least 30 psi of oil pressure. Use proper fully synthetic oil and don’t suffer the poor characteristics of mineral oils

We share the 4.55 axle ratio and your 650 x 16 tyres do give a lower gearing with 724 mm rolling diameter against 740 mm with the 5.50 x 18 tyres on the SS so you have 19.7 mph/1000 against 20.1 mph/1000 in the SS. A change to the 4.27 axle ratio of the 3½ litre would bring you up to 21 mph/1000.

Braking is certainly not in the same league as modern cars so you do need to drop back when someone overtakes you and steals your front clearance.

The main problem with long runs on fast roads is noise level. In the SS the largest source is wind noise due to less than hermetic sealing from the opening windscreen and also from the sun roof and side windows. Engine and transmission noise is negligible. Your fixed windscreen will give you a big advantage in noise over the SS although you miss out on the ventilation.

Main road gradients are almost never a problem for maintaining traffic speeds with the 2½ litre engine.



So Wayne ! consider a Ford type 9 five speed and converting to disc ( if only the front) using MK9 / XK150 components

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Could you elucidate on that?

Mineral oils break down under much lower stresses than synthetics and also drain from bearings in engines left standing. Synthetic oils have been designed to alleviate these and other problems. I have been using fully synth in the engine and gearbox of my SS Jaguar for at least 20 years now and am very happy with the results. I have also been using it in turbo-diesel moderns for even longer and can detect no wear. This contrasts with my experience of driving cars on mineral oils over at least the previous 30 years.

When I rebuilt the gearbox in my SS back in 1996 I initially filled it with mineral oil because I was uncertain whether the thinner synthetic oil would adversely affect my synchromesh but not long thereafter I drained it and filled with fully synth. If there was any change in the synchromesh it certainly was no less effective.

Fully synthetic oils were significantly more expensive than the mineral varieties but the price difference is much less dramatic today.


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Peter, have you placed one of these tranny’s in a MK V? I’m interested in it if it will fit.