2.5 litre engine torque settings

(John Mellor) #1

Hi all,

Continuing with my MKIV 2.5 engine overhaul, I will shortly be fitting the crankshaft and thank to the valuable technical information recently received via this forum (thank you again Roger) I now have more tech infomation on the (MkV) engine that was strangly missing from my MkIV manual, such as cylinder head torque figures.

However I can’t find the torque figures for the crank main brg’s or big ends (steel rods) in the MkV manual or any other torque figures, for example, the camshaft sprocket or tappet block bolts,

I would be prepared to tighted the tappet block and sprocket bolts by estimating the torque but would not be happy at estimating the torque’s for the crank and big end bolts.

Any help would be much appreciated.


(Rob Reilly) #2

Main bearing nut torque is given in the Mark V Service Manual on page B36 as maximum 1000 inch pounds.

Connecting rod nut torque is given on page B52 as maximum 350 inch pounds.

The reason for saying maximum is that you go to somewhere near this figure, then back off a little bit until you can get the split cotter pin in.That’s considered good enough, no need to be super accurate as you would with normal nuts, as there is no possibility of the slotted nut working itself loose if the new split pin is properly sized and properly installed.

There doesn’t seem to be any torque spec for camshaft sprocket and tappet block bolts. Apparently it is not as critical, as the sprocket bolts have a fold over tab to keep them from loosening, and the tappet block bolts have lock washers and do not typically loosen.

(John Mellor) #3

Thank you for the info Rob.

I went back to the manual and I now relise that I must learn to read through it more closely because some of the tech information can be found in either the data spec sections or the assembly proceedure sections .
So, studying the pages more intently now, I discovered a reference on page B35 where it gives the con rod torque figure of 350 lb/in but it only refers to the Duralmin rods.
This torque figure does appear to be on the low side for 3/8" high tensile bolt, although I checked the torque for the XK 120 engine,as a comparrison (which of course has steel rods) and the figure is 450 lb/in. For what its worth, the main bearing figure for XK 120 engine is the same as the 2 1/2 litre at 1000 lb/in.
Any thoughts?


(Ed Nantes) #4

I would suggest to never use the old conrod bolts but use modern ones which will come with their settings.
I once saw someone who had a rebuilt engine that the conrod bolts hadn’t been replaced. When the car came to a halt the engine looked as though someone had cut across the block with a chain saw and you could see down through to the roadway. The half a bolt with the nut still on it was a few hundred yards back down the road.

(John Mellor) #5

I couldnt agree more Ed, hence my desire to to determine the correct torque figures.
When stripping the engine down there were a number of signs that suggested the previous builder was rather careless and one concern was the evidence of compression damage to the plain washers under the big end nuts, which of course will also be replaced.
I havn’t got around to sourcing big end bolts, nuts and washers yet but could you please advise where I could get hold of these “modern” ones.

Cheers, John.

(Paul Wigton) #6

Con rod bolts should be treated the same as a cotter pin.

(Rob Reilly) #7

The difference between con rod nut torque in the pushrod vs the XK engine might be related to a number of factors.
The pushrod bolt is 3/8-22 BSF where the XK bolt is 3/8-24 ANF thread, so there is a small difference in stress area at the root of the threads.
The composition of the steels used and thus the proof loads are unknown and may be different. Proof load is usually about 2 percent below the yield load, the point at which the bolt stretches in the plastic range, i.e. permanent deformation.
The pushrod torque was specified by the engineers at Standard Motor Co. where with the XK engine it was Jaguar engineers, who may have had a different philosophy on the subject.
The pushrod engine is intended to run at under 4500 rpm, where the XK engine peak is 5200 rpm.
Torque is actually a secondary method of achieving the primary goal, which is a certain preload. Some engine builders measure bolt stretch.
The greatest inertia load on the bolt occurs on the intake stroke, and this plus the preload is the maximum loading condition on the bolt. The maximum load times the number of cycles is the fatigue loading. If the fatigue loading always stays under some value known as the endurance limit, usually 35 to 60 percent of the ultimate tensile strength, the bolt can last indefinitely.
However, running with worn out knocking bearing shells will increase the inertia loading and reduces the fatigue strength. This is the reason the factory recommended replacing them when replacing worn bearing shells.
reference Mechanical Engineering Design by Shigley and Mitchell, chapter 7 Design for Fatigue Strength.
Search the subject on the internet and you will find many studies, recommendations, and opinions, some of which are colored by catastrophic experiences and some by sales pitches. The bolt sellers are all in favor of replacing them as often as possible, as are race engine builders working on somebody else’s ticket and having to guarantee their work. I found one drag racer that likes to change them after every weekend at the strip. But by contrast there are non-racers running low stress engines that feel that with new bearing shells and careful measurement and assembly, they are not worried about con rod bolt failure, and would rather spend their money elsewhere.

Notice I’m not giving my opinion here, just reporting engineering facts for which I have great interest, and the existence of other opinions for which I have less interest.

(Ed Nantes) #8

Attached a pic of a MK IV block when the gamble of new be vs old be bolts didn’t pay off.
At the bottom of the pic , the steering box, to the right the front engine mount and pulley. to the left the rest of the engine block, and in the centre, the roadway looking down where the engine block should be.
I should oint out also that supercharging an engine actually reduced the load on the Be on the inlet stroke. As long as it’s not some extraordinarily high boost.
We have new bolts supplied b the machine shop. After all the rods are going there anyway to be checked [ and corrected] for Length, straighhtness, ovality of BE hole, bolt holes being parallel, crack tested and balance checked etc I presume the bolts supplied are sourced for XKs.although I can’t comment on 2 1/2 litres as we always change to a 3 1/2 litre crank.

(John Mellor) #9

I agree it would be a no brainer not to replace the big end bolts, nuts and washers. As you photos shows Ed ( thats what I call serious crankcase ventilation) it can be a false economy not to.
I raced a Super Sport car in the 70’s ( Mallock) where your left knee was inches away from the crankcase and the shrapnel from a catastrophic failure in the con rod department, would really hurt. Fortunatly I never has such a failure so I can still walk but needless to say, those expensive bolts were renewed at each rebuild.
As you pointed out Rob, there are numerous factors that effect the bolt torque, or should I say tension.
My main reason for renewing them would mainly be due to the unknow stress’s the engine may have endured previously, such as over/ under torquing the bolts or over revving the engine.
Apologies for preaching to the converted here but as you know, there are other factors such as oiled or dry threads that can have a significant effect on the torque / tension. For instance Porsche and Continental (aero engines) state their figures are for oiled threads and how often to people have theit torque wrenches calibrared? The higher friction on the threads on new bolts/nuts can also have a significant effect on the torque too. Then there is the angle measurement gauge. I recall these were needed to tighten the 12" long through bolts on the Rover “K” series engines and it was important to check their lengths were inside the limits before re-use. Boy, did they stretch.

Cheers, John.

(Paul Wigton) #10

Allow me to offer yet another opinion:
-engineering is a good thing: I deeply have faith in the process.
-on 40-50-70 yrar old con rod bolts?

Replacing them is cheap insurance.

(Paul Wigton) #11


F**kin’ OW.

(Paul Wigton) #12

PREEEzactly. A parameter almost always unknowable.

(Peter Scott) #13

Ed, that’s a very interesting point that hadn’t occurred to me previously.