2 1/4 crankshafts

[Julian]

Looking at the official LR workshop manual on the net yesterday I was looking at engine specs and noticed that the 2 1/4 petrol crank could be ground and used with undersized bearings while the 2 1/4 Diesel crank was not allowed to be ground undersized.

Any ideas why that would be? I thought they were the same....

Julian.

[Tanuki]

I'm guessing it's because the stresses are greater on a Diesel engine (higher compression!) and that re-grinding the crank without subsequent re-case-hardening and fillet-rolling the journals (which aren't things usually done outside the factory) might make it more susceptible to failure.

A petrol-engine crank has an easier time of it and so you'll [probably] get away with a regrind.

Personally I think a 2.25-litre engine has rather too large a reciprocating mass to be running on a 3-bearing crank!

--Tanuki

[Gazzar]

Agreed regarding the 3 bearings!

The primary difference is the diesel crank is forged, and the petrol is cast, the diesel is stronger, but it has to be, thus, as Tanuki says, it's not up to the job. I believe that a reground diesel crankshaft can be used in a petrol. But not vice versa.

Oh, as I learned from here:

Forged - a lump of hot steel whacked into shape

Cast - hot steel poured into a mould.

[steve b]

And also the diesel is nitrided iirc which you lose if you grind u/s

cheers

Steveb

[Julian]

Interesting comments. I initially thought along similar lines and thought that no manufacturer stresses components to the limit like that, there's always about 30-40% overkill regarding strength to cater for production tolerances, abuse etc.

So is the Diesel crankshaft 'living on the edge?' I've certainly never heard of them cracking or snapping, but I'm not really that experienced when it comes to this engine.

I know a jolly good crank grinder near me who would do a lovely job, keep the journal radius's, and re-nitride if I asked. Are the petrol bearing shells the same as the Diesel shells?

I'm not actually going to do any grinding, I was just thinking out aloud here regarding a potential project which is to buy a scrap 2 1/4 Diesel and re-con it over the winter, obviously this can only be done if the crank is OK as a Britpart crank would only obviously be good as a door stop.

Julian.

[Gazzar]

I know what you are saying, but the LR diesel had a bad reputation, despite being one of the first small diesels. How much of that was abuse, and how much was a outdated engine being compared with a modern unit, I don't know, but I never liked mine.

G.

[secondjeremy]

Rover only supplied the diesel crank as a spare - presumably to avoid the temptation the use the (Cheaper) cast crank in the diesel.

I'm sure many have been ground undersize.

I think the 5 bearing petrol and diesel use the same crank - probably cast which works as the techniques have improved over the years.

[Julian]

Thinking some more I suspect steve B may be closest with the nitriding. LR are probably scared of machine shops grinding the crank and not re-nitriding or maybe nitriding to a poor standard thus negatively effecting the crank's longevity. I struggle to see how shaving a few thou of the main and BE journals will materially effect the crank's strength.

Any engine designers here?

Julian.

[tugboat]

Nitriding will only affect the surface hardness, ie, it's done to reduce journal wear, not to improve the shaft strength. It's a long time since I looked at the manual but I seem to remember very twitchy comments about radiusing the journals and I suspect that that's been the sticking point - sharp transitions give localised stress raisers of 100s of percent, add in zillions of oscillations of load and you have to worry about fatigue life. Seems like Land Rover thought it was too much.

[sean f]

Nitriding will only affect the surface hardness, ie, it's done to reduce journal wear, not to improve the shaft strength. It's a long time since I looked at the manual but I seem to remember very twitchy comments about radiusing the journals and I suspect that that's been the sticking point - sharp transitions give localised stress raisers of 100s of percent, add in zillions of oscillations of load and you have to worry about fatigue life. Seems like Land Rover thought it was too much.

This was pretty much what I though, the radius that the manufactures carefully create to reduce stress concentrations and fatigue failure were often removed during regrinding of journals particularly in higher stressed engines. I suspect land rover didn't want claims for unreliable engines or failures after bad regrinding so issued a general guideline don't do it to cover themselves.

My old diesel had the crank heavily reground afer locking up (dropped hot spot in the header at 60mph) and carried on for several more years of being run hard in a LWB before I sold the vehicle.