cooper s mk III crankshaft questions

[almondgreen]

I got a crankshaft today.
I bought it as a cooper s mk III crank.
I am confused about the number.

Here a picture:



Is it right that the number is not cast on the crank ?

Andreas

[ivor badger]

Probably, but the journal looks a bit shabby. Remember that the only safe regring size is - 0.010", after that you will most likely be through the nitriding.

[almondgreen]

Thanks for the fast answer.

What do you mean with " the only safe resize" ?

[ivor badger]

At - 0.020", you will most likely not have nitrided journals.

[251 ENG]

The mk3 cooper crankshaft is produced from a 1300 type crank forging , part number 12G 1505.

The forging number is normally ground off and AEG 623 is stamped on .

The forging in NOT EN 40B and is not nitrided but tuftrided .

Tuftriding is only about 0.001" deep so will be gone at the first grind.

It is complete rubbish that EN 40 B cranks fail when you grind through the nitride case . Cranks normally brake because a crack starts on the surface .

The most critical thing is the size and form off the fillet radius in the corner of the journals.

Bad grinding kills cranks NOT the size of the journals .

[ivor badger]

Every 1275 S crank I have broken has failed across the flat webs at sides of the main bearings.

[251 ENG]

From a crack that started in the fillet radius

What are you doing to have so many cranks brake ?????

[ivor badger]

Using them hard. Know why the first probably broke, but never worked out a reasonable reason for the 2nd.

[littler]

probubly 12 out of the 15 or so cranks ive had fail in race engines ive built ect (most not A series) have gone on the journal radius. the radius attention to detail is definatly the make or literaly brake of a race engine!

allthough the last one early last year, a scat forged one in a small block ford race engine went straight through the web half way between no1 main and 1,2 big end! we put it down to a material issue and not so great harmonics and stresses of an externaly balanced v8 engine.

is it true that the S cranks where machined by someone to do with rolls-royce ?

[251 ENG]

The rumour is that they were nitrided and finished ground at Rolls Royce or Alvis . Both companys had large factory,s in Coventry at the time .

Both would have had considerable experience with EN 40B cranks for aircraft engines .

There not easy to grind , as the nitride case is very hard but brittle and prone to heat cracking if is abused by poor grinding .

Batches could have been ground by both companies due to the numbers needed