mk1-forum.net

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

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.

Thanks for the fast answer.

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

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

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 .

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

From a crack that started in the fillet radius

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

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

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 ?

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