7 replies to this topic

[Sputnik]

I know it might be a stupid question, but I was informed otherwise !

Is the stroke of a Cooper S with an EN40b crank and cooper S conrods the same as a standards 1275 crank and standard conrods ??

I was told the Cooper s setup has has a longer stroke so with 73.5mm piston you will get more than a 1380cc engine ?

[MRA]

You where told wrong.....
all 1275 engines have the same stroke.....

73.5mm pistons will give you 1380 on any 1275 with the standard 81.28mm stroke crank

With a larger journal crank and the "S" or Midget conrods you can get longer or shorter strokes, by off set grinding the big end journal to fit the small conrod

Edited by mra-minis.co.uk, 16 February 2013 - 04:39 PM.

[Cooperman]

The 1275 A-series engines, including the 1275 Cooper 'S' are all identical.

[iMurray]

i think we answered your question well enough..... *trying to sound smart*

[Sputnik]

Thanks chaps, I thought the same,

At least I have a EN40b crank to build a decent 1380 should my 1430 project blow up

[Guess-Works.com]

They may have the same stroke, but the cranks are different..

Both run a 2" main journal, however the cooper S crank ran a 1.625" big end, and the std 1275 run a 1.75" big end. This is why you can get offset ground cranks to do what your original post is about, where a std 1275 crank ( A series ) is offset ground to give a longer ( or shorter ) than standard stroke with a 1.625" big end.

I also believe the length of the main bearing also changed from the Cooper S crank to the Std 1275 crank, so putting a cooper S crank into a non S block will require some special thrusts being used, can't check for sure as I no longer have an S crank to hand, but the S should be 1", where as the std 1275 is something like, 0.975"

*edit*

Just noticed you're in South Africa....

I think the South African cooper S ( also called GTS ? ) used a different crank altogether.

Edited by Guess-Works.com, 16 February 2013 - 05:17 PM.

[Cooperman]

Spot on ther about main bearing widths.
To fit an EN40B 'S' crank in a non-'S' block you use the +0.030" thrusts milled/ground down on the back fact to give the correct end float.
Then you use 1275 'S' rods with the 1.625" big end journals.
That way you get a strong block and the ability to really rev without the crank breaking.

[Sputnik]

Thanks chaps, learning something new every day on this forum.

@ Guess-Works, our GTS in South Africa is a normal 1275 crank, with a 'stronger' con rod and ported head. The Cooper S stopped in SA in 1969 and spares are very difficult to get hold off.

I will rather keep the steel crank for future use and keep to my current 1430 project. I need torque and not KW at this moment in time for the track. The other cars are pulling away from me after each corner so torque is what I am after. I am happy with my top speed at this moment in time and the associated revs.