2cams70 wrote:I recommend doing yourself a favour and discussing your project with a good engine machinist first. The Lotus/1500 crank is 72.75mm stroke. The 1600 crank is 77.62mm stroke so with those combination of parts your pistons will be sticking out of the block by 5mm.
That's not correct as you may have forgotten that stroke means you end up with half the difference (the stroke is the difference between the pistons at TDC and BDC, or the complete stroke) so the pistons will be sticking out of the top of the block at TDC by approx 2.435mm depending on other tolerances such as whether the block has been skimmed or not etc.
Conversely, the pistons will sit 2.435mm further towards the bottom of the bore at BDC. That equals the total stroke difference of 4.87mm.
If crossflow rods at 125.222mm were used over 125e rods at 122.5804mm that would give you an additional 2.6416mm of "poke" out of the top of the block - this would put you at around the 5mm you suggest (5.0766mm)
The 2737e crank is actually slightly heavier (I've weighed them) and considered by some tuners to be the stronger crank than the 711 one. The 2737e was the original crank as fitted to the transit van engine (2737e 6015) so perhaps a stronger one was thought necessary for a "load lugger". The 711m crank is still a perfectly suitable crank whatever it's history.
Chrisis wrote:i'm building my engine using a 1600 crank in a 1500 701 L block together with 125E rods.
As i have high compression twink pistons if i machine the raised bump it will work or the low compression height piston is very different?
You could machine approximately 2.435mm from the top of your pistons, but you will then likely need to machine the valve pockets deeper to compensate which will in all likeliness weaken the pistons possibly beyond safe use. Plenty of people have done it though but it's not something I would want to do for piece of mind.
Better to use a thicker head gasket and machine the pistons less to save their strength. Of course forged pistons would be better overall - custom ones even better.