pistons for a 711m conversion

[pauljones]

im soon going to look at pistons and ring sets for my botton end.i have a 1600cc 711m in standard bore,the crank and rods from the same.i want to use the biggest pistons i can to give more cc's.am i right in thinking 86mm is the lagest i can go?

Has anyone done this conversion recently or know the way to go?

i will be useing the webber twincam head in standard form


Also a second question reference diff ratio's,i want to swap the diff internalls for the 3.55 type,to give me lower revs at motorway speeds.is this as easy as getting some gears from web sites and swaping them in my diff.any advice would be appreciated



Paul

[512BB]

Hi Paul,

Your block i dont know about. But your diff i do. The answer is yes, you can simply just swop your 3.77 CW&P for a 3.54 or 3.55 depending on what you can get hold of, and providing they have come from an English diff. I understand from recent discussions that the 3.55 is the true Lotus ratio. Obviously the whole internals will need setting up again, and it would be well worthwhile in investing in new seals and maybe new bearings, depending on how your old unit was running, and what history you know about it.

Good luck

Leslie

[rgh0]

To detemine how big the block you have can go you need to ultrasonically measure the wall thickness and then offset bore the block to maximise use of the available wall material.

Ideally I would aim for a minimum of 3mm or greater wall but you can go down to 2.5mm with some risk of splitting the bore overtime. Less than 2.5 mm and the bore is almost guarranteed to split quickly.

I spend most of my time measuring 1500 blocks for historic racing use but I have measured a few 711m blocks and the typical wall thickness and outside diameter of the bore castings is no different on avergae. You should be able to get 83.5 mm out of most unbored 711m blocks if they have minimal corrossion on the outside of the cylinder bore casting and you may get 74 or 74.5mm in a few cases. Getting 85mm is rare and generally requires a 2.5mm or less wall thickness which in turn generally means you will end up with a split bore

If going above 83.5 mm you also neede to pay close attention to the oil passge between cylinders 2 and 3 and make sure you dont break through into it also.

cheers
Rohan

[garyeanderson]

Paul

Take some time an start reading the past threads, there are a lot that are what you are looking for on the forum, if that sounds like a lot of work, maybe do a search on 711, Tall deck, 1700 or any other thing that you can think of. This site has a search function and works sometimes, if you still have not found what you want than try a search on google

1700 site:www.lotuselan.net/forums

substitute other words for 1700 and you will be reading all night.

after a couple of days you will have some questions i am sure

Gary

[tdafforn]

have a V cheap supplier of standard sized pistons (original spec) if you need them!
Tim