23 replies to this topic

[richardsoniscool]

ok here it is- can you bore out a 850 engine to fit 70.6mm pistons from a 1275 to make a 1071cc engine??

i assume you can because the 1275 block wasnt used untill after the 1071 was used.

and if the answer is yes has anyone done it or know what there like to drive? thank you

michael

[Jordie]

Wouldnt boring out to fit those pistons make it a 1275cc?

I always thought the 1071 was due to crank stroke?

[Ethel]

An 850 is the same casting as a 998 the Cooper 1071 S used the same casting as the 1275 & 970.

[not for sale sorry]

Why dont you just fit a 1275?

[richardsoniscool]

the 1071 has the same stroke as the 850 and the bore of the 1275, so what im really asking is can u bore out an 850 to fit the larger pistons?

i was think about fitting a 1275 but then i have a brain wave the 1071 can produce the same bhp as a 1275 but will rev better

EDIT - sorry ethel didnt read ur post properly so i would need the crank from an 850 and the block from a 1275? but wouldnt the 1275 block be too tall

Edited by richardsoniscool, 16 January 2009 - 01:16 PM.

[RobG295]

Is the 850 crank any good though? I thought they were pretty much a bent piece of wire lol!

[richardsoniscool]

hahaha your probably right but i would get a better one but use the 850 conrods, maybe, i dont know

[Sammy D]

hahaha your probably right but i would get a better one but use the 850 conrods, maybe, i dont know

Con-rod length wont alter the stroke.

[Ethel]

The point I was making is that you can treat an 850 as a 998 for overboring. I don't know how the compression ratio would work out (you'd need to lose a fair few cc's from the chambers. but there are some large pistons available for 998's.

[Guess-Works.com]

the 1071 has the same stroke as the 850 and the bore of the 1275

This maybe true, but you'll also find they are completely different cranks. First of all the main journal size of the 850 and other small bores is 1.75" where as the big bore cranks have a 2" mains journal, therefore using an 850 crank in a big bore block is not really possible. Secondly the 850 crank is probably made of EN16 at best, the cooper S crank is EN40.

The small bore block and the S block bear similar traits, being the tappet chests and core plug placement, however that's where the similarities end, they are completely different castings in terms of the water jacket and oil ways, which would make boring out rather difficult, if it were possible there would be many stories of small bore engines being bored out to take large bore pistons.

If you want a 1071 ( or short stroke engine ) the cheapest way is probably to buy one...

[richardsoniscool]

O well thanks for clearing that up i was getting a litle confused but to buy a 1071 were talking alot of money as they only made them for a year?

[Guess-Works.com]

and if the answer is yes has anyone done it or know what there like to drive? thank you

I'll tell you when I've finished building mine

Well wont be a 1071, but instead will be a 73.5mm bore on a 1071 crank so will be 1160cc (give or take)

[richardsoniscool]

then you can sell it to me on the cheap, what a great idea why didnt i think of that lol

[mk1leg]

well I did this twenty years ago and max bored an 850 anf fitted omega pistons and tuoghrided crank and fitted a 731cam stage 3 head sc/cr g/box 3.765 diff and it went like s**t off a shovel...........

[richardsoniscool]

WHAT and it didnt blow up ? now im confused they say i cant you say i can and have????