34 replies to this topic

[JetBLICK]

i started a topic the other week about doing a very big bore short stroker... maybe even an 8 port head, just to see what it'll do... the thing is finding a gearbox suited to the job.... i'm not the kind of person who wants to rebuild an engine everyfew thousand miles just to make sure... so i'm sticking with the 1380 and giggle gas for the moment.

i know, i wrote an essay at the end explaining how to do it

[edjohnson]

silly question, that is kinda on topic,

why do my two a+ engines have the same casting number, when one is a 1275 and t'other a 998?

[Guess-Works.com]

Here we go again....

Cooper S blocks have tappet chests the same as 998's ( aka small bores ), in general ( very general ) the block have more in common with small bores than they do with the stronger 1275 solid walled blocks.

When an engine is bored to +60, unless the bores are offset then there is little or no room left to overbore again to 73.5mm as the new bore may not cover the bore on the internal ( between pistons 2 and 3 ) side.. thus making it either very weak between 2 and 3 or not round !

The additional problem is the amount of metal between the bores is getting towards the limit and without the rigidity provided by the solid wall, there's a possibility that the block will basically fall apart under load.

Even the bible recommends not boring S blocks too far...

....As for using the S block as a basis for a big bore engine beware, Failure to bore over 2.9" is common and failures do occur at 73.5mm....

[JetBLICK]

i read the bible several times over looking for an aswer to this question before posting, must ave missed it... chars guessworks

[Sprocket]

It is also stated in TBASE that it is roughly a 40% succes rate on the Cooper S blocks, but not impossible. Just depends how much money you have as well as how many Cooper S blocks you own to achieve success

[Road Runner]

If you want to go 1380 then why not just use an A+ block it's a stronger block and at least it means you still have the S one.

[miniboo]

because i think that a 1071 crank wont fit into a 1275 block without a fair bit of work

[JetBLICK]

not a lot of work really, just shave 16thou off the trust washers. Its just that its kinda nice to have a proper 1071 motor, with the proper block. But if this block has to be sleeved mite have to do that because sleeving costs a fortune!

Edited by JetBLICK, 25 April 2007 - 08:38 PM.

[Road Runner]

Never tried so can't comment, what is wrong with an A+1275 crank or A+ engine come to that.

Edited by Road Runner, 25 April 2007 - 08:40 PM.

[JetBLICK]

already sorta answered that question deary me lol

[Sprocket]

The theory is to increase the bore size but keep a short stroke. going to the max bore on a 1071s crank makes for a high reving engine of around 1299cc which falls in motorsport classes nicely.

I have heard of an A+ block being used with the cooper S crank, but, thi is the bit i dont like, you have to skim a whole chunk of the top of the block leaving not that much on the top. Some say they have no problems, I personaly on a high reving engine wouldnt go there. This is the reason the Cooper S block, though not considered the best big bore block for rigidity, the 1275s block has a thick deck that can be skimmed and still have plenty on top, after all the 1275 block is actualy a taller 1071 block. The 1071 apeared before the 1275

Its all about comprimise with open tappet chest 1275 Cooper S blocks. you take your choice and you make it. I realy dont think bore and block flex is that much of a problem on an engine that tollerencies are not that tight anyway. Theres loads of slop in the A series.

[Road Runner]

not a lot of work really, just shave 16thou off the trust washers. Its just that its kinda nice to have a proper 1071 motor, with the proper block. But if this block has to be sleeved mite have to do that because sleeving costs a fortune!

That is the easy bit, did it once when I put an S crank in a 1275 block (not the S one)

[Dan]

I realy dont think bore and block flex is that much of a problem on an engine that tollerencies are not that tight anyway. Theres loads of slop in the A series.

Ever seen that stroboscope video someone shot of a Cooper S block running so that they could investigate this open sided block problem? It makes you think more than twice. You can see the ends of the block rotating around each other.

[minimad998]

[quote name='GuessWorks' date='Apr 25 2007, 08:50 PM' post='513587']
Here we go again....

Cooper S blocks have tappet chests the same as 998's ( aka small bores ), in general ( very general ) the block have more in common with small bores than they do with the stronger 1275 solid walled blocks.

When an engine is bored to +60, unless the bores are offset then there is little or no room left to overbore again to 73.5mm as the new bore may not cover the bore on the internal ( between pistons 2 and 3 ) side.. thus making it either very weak between 2 and 3 or not round !

The additional problem is the amount of metal between the bores is getting towards the limit and without the rigidity provided by the solid wall, there's a possibility that the block will basically fall apart under load.

Even the bible recommends not boring S blocks too far...


yep i thought that i had herd this somewhere recently, i can remember reading it in the David Vizard bible the other day.

[Sprocket]

Nope, but its not exactly going to do this http://www.innovatem...mp;d=1171306965