23 replies to this topic

[richardsoniscool]

?

[Ethel]

He's saying you can overbore an 850, not turn it in to a 1071 using "S" internals. you can use S conrods on a smallbore crank but they need a fair bit of fettling to fit. You'd be better off starting with a 998.

[richardsoniscool]

o ok thanks ethel

[Ethel]

The hurdle is the 850 crank throw is roughly 8mm shorter than the 998's. There are lots of 998 pistons available, but fit them in an 850 and they'd come 4mm less far up the bore than they should, so you'd have to modify the block.

[Pursey01]

i think the 850, 998 and 1098 are small block engines
and the 1071 and 1275 are big block something like that

[DeadSquare]

Fifteen years later .............

 

I offset bored and sleeved an 850 block to take 68.25mm pistons and used S rods. It wasn't quite up with the 970s, but was well ahead of the 998s on the straights ............ until somebody stole it.

 

But I digress :-

 

Are 970, 1071 and 1275 blocks all the same height ? or is the 1275 block a bit taller ?

[Spider]

The 970, 1071 and 1275 are fundamentally, the same engine / block, all being the Big Bore variety.

Starting with the 1275, it was (if memory serves) 3/8" taller than the 1071 and 970 blocks. You can deck a 1275 S Block to turn it in to a 970 / 1071.

The 1071 used a shorter stroke crank, 1275 rods and 1275 pistons.

The 970 was a short stroke than the 1071, used it's own longer rods (based in 1275 forgings) and the same piston as the 1275 and 1071.

The 970 and 1071 were not the same height as the 998 / 1100 blocks, but were very close.

The 1071 blocks were based on the Formula Junior Blocks, in fact, the very early 1071's were FJ Blocks.

The 1071 was the first released, a year-ish later that ceased. About a month prior to the 1071 ceasing production, the 970 and 1275 was released, with the 970 ceasing about a year later.

[nicklouse]

And then you can also add the SA blocks and cranks to totally mess with peoples heads.

 

1275 and 1098 were the same blocks but different cranks and pistons.

 

this is the bit I am unsure about.

 

there 998 was also I think the same block but it was heavily skimmed. All used the 12g949 heads.

 

love the confusion you see on Facebook.

[DeadSquare]

The 970, 1071 and 1275 are fundamentally, the same engine / block, all being the Big Bore variety.

Starting with the 1275, it was (if memory serves) 3/8" taller than the 1071 and 970 blocks. You can deck a 1275 S Block to turn it in to a 970 / 1071.

The 1071 used a shorter stroke crank, 1275 rods and 1275 pistons.

The 970 was a short stroke than the 1071, used it's own longer rods (based in 1275 forgings) and the same piston as the 1275 and 1071.

The 970 and 1071 were not the same height as the 998 / 1100 blocks, but were very close.

 

The 1071 blocks were based on the Formula Junior Blocks, in fact, the very early 1071's were FJ Blocks.

The 1071 was the first released, a year-ish later that ceased. About a month prior to the 1071 ceasing production, the 970 and 1275 was released, with the 970 ceasing about a year later.

Thank you for that comprehensive precis.

 

3.2" less 2.687" = .513", so the 1275 block is probably about 1/4" taller.

 

I can remember the happy smiles, on the evening that I Cycled up to Goodwood, on the faces of the Cooper FJ drivers.

 

The FJ formula, which didn't permit much in the way of special parts, had moved up from 1,000cc to 1,100cc and the, already outclassed by the Ford Anglia engine, BMC 948 engines had been braking cranks and conrods, and cracking blocks because so much metal was being removed to try and increase the capacity.

 

The 1071cc was also perfect for the 1,500cc class drivers, because it enabled a 1,499.4cc capacity under the 40% rule for supercharging at the time.

Edited by DeadSquare, 12 April 2024 - 07:31 PM.