How big is a 1275cc engine, really?

[ianh1968]

When I do the maths, I can only get a capacity of 1273cc...

Bore is 2.78" = 7.0612cm
Stroke is 3.2" = 8.128cm

PI x R x R x S x 4 = 77.69 CU or 1273.18cc

Where am I going wrong, or have we all been short-changed?

[chichestermini]

Interestingly, i was reading a similar topic a while ago on another forum. Here is what was concluded:

"The original BMC Workshop Manual gives the stroke of a 1275 as 3.20". This equates to 8.128 cm
The bore is stated to be 2.78" which is 7.0612 cm.
Thus the nominal capacity is 1273.347 cc."

"I'm not convinced about the accuracy of the various figures. For example, if you do the arithmetic on a 1071, you get 1069cc. BMC might have rounded this to 1070, but surely not to 1071?"

"Who will ever know what BMC did to get those figures. The A-series was designed and drawn using inch measurements, so already they have been converted to metric.
If you have ever measured the accuracy of crankshaft throw you will know how far out they actually were, up to around +/-0.004".
Maybe the capacity quoted is the maximum you can get with all the tolerance considered. 2 x 0.004" on the stroke would add about 1 cc to the swept volume. Add a maximum bore tolerance and you could get a bit more.
Oh, the wonders of 1960's manufacturing technology."

[davidnutland]

What about the cc's in the head?

[ianh1968]

What about the cc's in the head?

The capacity of the engine is the "swept volume", it is literally
the bore by the stroke times the number of cylinders...

The actual calculation is as per my original post!

The capacity "above the piston" is still there regardless of
whether the piston is at the top or the bottom of the stroke
so it is only used when calculating the compression ratio.

chichestermini, I think you may be on to something with your
tolerance answer. Or maybe it's just "rounding"?

If you look at the 2.78 figure, is this really 2and25/32"?
which is 2.78125" (70.64375mm)...

... and is 3.2" actually 3and13/64"?
which is 3.203125" (81.359375mm)...

Result = 1275.57cc!!!

Most engineering sizes that I have come across are based on
64th sizes. If you cut an inch in half 6 times, you get a 64th.
My assumed increased sizes are similar to the "official" sizes,
but are rounded up to what would be the most probable next
64th size.

3.2" should not really exist at all as an imperial measurement
and if anyone ever refers to something like 1/3 of an inch,
you will know that they are very unlikely to be an engineer...

[ricardo]

This is an interesting topic. Our (Portuguese) cars were registered with the following capacities:

- Early 850s (until late 1960) - 849cc
- 850s (late 1960 on) - 848cc
- 997 Coopers and early 998 Coopers - 994cc or 995cc
- Late 998 Coopers - 998cc
- 1071 Cooper Ss - 1069cc
- 1275 Cooper Ss - 1273cc (check the 'cilindrada' below in my cars registration paperwork)
- MK2 998 Minis - 998cc

[Spider]

Hangem on guys, you've got it quite wrong here.

I've got badges on 2 of mine and a sticker on another that definitely say they are 1275 - even the ones which have been bored

Yeap, all 1275 cc (or is that ci?) I'm afraid.

[ivor badger]

What is not being allowed for here is that the original capacity mathematics were done on a slide rule or logarithmic tables. No hand held calculators in those days.

[mk1]

As far as I am aware most manufacturers quite swept volume that is a couple of CC's higher than actual measured volume, this avoids anyone being caught with a car that is over capacity.

Rather like most factory speedos run about 10% fast to help you avoid gathering speeding points.

M

[guru_1071]

Rather like most factory speedos run about 10% fast to help you avoid gathering speeding points.

hows that work out for you then mark............

[mk1]

Its not much use when being caught doing 34% over the speed limit I will grant you.

M

[guru_1071]

I will grant you.

ive never been Will Granted before, sounds kind of kinky........

[mk1]

Woo Hoo!

M

[guru_1071]

Woo Hoo!

M

that's the noise will makes......


apparently

[Etienne B.]

Woo Hoo!

M

that's the noise will makes......


apparently

Oh boys !

[Spider]

I will grant you.

ive never been Will Granted before, sounds kind of kinky........

And we all know how the guru likes happy endings

[guru_1071]

And we all know how the guru likes happy endings

hey, you pay your money, you makes your choices

[Etienne B.]

And we all know how the guru likes happy endings

hey, you pay your money, you makes your choices

Ah but you must be talking about difficult calculation of big (blocks) blokes capacity, aren't you ?