18 replies to this topic

[nath]

i bought a engine and not sure what it is can people tell me by the markings on the pistons?
they ar Ae 21251 +30 (CBD) <<< think its CBD not too sure tho
thanks

Edited by nath, 14 April 2011 - 03:28 PM.

[MRA]

21251 are a fairly standard 1275 piston, +0.030" is 1310cc

[Cooperman]

21251 are a fairly standard 1275 piston, +0.030" is 1310cc

Martin, I think +0.030" is 1301 cc.

+0.020" = 1293 cc
+0.040" = 1310 cc
+0.060" = 1330 cc

[Cooperman]

21251 are a fairly standard 1275 piston, +0.030" is 1310cc

Martin, I think +0.030" is 1301 cc.

+0.020" = 1293 cc
+0.040" = 1310 cc
+0.060" = 1330 cc

Cooperman, thats correct but +0.040" is 1312cc

Actually it is 1309.815 cc, but I reckon I'm being a bit pedantic. Call it 1310 cc.
+0.060" is actually 1328.463 cc.

[MRA]

Yes of course it is my mistake .... I feel like I have a hangover and my memory has g...... what was I talking about ?? who am I

Yes Peter your answers above are spot on, thanks for the correction.

[MRA]

I measured mine and it was 1317cc...... it was a b****r of a job measureing all that wear

[cooperrodeo]

Just what are the standard exact figures for 1275 bore and stroke? Conversely, what is the exact capacity of a 1275? The Haynes manual says bore and stroke are 70.6 and 81.33, but these figures give 1273.53cc. Is a 1275 really 1275 or have we been diddled out of 1.5cc?

[Cooperman]

Just what are the standard exact figures for 1275 bore and stroke? Conversely, what is the exact capacity of a 1275? The Haynes manual says bore and stroke are 70.6 and 81.33, but these figures give 1273.53cc. Is a 1275 really 1275 or have we been diddled out of 1.5cc?

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.
Are we getting too pedantic, or should we go to the widely accepted sizes of 1275, 1293, 1301, 1310 and 1330 cc for the standard, +20, +30, +40 & +60 sizes respectively?

[Cooperman]

My mistake.
Should we also be adding 73mm (1360cc), 73.5mm (1380), 74mm (1399), 74.7mm (1425)

Quite right we should. Maybe a 'sticky' for this as so many people are confused by what overbore gives what capacity.
Maybe one of the Moderators could do this for us all.

[AndyMiniMad.]

I did a comp ratio calc on my 1380 and it came out at 10.7-1, With 1379cc....Oh no! ive been diddled out of 1cc as well. How am i going to hold my head up in public again after telling everyonre its a 1380. Im off to top myself......

[MRA]

I found that if you use Microsoft Excel you can end up with alsorts of slightly different answers dependant on the way you write the equation.....

And like all things, there is a tolerance, now if you actually go and measure the bore and stroke you may find that you have lost more than 1cc

I don't mean due to wear either.

[Spitz]

If making a sticky.....please add small bore engine overbores ( 848, 1098, 998 )

[998dave]

Heya,

Pop it all together as a concise summary on one entry and I'll happily stick it up for you.

Please include overbores for the smaller capacities too.

848, 997, 998, 1098, and any others.

[AndyMiniMad.]

I found that if you use Microsoft Excel you can end up with alsorts of slightly different answers dependant on the way you write the equation.....

And like all things, there is a tolerance, now if you actually go and measure the bore and stroke you may find that you have lost more than 1cc

I don't mean due to wear either.

Well im still alive!! misscalculated on the rope lenght...just goes to show that some times measurements can be inaccurate......More than 1cc you say.....Im off to try again.

Edited by AndyMiniMad., 15 April 2011 - 04:25 PM.

[nath]

iv measured that te bore is 71.5 aswell:)