19 replies to this topic

[DeadSquare]

My BMC Workshop Manual shows all three Cooper S bores as:-   2.780 inches (70.6mm).

 

I just know, and have known for over 50 years, that the "compression height", the term used for the distance from the top of the piston to the center line of the gudgeon pin, for an 848 mini and a 1071 mini, is 1.5 inches  (38.11mm).

 

The South African '1100' engine uses the Cooper S bore, but has a slightly longer stroke which brings the engine size up from 1071cc to 1098cc,

[PACINO]

Thank you very much Deadsquare!So, (and sorry for the inconvenience I am causing you), what is the final cubic capacity that the engine now has after being rectified and mounted with +0.20 Oversized Pistons? And what cubic capacity did the engine originally have before being rectified?

Regards, Luis

[DeadSquare]

Let us go through the math together. (it doesn't work perfectly)

 

70.6mm =.............................................................Cooper S bore

35.3mm X 35.3mm = 1,246.09 = .........................The bore radius squared

times pi = 3,1416   =  3,914.7163........................The area of the bore

times 68.26 = the stroke, gives 267.21853cc = ...The volume of one cylinder

times 4 = 1068.8741cc, should be the size of a....1,071ccCooper S engine, but as I say, it doesn't seem to work out correctly.

 

For some reason, this works out at 2.126cc too small.

 

 

Your measured compression height of 37.15mm is .97mm less than my recollection of the 38.12 1071cc Cooper S compression height, .....so,......let us see what happens if we add .97mm to the 1071 cc Cooper S stroke which then becomes 69.23mm.

 

 

70.6mm =.............................................................Cooper S bore

35.3mm X 35.3mm = 1,246.09 = .........................The bore radius squared

times pi = 3,1416   =  3,914.7163........................The area of the bore

times 69.23mm = the new stroke, gives 271.0158cc = the volume of one cylinder

times four = 1,084.0632cc, which could be the theoretical original capacity of your original engine, depending on how accurately you managed to measure the compression height, but unfortunately doesn't come out any where near the South African "1100", which (someone in S.A. will correct me) is 1098cc;  in which case it is 14cc too small.

 

 

 

Now let us add .020" (.51mm) to the rebored engine and recalculate.

 

 

70.81mm..................................................................New bore size

35.405mm X 35.405mm = 1,2535.514 =..................The bore radius squared

times pi = 3.1416 = 3,938.0395...............................The area of the bore

times 69.23mm = "the stroke" gives  272.63047cc.=....The volume of one cylinder

times four = 1,090.5218cc, (an additional 6.5cc) which could be the theoretical capacity, depending on how accurately you managed to measure the compression height.

 

 

 

 

 

P.S.  I have found a note in my workshop manual, which probably indicates that the South Africa 1098 engine has a stroke of 69.85mm.

Edited by DeadSquare, 21 January 2020 - 04:49 PM.

[AP2020]

As most people will know in the meantime Federal Mogul has concentrated piston manufacturing with Nüral (Nürnberger Aluminiumwerke). Some of the old Hepolite designs are still available now with Nüral numbering.

 

For the big engines those are:

Hepolite Nüral Interchange.png

 

Edit: Yes, sorry, not deck height but compression height is correct.

 

@Pacino: perhaps you can find a number somewhere, maybe in the inner part of the piston.

Nick is correct in this not being performance-oriented. In my list the only performance pistons are the 73.5 as they are the old STR310 Powermax ones.

 

Yes and with the move went the quality... sad really as the 21253's where a good piston at high revs for a good cheap fast road engine

[PACINO]

Hello guys! This is what I have done. Formula to find the engine displacement. Pi (π ) per square piston diameter, divided by 4. This result multiplied by the piston stroke by number of cylinders.


Pistón diameter 71,10mm
Piston stroke 81,3mm (in general 1275 engines)

(π x Diámeter²)/4 x Piston stroke x n° cyl.


(3'14 x 71,10²mm)/4 x 81,3 x 4 ➡ 1290504,11922 mm³.

1290cc.