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Stroking crankshafts
https://www.ausmini.com/forums/viewtopic.php?f=5&t=63653		 Page 1 of 2
Author:  850man [ Fri Jun 11, 2010 4:39 am ]
Post subject:  Stroking crankshafts
Can someone please fill in the blanks for me or tell me the formula to work it out, I am thinking about building a stroker 1275 motor and would like some capacitys, bore size Vs stroke. The block is currently Std bore.

78mm
79mm
80mm
81mm Std bore 1275 +030 1300 +060 1330
82mm
83mm
84mm
etc

Thanks.
David.
Author:  1018cc [ Fri Jun 11, 2010 8:28 am ]
Post subject: 
The formula is as follows:

Bore in inches
Stroke in mm

((((pi x ((bore + overbore) x 25.4)^2)/4) x stroke) x 4) / 1000

Basically you use pi x D^2 / 4 to find the area of a circle, multiply that by the stroke, multiply by 4 (4 cylinder) and divide by 1000 to the numbers to cc's.

Hope that helps.

Otherwise, here is a table I prepared just before:

Bore (in) Bore +0.030" Bore + 0.060"
Stroke (mm) 2.780 2.810 2.840
78 1221.4 1247.9 1274.7
79 1237.0 1263.9 1291.0
80 1252.7 1279.9 1307.4
81.28 1272.7 1300.4 1328.3
82 1284.0 1311.9 1340.1
83 1299.7 1327.9 1356.4
84 1315.3 1343.9 1372.7

<edit> if you "quote" this post the table formatting comes out heaps better.
Author:  awdmoke [ Fri Jun 11, 2010 9:39 am ]
Post subject: 
or use this calculator
http://www.ajdesigner.com/phpengine/engine_equations_cubic_inch_displacement_b.php
Author:  850man [ Fri Jun 11, 2010 9:41 am ]
Post subject: 
Thanks guys, That was just what I was after. :D
Author:  justfanginaround [ Sun Jun 13, 2010 5:11 pm ]
Post subject: 
sorry for the hi-jack but can you de-stroke a crank ?
Author:  Morris 1100 [ Sun Jun 13, 2010 5:20 pm ]
Post subject: 
justfanginaround wrote:
sorry for the hi-jack but can you de-stroke a crank ?

Yes.
Author:  justfanginaround [ Sun Jun 13, 2010 5:33 pm ]
Post subject: 
if i use an uncracked 1100s b/j crank how far un-stroked can i go ?
Author:  mickmini [ Sun Jun 13, 2010 8:30 pm ]
Post subject: 
justfanginaround wrote:
if i use an uncracked 1100s b/j crank how far un-stroked can i go ?


depends on the wear of the big end journals, but basically the maximum offset is the difference between large and small journal sizes for the diamter, with the centre of the journal is then offset by half this number.
Author:  Asphalt [ Sun Jun 13, 2010 9:01 pm ]
Post subject: 
'Bohrung' means bore, 'Hub' means stroke :)
Image
red - std. cc
green - std. overbore
blue - other popular oversizes

Helpfull? :) (may contain the odd rounding error)
Author:  drmini in aust [ Sun Jun 13, 2010 9:26 pm ]
Post subject: 
mickmini wrote:
justfanginaround wrote:
if i use an uncracked 1100s b/j crank how far un-stroked can i go ?


depends on the wear of the big end journals, but basically the maximum offset is the difference between large and small journal sizes for the diamter, with the centre of the journal is then offset by half this number.

If you are prepared to grind the crank more undersize than usual, you can get more stroke (or less stroke if destroking).
Matt Read aka TheMiniMan holds the record I think, he did a couple with the crankpins at -.080". :shock:
But to do this, he had to corner the world market of -.080" bearings... :lol:
Author:  david rosenthal [ Sun Jun 13, 2010 10:46 pm ]
Post subject: 
justfanginaround wrote:
if i use an uncracked 1100s b/j crank how far un-stroked can i go ?

With a good [std size] 1.750" B/J crank you can shift the center by 3mm to 1.625 [allowing for clean up] and this X by 2 will add 6mm to the overall stroke.
If you want to de-stroke then just use a 998 crank[1275 crank 81.28 -6 =75.28 and 998 is 76.2 mm] with spacers in the block and caps[2" to 1.750"] or new steel caps.
Author:  justfanginaround [ Mon Jun 14, 2010 4:07 am ]
Post subject: 
so if i put a 998 crank into a 1275 block with a 120" over bore i would get 1298cc.

thats cool
Author:  drmini in aust [ Mon Jun 14, 2010 8:31 am ]
Post subject: 
justfanginaround wrote:
so if i put a 998 crank into a 1275 block with a 120" over bore i would get 1298cc.

thats cool

Might be cool for the track (if 1300cc limit) but give me more cubes for road use. A 1445 stroker's torque would suck the doorhandles off your 1298. :lol:
Author:  david rosenthal [ Mon Jun 14, 2010 9:28 am ]
Post subject: 
justfanginaround wrote:
so if i put a 998 crank into a 1275 block with a 120" over bore i would get 1298cc.

thats cool


And stick a 16 valve twin cam head, fuel injected and a blower on it then you will make some HP :)
Author:  justfanginaround [ Mon Jun 14, 2010 1:40 pm ]
Post subject: 
s/sedan rules not force fed and under 1300. So i figure if I have a big bore and a small stroke I should have the best of both.
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