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Pistons don't reach top of block
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Author:  suffolk [ Thu May 24, 2007 5:41 pm ]
Post subject:  Pistons don't reach top of block
My cooper s 1275 pistons don't reach top of block,they stop short about 12mm,it has german pistons which are correct hieght. What does this mean is it normal or is it to do with stroke?????Ive had engine running before stripping it and it floggs along nicely .It has cooper s crank .........any ideas cheers
Author:  sports850 [ Thu May 24, 2007 5:45 pm ]
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Have you still got the head off ? If so , spin the motor till one piston is right at the bottom of it's stroke and measure the distance from the top of the block and then spin it to the top and measure again to see what the actual stroke is (just make sure you measure them at the same point of the piston) . I would have thought they should come higher than that though , normally they are nearly flush .
Author:  Lillee [ Thu May 24, 2007 5:55 pm ]
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12mm from the top of the deck will give you something like 4:1 compression :? That will severely kill your power output.

As said above are you sure they are below the deck? turn the motor over until one piston is top dead center (piston raises to maximum height before starting to come down again) and measure it from there.

If it's still 12mm and it's an S crank, are the rods AEG521 S rods? if so what are these pistons? as they are clearly the wrong hieght.
Author:  suffolk [ Thu May 24, 2007 6:09 pm ]
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I had pistons measured at engine reconditioners and they said they are up to specs, i'll have to check conrods tomorrow,seems strange motor goes really well, some one said to me it might have been low compression for a super charger but i don't know anything about them!!!!! I spose the recomendation would be to get them flush with the deck?
Author:  sports850 [ Thu May 24, 2007 6:12 pm ]
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What's the part number of the crank , it's not a south african 1100 one is it , much the same as a 1071 from memory in a 1275 ?
Author:  mason [ Thu May 24, 2007 6:16 pm ]
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12mm you reckon thats about 480 thou You have got some major problems. The last motor i built was a 1275 bored to 1330 and those pistons were 1mm (40 thou)from the top of the block deck to give a idea of what it should be and then i milled about .55 mm off the block again.Sounds like you may have the wrong pistons.With the c/r that low it wouldnt even run any good even with a turbo bolted on :cry:
Author:  suffolk [ Thu May 24, 2007 6:17 pm ]
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en40 or enb 40 something like that can't remember but my mate at the mini place was told my crank number and he said its defently cooper s crank! must be wrong rods or pistons or both? What does stroking do lengthen or shorten stroke?
Author:  mason [ Thu May 24, 2007 6:25 pm ]
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Yep EN40 is a S Crank. Stroked means longer sounds like wrong rods or pistons.Did you take complete engine into shop.If you did i would hope that its still got the same rods in the engine so most likley pistons are at fault
Author:  suffolk [ Thu May 24, 2007 6:50 pm ]
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Just went to have a look crank is en40b ,conrods are aeg521, total hieght of piston is 59mm,,from top of connecting rd hole to top of piston is28mm,,pistons are MAHLE MADE IN GERMANY HELP ARE THESE PISTONS RIGHT HIEGHT??
Author:  mason [ Thu May 24, 2007 8:49 pm ]
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EN40B crank and AEG521 Con rods Are 1275 S Parts so nothing wrong there.Have you checked all of the cylinders are the same or is there just one like that (Bent conrod).Are you sure its 12mm not 12thou its got me lost at the mo.If its 12mm it has to be a fault with the pistons
Author:  Mick [ Thu May 24, 2007 9:10 pm ]
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Piston height and wrist pin to top height are the same as the mahle I have here.

The difference in strokes between a 970 and 1275 and a 1071 and 1275 are 19.42 and 13.07 mm respectively.

All early S cranks are EN40B badged (970,1071,1275, it is the steel type not the partnumber which is elsewhere) so check the stroke to make sure it is 81.33mm
Author:  paul_ewins [ Fri May 25, 2007 10:14 am ]
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One possibility is that you have a 1071 crank, the other is that your pistons are short, the third is that your rods are too short.

The 1071 was introduced first. When the 1275 was introduced the block height was increased to compensate for the longer stroke. For the 970 the rod length was increased. So if a 1071 crank is put into a 1275 block the pistons will be below the top of the bore. (I've no idea what happens if you use a 1071 crank with 970 rods in a 1275 block).

A 1071 crank should have the number AEG 171 (see http://www.hotbricks.org.au/techtips/ar ... cranks.txt for a list of the 1275 possibilities)

Short pistons seems a remote possibility if they were sold as Mini pistons. Putting Hillman Imp pistons into an 1100 Mini had this problem as the Imp pistons are shorter (i.e. between the top of the piston and the pin, not the overall height) than Mini pistons. Imp pistons won't fit a standard 1275 block however (either 68mm or 72.5mm diameter depending on type). If the pistons didn't come in a box marked "Mini" then maybe they're from some small Korean or Japanese car.

Short rods seems even more unlikely, since the only conceivable use for them would be to put a 1275 crank into a 1071 block or, as noted in an earlier post, to create an engine with stuff-all compression.
Author:  suffolk [ Fri May 25, 2007 1:13 pm ]
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Problem solved,my crank has AEG171 on it,,,WRONG CRANK,, Any one have 1275 cooper s crank for sale??
Author:  Mick [ Fri May 25, 2007 1:14 pm ]
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a big oops!
Author:  Lillee [ Fri May 25, 2007 1:46 pm ]
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That 1071 crank is EXTREMELY rare so oil it up and keep it well :wink:
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