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Location:
Melbourne
Registered:
August 2003
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Re: 2Ltr 2tg questions (NEW ONES!) I think..
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Sun, 22 February 2004 09:05
 
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If you want to use 18R-G pistons, you can but you will need to keep the 2T crank I think. This is still a good thing and was common back in the day. They are 88mm giving 1703cc I think. Most people to go with the 3T crank.
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Something to keep in mind here is that in standard form the 85mm/70mmcylinder dimensions of 2T lead to a very rev-happy motor which needs to be kept 'on the boil' to perform.
This is because for each revolution a 397CC volume is swept and ignited, driving the rotating assembly through only a 70mm stroke. Proportionally, the bore/stroke ratio in 2T is 1.21:1.
When lengthening the stroke to 78mm on the standard bore, the resultant 443CC volume drives the rotating assembly through a 78mm stroke. The bore/stroke ratio in this case is 1.08:1, so much closer to square. This motor would, all else equal, be more 'torquey' in its delivery than the over-square 85/70 motor given above.
With me so far?
If you now increase the bore of 2T to 88mm deliver 1703CC with 18R pistons and 70mm (2T) crank, your bore stroke ratio will be 1.25:1. (Can you hack camming up then revving out the motor to 8000+ RPM every day to make it perform? Can you live with a 4.3:1 (originally available in D or T axle I think?) or higher short final ratio to keep the motor on the boil through the gears? etc. etc.)
Alternatively, the 85/78 motor described above will not require reboring to suit your pistons, only the 3T crank is needed. Standard 3T-C pistons can be used (with machining for valve releifs). The resulting bore/stroke ratio will be 1.08:1 and the total capacity 1770CC. It's a larger displacement (and a much more usable one) with no overbore required. Think about it.
For interest's sake, the 1mm overbore to fit off-the-shelp cast 86mm 3T pistons (1mm oversize) will net an increase of 42cc to 1812cc. I'd not go to the expense of a rebore unless the block required it, as 1770CC vs 1812CC after the cost of the bore is not bang-for-buck, the difference will be three-fifths of stuff all.
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