Bristol Engines

[UK6]

Tom,
Re overboring cylinders whilst keeping the external dimensions of the
block the same - I think that you will have insufficient sealing area
between the bores. The original engine is already marginal in this area!
The other issue, as correctly pointed out by Geoff Dowdle, is that a
substantially overbored block from say 66 to 75mm diameter would
encroach/erode your main bearing area of your block.
In short, if you increase your bores to enable 2.5L or more you need a new
longer block casting and crankshaft. Hence my TR 2500 engine block and
crankshaft suggestion earlier. Can I suggest that you have another look
inside any block and you will see what I mean.

I guess you could design a "modular" engine with different stroke
crankshafts and vary the rod length. Unfortunately the 6cyl Bristol motor is
already substantially undersquare and thus increasing the swept volume by
increasing the crank throw and fitting shorter rods would be a backward step
in efficiency. I think that most designers just set out to optimise the bore
to stroke ratio and allow say, a 3mm overbore capacity.

Regards,

Brett