Midget engine swap,1275 to 1380cc,anyone done it?
thinking about boring the 1275cc A-series out to 1380cc for more capacity. the standard bore is 70.6mm and the 1380 needs 73mm. is the block strong enough for the overbore or am i asking for trouble? the alternative is fitting a 1275 GT head with larger valves on the standard block. which gives more real-world power for a hillclimb car?
The A-series block is strong enough for the 73mm bore if the block hasn't been bored before. The key is having the bore measured by a proper engine machinist first. If the original bore is still at standard, you've got enough material for the 73mm overbore. The 1380cc engine with the 1275 GT head and a proper cam is a brilliant combination for hillclimb. You'll gain torque across the whole rev range which matters more than peak power on a short hillclimb course. Budget about $1,200 for the machining and rebuilding with new pistons.
The A-series engine responds very well to increased capacity. The 1380cc conversion is well-documented in the Mini and Midget racing communities. For hillclimb work the extra torque from the larger bore is more useful than the higher-revving characteristics of the standard bore with a wild cam. The 1275 GT head with larger valves is an excellent companion modification. Both together will give you a proper little competition engine.