You can offset grind the large journal crank to give more stroke. You will need to by small journal rods to suit after they offset grind it

I thought you used a non standard crank and oversized pistons to get to get to 1380

Standard crank but overbored to 73.5mm gives you 1380cc.

The downside is you don't really have any more rebores once the engine wears. If you went +020 over, then next time you can bore to +040 etc

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don't bore a standard block to 1380!!!!!!! or even up to 060

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The bitterness of poor quality (or Crappy Workmanship) remains LONG AFTER the Sweet Taste of Low Price is forgotten.

Then what's the best way to get more power other than twin su carbs or a 45 dcoe webber, a cam with higher lift or a better cylinder head

You need to match all the components to get a good result. Eg
Compression ratio
Cam
Cylinder head
Carb(s)
Ignition system
And also decide what your goal is regarding power/driveability etc

The difference from 1275/1293/1310/1380 won’t make as much difference as getting everything matched and working together.

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Timmy is 100% correct

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The bitterness of poor quality (or Crappy Workmanship) remains LONG AFTER the Sweet Taste of Low Price is forgotten.

Hi Woody. Without trying to be rude, do you know what a standard mini feels like on the road - a 1275 or thereabouts having about 60 horsepower at the wheels? I am only wondering because if you go to a lot of time money and expense to get approx 80 hp at the wheels you may still be disappointed or you may be delighted.
I am with the others regarding the block, DON'T overbore to the max on your first go. To add to Timmy's list a good exhaust manifold and exhaust helps flow for relatively small change ( commonly called a stage one kit in the UK)

Yeah I do, my grandfather use to have a cooper s with a 1275 and I enjoy the power, but I still feel I need a bit more just because I live in an area with lots of steep hill and more power and torque would go along way to making it a bit better.

Hmmm. A 1275 Cooper S should eat up steep hills.

The engine that you have if bored to 1380 would probably have less response than the 1275 S.

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The bitterness of poor quality (or Crappy Workmanship) remains LONG AFTER the Sweet Taste of Low Price is forgotten.

It did but I have a 1977 Leyland Mini sunshine with a 998. Abd the 1275 I know isnt a cooper s engine so I new I would need to make more power some other way and I instantly went to boring it out to make more power. Now I know that keeping it a 1275 and doing cam and head upgrades would be better.

Good to keep asking questions and good that you are armed with info beforehand.. good luck and keep us informed.

Don’t know if this helps, but the 1275 I just had done was already bored to 0.040” and fortunately in excellent state, so no need to bore it further. This was then matched with cylinder head work, cam, carbie, exhaust, 3.4 diff etc. to produce a great road going 1310cc giving good torque from relatively low rpm for around town, and good acceleration on the highway and pulls like a train up hills.

So definitely agree with what Timmy was saying that it’s the sum of all parts that are correctly matched that will provide the biggest bang for the $

4.3 diff???? Not with 10 inch wheels.

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The bitterness of poor quality (or Crappy Workmanship) remains LONG AFTER the Sweet Taste of Low Price is forgotten.

I have 12" rims with a higher profile tire arm. I'm got go down to a lower profile due to my tires rubbing if I go over a bump with more than 2 people in the car.