Thank you Andy841. Your explanation does actually make some sense. Although I didn’t measure them those QED pistons must have standard offsets. Thank you Craven also for raising a topic from which we can learn.
OK, so in the spirit of full disclosure, I have just tried some back of the envelope maths.
Assuming that the piston is at its maximum height when the centre of rotation of the crank, the crank pin and the gudgeon pin are all in straight line, it should be easy to approximate the change in angle of the crank required to move the gudgeon pin by a certain amount by considering the system as a right angle triangle.
With a conrod of 125.171mm and a stroke of 72.75mm, that gives a hypotenuse of 125.171 + 72.75/2 = 161.546mm
Assuming a 1mm offset, the ‘opposite’ side of the right angle triangle is 1, so as sin = opposite/hypotenuse, the angle will be:
angle = arcsin(opposite/hypotenuse) = arcsin(1/161.546) = 0.355 degrees
What am I doing wrong?
Edit: I got the conrod length wrong, I think I used conrods for a Kent block. The LTC conrods are 121.922mm, but that still gives an angle of 0.361 degrees, not the 4 degrees Rohan calculated.
mmmm… need to go back and recheck my spreadsheet. Maybe I got an order of magnitude wrong somewhere. 
Dammit…and I was just about to hand out the grumpy uncle awards!!
Andy
My calculations were correct and I actually said in my post above with the calculations the difference was 0.4 degrees in TDC. Somehow this went to 4 degrees in my mind in later posts 
The measurements of my pulleys reinforced that 4 degrees in my mind but given the accuracy of TDC markings on the pulleys this difference is due more to marking and measuring errors on the pulleys not due to offset versus non offset pistons
So in the end the TDC shift versus the crank is small at 0.4 degrees and the error due to the asymmetric curve when setting the TDC by measuing distance down the bore is even smaller at 0.05 degrees for offset versus non offset pistons
Sorry for the confusion
cheers
Rohan
Andy gets the grumpy old uncle award with consolation prize going to Rohan.
Craven gets a point toward membership of the honorary grumpy old uncle life membership award.
I will try to do better next time so I can get into first place. 
Interesting discussion, thanks for doing the maths!
Very interesting, and everyone learning and progressing still after 70 years eh!
Thanks 
Hello to you all, and to grumpy old uncles too
I’d be happy to give feedback on how the engine works with the QED 420 camshafts, but you’ll have to wait a bit!
To come back to the subject of determining TDC, without this inflaming the debate like the doughnuts and CVs, I tried several methods by demonstration (calculations bore me) according to your different opinions.
First of all, I don’t know if my pistons have an offset pin (QED/Omega forged std), I didn’t want to dismantle the lower engine to check.
But the result is that TDC always ends up in the same place, plus or minus 0.5°. I don’t think that the method is decisive to have an accurate TDC, in this case for a street engine.
The engine assembly is complete and will be re-installed in the chassis this weekend. I’m thinking of starting it up to check that everything’s OK before reassembling the body.
Regards
Stéphane