Nick
As I think I had said earlier in the post, this guys plans are excellent and nicely detailed. I found one error on the crank and the dimensioning on the A frames wasn't sufficient that I had to re-draw them to work out the cutouts. (see attached). I also converted all the threads to BA as I have a stock of small head BA screws.
The place where things could go wrong, and I only realised this after I finished it and was reading an article on a grasshopper in a recent ME. The geometry of these engines is such that if it is not built accurately enough it can lock up as there is no cross head on the piston so there can be a tendency for it to cock over. The piston yoke is adjustable so I think that's where any errors are resolved.
That's two of his I've done now and will no doubt do another at some point.
Pete