All great to know! I got a couple bottoms and a box of pistons. Right now doing bicycle prep I'll be referencing all the above info when the time comes to do the engine. I have put a flying horse piston in a Grubee engine before with great success. The engine and bike were completed 3/4 years ago. It's my kid sisters bike which still runs and goes like crazy. I can't recite the science but I had to plane down the cylinder Head and trim the piston skirt.I got pics of this in my album titled flying horse in a grubee. Be awesome if one of you guys would check it out and see what ta heck I did. Please feel free to leave a comment. Thanks you guys!
http://motorbicycling.com/album.php?albumid=808
After looking at the pix, I noticed something right away, the piston you're labeling as a GT5 isnt a GT5 piston, its a pk80 low pin type piston, one reason I know this is because wrist pin locating on the Flying Horse engine and a GT5 is the same, they both use the high pin piston.
so what you actually built was a super short stroke engine, because the lower you made that engine out of is a pk80 geometry that uses the 110mm short rod and thats why when you put the GT5/Flying Horse TypeB piston in it the skirt hit the cranks counter weights at BDC, what you sorta done back then was make another form of a hybrid/ Half Breed engine we seem to be seeing more of lately, difference is that the Half Breed engine use a 38mm stroke crank that has the 40mm stroke 110mm rod, it uses the low pin piston and this makes the piston .062" shy of coming all the way up to correct deck height when piston is at TDC, so it actually needs the deck lowered right out of the box inorder to get compression up to a good level.
What you did is use a lower assembly designed for being a 40mm stroke and you did all those so that you could make a GT5 geometry upper work on a pk80 lower, you could take that same lower, put low pin piston in it and use a standard pk80 jug/cylinder on it and you'll have a standard 40mm stroke pk80 geometry engine.
Not a thing wrong with what you done here, and what I think you may have had originally if im not mistaken was a GT5A type engine which I think was a 40mm stroke engine, I cant be possitive and if Im wrong someone can correct me here, but either way the fact is that you forced a GT5 38mm stroke setup to work on a pk80 40mm stroke lower engine assembly and if it works as good as you say, I see nothing wrong with that at all, what you may have here is a 34mm engine now....
at any rate the fact that you got it all to work and run good is a cool thing to me, Id call that your "Mad Scientist" Geometry engine.....lol!!!
I remember when you did all of this, but back then I wasn't as familiar with the different engines and there geometries as I am now so I didn't realize what it was you had actually accomplished.
One thing you can remember is that if the upper ever gives up the ghost all you'll have to do is drop a standard pk80 type jug and piston on that lower and she'll be ready to go again without having to do all the mods unless you just reall wanted to do the tinkering.
looks like a fun project, at somepoint I might even make me one of them just for the fun of it.