@ninja As a general rule of thumb, the greater the compression ratio the lower torque spread but the greater the peak power. I don't know the figures, but I'd look to go with a domed head.
All depends why reboring it, if the cylinder measures size you say then never been rebored. The cylinder on your year although called 56mm is actualy 55.95mm, if the cylinder is not damaged then just get it honed. If honing the cylinder then this piston is what you need.
https://www.ebay.co.uk/itm/Yamaha-DTR125-DT125-R-DTR-125-56-25mm-Bore-Namura-Piston-Kit/292513505226?hash=item441b29dfca:g:jyEAAOSwwbdWQwnA
Whatever size piston you buy whoever is reboring it will bore and hone to match what piston you supply. The best reboring is done by Karl at KPM as had them do a
few and also by PJME. Karl charges £30 for a bore and hone with return postage at £10-16 postage depending on courier and insurance required.
He will also fully port your cylinder for £100, just have to tell him what your after like more midrange etc or more mid and topend like a YZ.
http://www.kpmotosolutions.com/
@sgt-robuck-5 doesn't work like that. Air pushes coolant up in the expansion tank filling it and then spits out on the road. Then returns to normal once cool. Just open the radiator cap and bleed it, problem solved.
Italian import, you get mega props for owning one. They have wicked Motori Minerelli Stamps where the original DTR engine doesn't. It's super cool engine.
Slow as balls though. It's just not comparable to the SP.
Right also, don't be so quick to slate Yamaha for using the bolts that they did.
I'm not going into here, you can easily Google that information, but trust me when I say there is a perfectly justifiable reason as to why they used the bolts that they did.
I've always done about 5 heat cycles then a gentle ish ride before riding normal, never had any of my rebuilds seize or fail, obviously you don't wanna be holding it wide open for 2 or 3 rides but don't be afraid to accelerate, with a good forged piston and a few heat cycles you won't cause any harm.
Took it into my college to ask my tutor what he thinks. (he builds race bikes) he said it looks fine and I could file it down very lightly. he also said the scratches on the outside were fine.