new forged 90.5 Mahle B&P`s measured them up (all digital/calibrated bore mics and micrometer so 3 decimal places ) but the total piston skirt gap - barrel is only 0.043mm and think it should be 0.07 minimum ! I know you shouldn`t run them straight from the box so should they be honed them to get them within tolerance ?
My knowledge is gained from motorcycles, I suppose the air cooling on the older bikes cant be that different from air cooled vw. For road use base line was 1 to 1 1/2 thou per inch of bore so a 4 inch bore would be 4/6 thou or 0.1 or 0.15mm. For high perf we would go tighter than that down to 0.7mm, but that would be with high quality oil and regular changes. I personally think 0.4 is way tight, but there must be more knowledgeable folks than me. If you do have to hone it to size I suggest a machine shop type hone rather than the stone type.
Paul thats what it says about standard 1600 but the minimum of 0.07 was from a volksworld tech article by Matt Keene on identical Mahle forged 90.5 type1 B&P`s . Its not alot 0.02 aside if you think alloy expands more than cast iron ?
I don't disagree with what you're saying, but initial start up is going to "shave" the piston and the bore, so as long as you don't go revving the tits off it you'll be fine!! I'm currently building up a 1.6TD motor for my GTD Mk2 Golf, and the start-up tolerance between the piston and the bore is 1thou!! and that is a cast steel block and ally pistons...
