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1. the ring on the top end of the 250 ccm cylinder centers the cylinderhead . Drilling the cylinder to 300 ccm makes the center ring disappear. How do you mange the cylinderhead to be centered on the cylinder ?.
In this simplest solution you haven't got this feature. Head itself is machined to make collision impossible by making it 77 - 77,5 mm wide at the bottom. This dimensions were taken by experience and it is the cheapest way that work. If customer is willing to pay more, of course it is always better to have it. like here for example. There is free 3D model of head for 300 that I made.
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2. You use an Almot Piston.This piston is known to expand more than the original piston.Do you have more than 0,05 mm clearance (as it was used on the original piston /cylinder) ?
Generally, piston clearance is not the case. Couple years ago we took several experiments to figure it out, and after an answer from almot insider we knew, how they make their pistons and what they skip. I assure you 0,05 is more than enough. It's all about cylinder roughness, cross-hatch angle and driver skills
If you have seizure at the top of the piston, where it is 0,25 smaller and have 0,3 mm of clearance, then 0,01 from one side to another is really irrelevant. Typical almot seizure is at the top. Almost never on area, where you have this 0,05 of clearance. What I made for prototypes, that have to take extreme treatment from the beginning - I took extra 0,1 mm from piston rings area to make it 0,35 - 0,4 mm smaller than piston, an little angled relief under rings. The piston had enough place to grow but it was very noissy and lasted not more than 8 - 10 kkm.
Unless you make it 0,1 mm which is quite safe, but there's no real power then and the engine is a ringing like bell. There's no sense to make 300 this way in may opinion.
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3. On most of the ETz 250 engines the 300 ccm kit is not "plug and play".Very often, the bottom of the piston comes in contact with the crankshaft or the cankshaft case. How do you eliminiate this problem ?
Some people take off 1 or 2 mm of the bottom of the piston which changes the port durations....
Yes that is true, but almost every aware MZ rider knows that or should know that. Here in poland it is not that typical. I guess we had maybe 2 cases over more than hundred MZ's. But it was years ago, when we found that and right now everyone should be informed by the machinist that this is possible. Crank time at intake is at its extremes even in stock engine if you want to make reasonable economical symmetric motor. Around 160 degrees. Generally it is common here to machine the crankcase. Most aware users make it, when they make general repair.
At the end I have to say - it's not an advert. I was forced to close my shop. It was not profitable enough in polish reality at the end (31.12.2019) despite the amount of work i had and I don't do it anymore. It's just entertainment and while it's about MZ, I thought I'll show you that
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