MTFCA Forum

I have heard that the Z heads have some size variation between the different cylinders. Especially compared to a Prus head. This was posted here many months ago so I decided to measure the sizes of my Z head.

I found that cylinders 2 and 3 were perfectly matched at 211 grams of water. #1 measured 210g and #4 held 208g. I think that is a difference a little less than a 1/2" diameter marble in size. One gram of water equals one cc.

Does anyone have any knowledge of the variation of other Z or Prus heads and What would be the max allowable variation?

My Z head has never been surfaced and I've had it for many years.

In such a large combustion chamber volume, the discrepancies are not great, and would have little effect on performance. Of greater concern with Z heads is the variation in combustion chamber shape at the gasket surface. They often need some finessing to give clearance on the gasket/piston and to accurately match the footprint of the gasket.

Allan from down under.

I would think that was very good! A roughly one percent variant in chamber size? You will have a greater anomaly than that due to the flow characteristics in the manifolds! Cylinders one and four flow better on the intake that do cylinders two and three. And cylinder number two has the worst flow on exhaust due to it following cylinder number one so closely. If you ever notice, exhaust manifolds usually reach glowing red between two and three first. Cylinder four also is restricted by cylinder two's passing exhaust ahead of it, however it dumps into a larger and expanding exit.
Your combustion space variation is minimal in comparison.