Will zero with the head bolted directly to the block with no gasket work, or am l increasing the compression ratio to much.
The current gap l have is no good at - 2.5 mm ( minus 2.5 mm).... head is touching and sitting above pistons above the block.
I have a customer who bought a Z head and the piston caused interference. I was busy and he wanted it NOW.. He used a little paint on the pistons and installed the head. The paint transferred to the head and he ground it off. He kept at it and eventually could place the head on the block with no gasket and no interference. Then he installed the gasket and torqued down to 50 lbft and is VERY happy with the result.
I think the original interference was due to the the head being decked too aggressively
I agree, the clearance you get from a compressed head gasket will be just about right. The Prus heads are machined in the squish areas above the pistons so I don't think you'll have any problems with that right out of the box in contrast to the interference problems reported with some Z heads. I decked my Prus head 0.100" though, to reach 5.8:1 in compression, so I had to mill 0.060" off the pistons too for clearance. Here I'm checking the clearance with some modelling clay..
I see that you have stepped the pistons where the head opens up.
Does that not cause an area where the compression has to work its way around the step ??
we are all good now, we have a good just misses with no gasket, all fine with a gasket... back to work... thanks.
Yes, but I don't think that'll be any problem. The piston moves down and the gap gets bigger fast - had some friends driving behind me last night, they said I was running 53 mph with a standard NH carb without opening it up fully so I think it'll be fast enough
good job ... and fast.
The "big guys" have figured out the .030-.060 is optimum for good "quench"
OK thanks for that Les, l have a slight understanding on what and why Quench is important, BUT there may be some out there that have no idea why and what quench is and why it is important.
Could you give us a better understanding on why, what and how this makes the firing better.
The flame can not burn in a gap that thin because there is all that relatively cold metal on each side. Thus it "quenches" the flame. This eases the potential of "spark knock" and makes the spark timing slightly less critical as you increase the compression ratio. Increasing the compression ratio speeds up the flame speed and so the quench becomes more important or at least handy. This is the fundamental problem with the spark plug location of the "leaping lizard" head
here's another l found ... thanks Les that is very informative...
"Quench, or squish area is typically the flat area on the top of the piston that's almost level with the top of the block deck. It must have a corresponding flat area on the deck surface of the head to qualify as quench.
If you look at a combustion chamber, you will usually see these flat areas, and they will have the volume of the actual combustion chamber between them. When the piston is compressing the mixture, as the piston nears the head, the flat areas on the head and piston come together and force the mixture from those areas to "squish" into the chamber, where the spark plug and burning mixture reside, so you achieve a more complete burn.
The quench area also runs cooler than the rest of the chamber / piston. These lower temperatures are where the "quench" comes from.
When properly designed, the quench areas can have a tremendous effect on the quality of combustion, and allow higher compression ratios, and due to this they are considered "artificial octane" by scientific types.
Bottom line is "properly designed, quench is good".