Measuring deck height on a dish, looks like a wrist pin bore issue Options

[bbrock]

I searched and couldn't find an answer about where on a dished piston to measure the deck height. I have Euro spec Mahle "flat top" pistons for 8.0 compression, but in reality, they have a slight dish in them. I'm following Tom Wilson's book and he just says to measure them "at the crown."

I measured deck height with the single metal gasket/shims that came with the gasket kit installed only because they are already on there and I just wanted to see if I was close. Deck heights at the center range from .052" to .07" which seemed like a decent starting point but height out at the edges is only .02". Of course, I can get any number in between depending on where I measure. I just want to make sure I have the right clearance.

Also, how important is it to cc and calculate precise compression for a street ride? I'm just looking for a reliable engine and not concerned about squeezing every HP out of the motor UNLESS it will improve fuel efficiency without sacrificing reliability. I'm happy to go through the drill if results in a better engine for my needs, but reading Wilson's book, it seems the important thing is to have enough clearance to keep the valves and piston tops safely apart.

Finally, is it normal/acceptable/desireable to mix and match barrel base shims to even out variances in deck height?

Any advice welcome. I just get confused trying to separate basic build best practices from performance tuning.

[MarkV]

Just to be clear, the "Barrel Height" I posted above is the distance between top and bottom sealing surfaces on the cylinders themselves. Three of them are within .004 of each other so basically identical. The fourth measures about 0.01 shorter than the other three which is also about the same as the difference in deck height.

I'm pretty sure this means I can't even things up by moving jugs around because that is only going to move the shallow deck to another location. If I had a little more variance in deck heights of the other cylinders, I might have some opportunity there, but as it is, the other cylinders are darn near perfect. I'm going to play with that some tonight to verify. I also don't think milling the case will solve the problem since the difference looks to be in the cylinders themselves. The only two ways I can think to even things out would be to add .01 to the short cylinder, or take a little off the tops of the other three to make them all the same length (sealing surface to sealing surface. I'm not sure what is allowed. (IMG:style_emoticons/default/confused24.gif)

I never measured the "Barrel Height".... never occurred to me to check that. I installed cylinders and tightened them in place and then checked deck height. Juggled them around until I had matching pairs. (IMG:style_emoticons/default/confused24.gif)