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.

[bbrock]

A new question/problem. I'll properly measure the combustion volume over the weekend, but for now, I remeasured deck heights with shims removed and registers barrel bottoms recleaned. I plugged in those numbers with an assumed 60cc for combustion volume which seems to be normal for a stock 2.0 head just to get a ballpark of CR numbers.

The numbers are below. The problem is cylinder #3 which is reading roughly 0.01 lower deck height than the other cylinders. I measured all of the barrel heights and found all very close except #3 which, coincidentally, is about 0.01 shorter than the rest. Ignoring #3 for now, the minimum shim for the rest of the jugs would be .03 which would give me about .043 deck height, 8.3 CR and max CR variance of 0.002. That's not the 8.5 Mark suggests but looks pretty good to me and gains over the stock spec of 8.0.



Now on #3... I suppose the proper thing would be to have the barrel tops milled so they are all equal and go with .04 shims all around, but I'm thinking an extra 0.01 shim would have the same practical result. It seems like that would both bring the deck height close to the other cylinders, and also push the head sealing surface up level with #4 for good head sealing. That would make 8.2 CR on that cylinder which is lower than the rest, but it doesn't seem the worst thing to have #3 a little lower. Is that amount of variance in barrels normal? Okay, let's hear how stupid I'm being. (IMG:style_emoticons/default/hide.gif)

And Rob, your thread on your own deck height issues was very helpful for getting me started on this. (IMG:style_emoticons/default/agree.gif) thank goodness Mark and others are here to help!

[Mark Henry]

A new question/problem. I'll properly measure the combustion volume over the weekend, but for now, I remeasured deck heights with shims removed and registers barrel bottoms recleaned. I plugged in those numbers with an assumed 60cc for combustion volume which seems to be normal for a stock 2.0 head just to get a ballpark of CR numbers.

The numbers are below. The problem is cylinder #3 which is reading roughly 0.01 lower deck height than the other cylinders. I measured all of the barrel heights and found all very close except #3 which, coincidentally, is about 0.01 shorter than the rest. Ignoring #3 for now, the minimum shim for the rest of the jugs would be .03 which would give me about .043 deck height, 8.3 CR and max CR variance of 0.002. That's not the 8.5 Mark suggests but looks pretty good to me and gains over the stock spec of 8.0.



Now on #3... I suppose the proper thing would be to have the barrel tops milled so they are all equal and go with .04 shims all around, but I'm thinking an extra 0.01 shim would have the same practical result. It seems like that would both bring the deck height close to the other cylinders, and also push the head sealing surface up level with #4 for good head sealing. That would make 8.2 CR on that cylinder which is lower than the rest, but it doesn't seem the worst thing to have #3 a little lower. Is that amount of variance in barrels normal? Okay, let's hear how stupid I'm being. (IMG:style_emoticons/default/hide.gif)

And Rob, your thread on your own deck height issues was very helpful for getting me started on this. (IMG:style_emoticons/default/agree.gif) thank goodness Mark and others are here to help!

Get the deck milled and don't worry about very slight variances, same with the slugs.
Trying to get perfect small tolerances quickly becomes a game of the dog chasing its tail, balancing rods end for end is a perfect example of this fact.