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.

Measure where the dish isn't. In this case its the crown around the edge that you have measured at .02.

When you compute the CR, you will "fill in" the dish by computing its volume (which you will need to measure unless you know that number already).

With a head gasket in place and dished pistons, you won't have a screaming CR. What you want to do is go for the CR you want and let that dictate your deck height (within reason - as you said you do not want valves to touch the pistons). If you are targeting a stock CR, that should give you a nice cool running engine (not knowing any other variables).

Zach

All the barrel base shims need to be the same. Make sure you get the registers completely clean and the shims completely flat. I found that there was minor sealer remnants on the registers that were that were causing mismatched deck height. You can also re-arrange cylinders to get a match.

8:1? You running Kerosene?

Thanks guys. This is helpful. And yes, I do need to pull the jugs and clean up the registers. Thought I toss on a pic of the piston tops. These are the OEM Mahle pistons bought back when they were still cheap. The dish is quite subtle. I actually thought they were flat all these years until this morning when I started measuring. I caught the light so you can see it in the pic. I'm not sure it will add significantly to the combustion chamber volume, but we'll find out.

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This is Montana man, we run on buffalo tallow! Just keepin' it stock

Not enough to worry about on a stock engine, IIRC the 2.0 US piston is .5 CR ...or was it 5cc, it's been a while since i've use them.
At any rate the amount of that dish is trivial. I'd aim for 8.5:1 with no less than a .040" deck.
I don't use head gaskets, just base shims, which have to be the same.

Glad Mark is here to help us

I had originally used valve relief pistons in my build, and I think I read from one of the experienced builders here (Mark Henry or McMark) that those reliefs added about 2 cc's to chamber volume. So I would think that dish would be closer to the .5 cc value, correct? Best to verify though, as the difference between .5 and 5 cc's added is going make a big difference on CR (all other variables the same).

I ended up replacing the valve relief with Keith Black flat top and with a deck height of about .042 I'm in the 8.5 CR ballpark. Excited to fire it up once I can clear some space in the garage

Good luck, Brent.

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.

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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.

And Rob, your thread on your own deck height issues was very helpful for getting me started on this. thank goodness Mark and others are here to help!

Prob need to get the case decked. It's common for one or more to "sink"...but I defer to the experts

dp

That was my first thought (and fear since that would require splitting the case and starting over), but it doesn't seem like that would solve the problem of a short barrel. It looks to me like the variance is in the barrels themselves rather than the case.

It's been a while since I did it. If I remember correctly I did a combination of things to get it set up right. I was using thicker shims and they had some small imperfections from manufacturing. I used a piece of granite and some 1000 grit wet and dry sand paper with oil to dress them. I also cleaned the registers as they had some minor remnants of the glue and paper from the oem gaskets. By switching jugs around I was able to get the both pairs to match each other although they were slightly different from bank to bank.

Common, I often see cases where the deck sinks between the two cylinder bores.

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.

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.

Sorry, posted that last one before seeing Mark's last post. So the only thing that really concerns me is that with the variance I have now (which I believe is due to the slugs), puts the one cylinder a tiny bit below the minimum .04 deck height. If I go with .04 shims, that drops the CR into the 8.1 range.

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.

Well, after a lot of measuring and checking that I documented on my build thread starting about here, it looks like the high deck on #3 is caused but the wrist pin bore being about .012" lower than the rest. I don't know how common that is on an unused OEM P&C set purchased in the 80s, but appears to be what I have.

What are thoughts on what, if anything, to do about it? Since this is only going to be for street use, I'm tempted to just set the deck on #3 as close to 0.04 as I can get and live with lower compression on the other cylinders. If I did that, should I leave the higher compression on #3 or move it to another location? If it's true that higher compression reduces exhaust gas temps, maybe that would be good for #3?

Alternatively, I guess I could take all the rod assemblies in to have that one piston corrected and all re-balanced. I don't know enough to understand the pros and cons of that though.