How do/did you decide on best pistons for a street motor?

[MIGHTY 8]

Would like to hear all opinions for a 2L normally aspirated.
1. Over bore. Start small? Or go right away to max (85mm?)
2. Max recommend CR. Planning on only using street fuel (~93octane)
3. Starting point. What is the recommended starting point where one begin an engine build? Start with either A, B, C, or D?
A. Pistons?
B. Cams?
C. Carbs? (Only Weber's. Either 40's, or 44's.)
D. Valves? All vendors sell same size intake& exhaust.

[vandor]

Overbore is a personal preference. Some people want to go as big as possible for more engine capacity, others want to be able to rebore the engine again in 20 years.

If funds are limited then spend it on the engine block (pistons, light flywheel, balancing, etc) first, as that's the hardest to modify later.

Generally you don't want to go over 10:1 CR on the street. It's been done, but often you end up running timing that's more retarded than ideal, so you loose the advantage of more compression.

[So Cal Mark]

start with the pistons, CR will dictate any other engine options. The first decision is to determine the parameters the engine will operate in. For example, I constantly get calls about using radical cams from owners that don't go over 5000rpms.

[djape1977]

don't go to max overbore. these engines often blow gaskets between cyl 2 and 3, overboring reduces ammount of material there and increases failure rate

[MIGHTY 8]

Thank you for your responses.
This is a street build, but without a doubt, this motor will spend most of its life in the 5-6K range! I don't drive as much as I would like, but when I do, after she is warmed up, I do not NOT take advantage of what Señore Lampredi has provided.

From all my research, it seems (pretty obvious now) that the pistons are at the heart of the "rebuild."
Have already heard enough recommendations to NOT go straight to 85mm, although I believe my short block is in good order, but that is just my guess.

From memory, it seems it's best to skip to the first 0.4, and maybe go to 0.6 overbore. This appears to be approximately in the "happy middle" before the MAX of 85mm. ( this would constitute the FIRST rebuild or this block.)

Needless to say, head, valves, exhaust, will all be addressed.

I just want to clarify from those that are willing to provide input- which direction to go with.

[SoFlaFiat]

Gabriel,
I went with minimal overbore, 80.4, to assure clean cylinder walls, and a middle of the road 4.5mm dome. I too didn't want to get too extreme with the compression. I'm a believer in "all around" improvement and therefore installed bigger valves, longer duration cams, good intake and exhaust.
In any case, someone will correct me if I am wrong but, don't get the compression too high is my opinion. And, increase it with piston height rather than diameter.

[vandor]

Alan,

I think you mean 84.4mm

[fiatfactory]

.

[SoFlaFiat]

Alan,

I think you mean 84.4mm

of course I did!
In any case Gabriel, as far as SteveC says, I have made bad choices... And obviously can't remember the ones I do make...

I'm betting it runs in the end!

[baltobernie]

I would start with choosing my camshafts. The amount of overlap determines the true compression ratio the engine experiences while running. (Google "Dynamic Compression Ratio")

After you've selected camshafts, then choose pistons. If you're going to use an aggressive cam profile for your 5-6k normal operating range, you'll need a high static compression ratio to offset the cams.

Backing up, keep Mark's advice in mind. You can rev the stock cams until your ears bleed, and they will continue to make power all the way up, as long as you've got fuel and exhaust to support them. Like Mark, I'm not sure an extra 10 or even 20 HP on the top would be a good trade for a crappy idle and no power (none) below 3K. Particularly in Manhattan.

[carl]

Most aftermarket HC pistons that are readily available have either 4mm dome or 8mm dome. There is a long history of guys with 8mm dome pistons in street cars having to really pull back on timing and most regret those pistons. I got the 4mm dome pistons in my stock 2L motor with IDFs and in warm weather I have to run 93 octane gas.

[MIGHTY 8]

Thank you all for your valuable input, and taking the time to respond.
It is fair to say, that not in one sitting of reading these pages will make an engine building Master! But, a little knowledge goes a long way. Thanks again.

Am trying to walk the fine line between "over-thinking" this whole rebuild process and simply going with the "Expert's" tried and true concepts. Confusion reigns, where such a significant difference exists, for example between "flat tops," and "domed" pistons. In this case when both are not only readily available, but recommended by their respective sellers. As far as I understand it, domed pistons where THE factory OEM for the European versions of our beloved early 124's. Talk about experts...Who can argue with that?

There may be in the end, no "wrong" decision as long as some have pointed out, I make sure I understand where I want this power band to flourish. (I. E. Drivability vs. outright performance.) and proceed from there.

Thanks again fellow Fiatsters