Definitive 903cc piston deck clearance...?

Matt Zerega

Passion, not Practicality.
Good day, All. I'm in the process of building an offset bored 903 turbo motor using a bare block I procured (I don't have the factory pistons, and I'll be using custom pistons). I can't find the factory piston-crown-to-block-deck clearance spec...which is critical to properly specifying compression height on my custom pistons, in order to achieve desired static compression when the motor is all buttoned up.

Here's what I understand thus far about stock 903cc engine specs:
  • 65mm O.D. piston
  • 68mm stroke
  • 110mm rod length
  • 24.3cc chamber volume
  • 30.75mm piston compression height
  • 1.14mm assumed head gasket compressed thickness
So...at 9.5:1 math suggests that the stock pistons set -0.41mm below the deck surface

Anybody know where I can find the stock 903cc spec for deck clearance? I'd rather not rely entirely on my assumptions and math.
 
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lezesig1

expatriot
Hello Matt,

On your behalf, I just posed this question to a long time and well known 903 engine builder.
Unfortunately, I did not receive the dimensional answer you were hoping for.

Their answer is "it depends". They further explained the myriad of available dome shapes,
compression ratios, head types, head materials, and so on.

Feel free to PM me direct if you would like the contact info for the company I spoke with.

lezesig
72 spi
 

fiatfactory

Steve Cecchele
You should double check a few of your assumptions.

CH is the distance from the gudgeon pin centreline to the piston crown - exclusive of any dome or dish.

You have the 30.75 mm CH part correct, but the 9.5:1 100GBC engine also has a raised dome across approx half the crown surface, and it's another 0.75mm high
pistons 850 coupe 65_4.jpg

Check your combustion chamber volume, that number doesn't look right to me... my recollection is more like 28.5cc, but it's been a while since I CC'ed one

I also just now measured a few OE Fiat 903 head gaskets, and the thickness at the fire ring of the stock gasket is 1.02mm, so it's going to be under 1mm compressed.

I've never seen the value your're looking for mentioned in a Fiat service manual, but how about rather than trying to work backwards from the compression ratio to figure it out (this induces too many errors), how about just measuring the block height from the crank centreline to the deck. take away half the crank throw, take away the rod length, take away the CH and what you'll be left with is the deck clearance (either plus or minus or zero) ... that will be pretty definitive.

SteveC
 

Matt Zerega

Passion, not Practicality.
Thanks, Steve and lezesig... I just got the block back from the machine shop; offset bore is done. And I've got a set of pistons that I'm hoping will produce acceptable static compression. I'll get the thing assembled...and then I'll measure. Nothing like the confirmation we get from measuring tools to yield a good night's sleep. I'll post a photo or two after it's together.
 

Matt Zerega

Passion, not Practicality.
Quick update: the cylinder head combustion chambers measure out at approximately 18.8cc, with the valves and a sparkplug installed. And with the 66.5mm bore, 117mm rods and lightweight hypereutectic pistons installed, there's 5.08mm of deck height at TDC. Assuming the Victor Reinz head gasket compresses to 1.14mm, a stock 68mm crank will yield roughly 6.5:1....so the crank is getting shipped off to be stroked to 70.5mm...yielding a usable 8.6:1 and allowing 14 lbs. of boost from the GT12 (Chinese copy) turbo (near ideal Pressure Ratio, as per "compressor map")...thereby yielding 12.0:1 effective compression under boost. Appears Peeeerrrrfect. Now...I wait 12'ish weeks for the crank to get welded, ground and shipped back :( When it's back, I'll assemble the motor, clearance the bottom of the cylinder walls and the camshaft (to clear the rods) and I'll start on the Speeduino NO2C fuel/spark/boost control computer integration. If you haven't read up on the Speeduino, check it out. It's a well thought out, tiny controller that's amazingly capable and inexpensive. I'll post more detail as I get farther along.
 

ramona300

True Classic
So what is the deck height going to be at the end so that you get the calculated 8.6:1 CR. I'm doing something similar at the moment and using Suzuki pistons and head and have been trying to find the stock piston height for a Suzuki with no joy. I have stroked the crank and presently the piston is about 4mm down the bore at TDC. I have decided to have the deck machined down to zero deck height. The present engine runs 10psi with a supercharger with about 8.5 CR. I run a Microsquirt for full EFI. I guess you will be using Tunerstudio with your Speeduino.
 

fiatfactory

Steve Cecchele
: the cylinder head combustion chambers measure out at approximately 18.8cc, with the valves and a sparkplug installed.

Your cylinder head has been surfaced quite a lot to have that volume in the combustion chamber.... the F.I.A. homologation documentation for the 850 sport coupe with a 100GBC engine says the measured combustion chamber volume should be 26.5cc. on a new cylinder head

Why the need to "offset bore" anything with a piston diameter of 66.5mm?

The disadvantage of hypereutectic aluminium alloys (those with a silicone content above 12%) for pistons is they become more brittle, so while they expand less and allow for a tighter piston / bore clearance, they are more prone to breakage if the engine detonates.

SteveC
 

Matt Zerega

Passion, not Practicality.
Your cylinder head has been surfaced quite a lot to have that volume in the combustion chamber.... the F.I.A. homologation documentation for the 850 sport coupe with a 100GBC engine says the measured combustion chamber volume should be 26.5cc. on a new cylinder head

Why the need to "offset bore" anything with a piston diameter of 66.5mm?

The disadvantage of hypereutectic aluminium alloys (those with a silicone content above 12%) for pistons is they become more brittle, so while they expand less and allow for a tighter piston / bore clearance, they are more prone to breakage if the engine detonates.

SteveC
That's a bit disconcerting. The head I've got measures 69.0 mm thick. The only other references I've found are fiatplus in Santa Cruz CA, that says a new head is 69.6 mm thick. And then there's a dude on fiat-850.nl that appears fairly diligent that cc'd his head with valves and sparkplug installed and found 20.5 cc. If I assume the head I've got was shaved by 0.5 mm at some point (reducing chamber volume by roughly one cc), and the valves and sparkplug occupy roughly 1.5 cc of volume...that would get me to 26.5 - 2.5 = 24 cc. Finding documentation on these little motors that lines up with what I'm experiencing...and measuring is difficult. Anyway...you've inspired me to order to brand new head casting, which I've done. Now who knows what that'll yield; it's purported to be for Panda, 127, A112, 903cc motor. Whatever. It's new, flat and clean...and there appears to be a lot (relatively speaking) of these new castings out there....so I'll build to that.

As for why to offset at only 66.5 mm: even at this bore, fire rings on the stock 903cc head gasket were too close to the new bore for my taste, so I bought a gasket for the 1050 motor. Fire rings are noticeably larger and spaced differently. Offsetting a bit put the 1050 gasket's fire rings outside the OD of the new bore. There wasn't much offset.

Finally..on the disadvantages of hypereutectic: These are pretty much de-facto OEM standard these days..and I've run this type of piston in several different motors (much larger motors.. Japanese and American) and I've never seen or head of anybody breaking one. I'm not claiming it doesn't happen; I'm just saying that I'm not deterred by their hardness. I like tight piston/cylinder clearances...and I'll be careful about timing and boost, thereby doing my best to avoid detonation and preignition.

For the curious..here's a comparison of stock vs. the rods and pistons I've chosen. Like crows, I like shiny things.

p1.png
p2.png
 

ramona300

True Classic
I guess you have weighed the two pistons for comparison. The Suzuki pistons and conrods for mine are about 2/3rd the weight of the Fiat items. The pistons are made in Taiwan and are downright pretty, a shame to hide them away!
 

Matt Zerega

Passion, not Practicality.
Agreed; these pistons are no-nonsense precision pieces of machinist art. I haven't weighed them, or the 117mm "Fiat" (Chinese) rods with factory 43.66mm big end...but I will, and I'll report back. I bought bronze bushings and had the small ends downsized to fit the Suzuki (Namura) pistons. Regarding rods, are you using F10A (45.01mm big end) or Hayabusa (41.00mm big end) rods....or something else? I'm intrigued... because I'm beginning to think I could've avoided the significant cost of welding and grinding (stroking) the crank; I see that F10A and Hayabusa rods are 119.5'ish long, and the Hayabusa rods have smaller big ends thereby allowing for downsizing the 903cc crank to fit!
 
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ramona300

True Classic
I'm using the stock F10 conrods. I had the crankshaft ground to take the smaller big ends of the Suzuki conrods and gained some extra stroke at the same time. It will be about 950ccs and with the supercharger adjustment of 1.7 jumps me into the next class. I was in the under 1600cc class.
 

fiatfactory

Steve Cecchele
That's a bit disconcerting. The head I've got measures 69.0 mm thick. The only other references I've found are fiatplus in Santa Cruz CA, that says a new head is 69.6 mm thick. And then there's a dude on fiat-850.nl that appears fairly diligent that cc'd his head with valves and sparkplug installed and found 20.5 cc. If I assume the head I've got was shaved by 0.5 mm at some point (reducing chamber volume by roughly one cc), and the valves and sparkplug occupy roughly 1.5 cc of volume...that would get me to 26.5 - 2.5 = 24 cc. Finding documentation on these little motors that lines up with what I'm experiencing...and measuring is difficult.
don't take my word for it, the FIA documentation is here

scroll down to page 5, item 143"volume camera di scoppio" is 26.5cc, now I cant find a definitive explanation of this volume, i.e. if it includes the head gasket thickness or not... (I think it may) so if this includes a 1mm thick compressed head gasket at 66mm ID fire ring (that's roughly 3.4cc) still gives a nett volume of 23.1cc in the combustion chamber itself.

in other models documentation Item 168 is usually the value that's important , the "volume camera combustione nelle testata" which is definitely the nett combustion chamber volume, but item 168 doesn't appear in this particular documentation.

on the hypereutectc alloy pistons used, agreed a tight piston to bore is a good thing, but I've seen plenty of breakages (I fix a lot of other people's f@ck ups) due to detonation

it's mostly the ring land between the first and second ring that breaks a piece out, fortunately the suzuki pistons you've chosen have quite a bit wider margin here than most (certainly wider than the 850 piston next to it) so the greater width means it's less likely to break than a narrow width...

SteveC
 
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Matt Zerega

Passion, not Practicality.
Oh my... Thanks a ton for the documentation reference! Many hours are likely to be burned perusing that resource. After digging through most of the 850 variant documents, including those for the 127... I'm prepared to conclude that the head I have is from/for a 127...which the documentation states is "23,3" for "Volume camera di scoppio" If the head gasket is included in the figure...and it's a little thicker when compressed than your estimate we're awful close to the observed ~19cc. It will be interesting to see what the chamber volume is for the new casting I ordered from Slovenia. P.S. Thanks to ramona300, I ordered a set of used Hayabusa rods...due to their 119.5 mm length...and 41.0 mm big end. Since I failed to ship off my crankshaft for welding and grinding ("stroking"), bushing down the small end of these rods might save me some money...and more importantly, many weeks of waiting; a great deal more shops can deal with re-bushing the rods and grinding (only) of the crank's rod journals. Thanks a bunch, ramona300 and SteveC. I sincerely appreciate your input and guidance.
 

fiatfactory

Steve Cecchele
I found some Panda 903cc documents that show 20.5cc for combustion chamber volume in the head (nett figure), and also gives a value for the head gasket thickness at 0.95 to 1.05mm compressed,

what you're getting from overseas is probably a 127 / panda type head, as they are the ones most commonly seen for sale.

Looks like there are some variations in combustion chamber volume thru the engines production run of 20 or so years..., as 850 sedan / special , 850 coupe , 127 and a112 all show variance in the combustion chamber volume values in the FIA documents.

SteveC
 
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