Understanding High Compression Pistons for 1608

[Daniel]

jim im still around and so are you

[fiat218]

jim im still around and so are you

[seabeelt]

Daniel, did you end using those pistons..........
R/

[thechadzone]

I've been trolling Europe for euro spec/high comp 1608 pistons, and the most trustworthy info in most of the postings seems to be the the label on the nos box of pistons designating 80mm and/or 1608, and the pics of the pistons showing at least somewhat of a dome with fly cuts for the valves. My US spec 1608 pics are flat topped.

I'm sure it's a tough question, but will any 1608 piston that isn't flat topped like mine yield a higher CR? I pulled out the pieces of my disassembled & painted 1608 yesterday, and I'm considering building it instead of the two liter from my Spider for my BC Coupe because I have very little time to pull out the two liter and get it to the state that the 1608 is at now. this new stay at home dad lifestyle is busy!



[url=http://www.flickr.com/photos/thech ... .jpg[/img]
Fiat Coupe engine paint latest progress by thechadzone, on Flickr[/url]


[url=http://www.flickr.com/photos/thech ... .jpg[/img]
Untitled by thechadzone, on Flickr[/url]

[Daniel]

Daniel, did you end using those pistons..........
R/

Many times

[Daniel]

thechadzone can you post some photos of the pistons in the block or not top and side views maybe i can tell
what you have by viewing them. BTW i like what you did with the valve covers with the little red racing stripes
seems fitting for an Italian car.

[So Cal Mark]

I've never seen a 1608 with flat top pistons, are you sure you don't have a 1592?

[fiatfactory]

.

[thechadzone]

My mistake. I think I was recalling the pistons from my former 850 Coupe, which were indeed flat. I just checked my Flickr, and my 1608's pistons are as I described the ones I've been seeing for sale around the world. Too bad, I was hoping for easier identification of higher comp pistons...

[[url=http://www.flickr.com/photos/thec ... .jpg[/img]
Fiat Sport Coupe engine tear down 6 by thechadzone, on Flickr/url]



*Thanks to Mark for letting me know what motor the flat ones go to.

*Thanks to Steve C. For chiming in as well. Two of Earth's foremost Fiat experts, can't get better help than that! Daniel is clearly no amateur either. I really like the friendly help around here, it's much appreciated!

I always copy my longer posts into Word or into the Notepad of my iPad before submitting them. I'm sorry to have missed out on some good Fiat Factory info!

[Daniel]

My mistake. I think I was recalling the pistons from my former 850 Coupe, which were indeed flat. I just checked my Flickr, and my 1608's pistons are as I described the ones I've been seeing for sale around the world. Too bad, I was hoping for easier identification of higher comp pistons...

[[url=http://www.flickr.com/photos/thec ... .jpg[/img]
Fiat Sport Coupe engine tear down 6 by thechadzone, on Flickr/url]



*Thanks to Mark for letting me know what motor the flat ones go to.

*Thanks to Steve C. For chiming in as well. Two of Earth's foremost Fiat experts, can't get better help than that! Daniel is clearly no amateur either. I really like the friendly help around here, it's much appreciated!

I always copy my longer posts into Word or into the Notepad of my iPad before submitting them. I'm sorry to have missed out on some good Fiat Factory info!

Thanks for that. As for the photo it would be a little easier if I had a side angle view of the piston's top so can you spin the crank and move the piston to the top then take a couple more photos from dif points of view thanks.

[fiatfactory]

.

[smbaird37]

SteveC,

Thanks for your last post on this thread - great info! I have been sidetracked on my project for a long time but finally getting back to it. Not sure what I'll do since the block is already bored out to .06 OS. But thanks to you, I have a clue what to look for.

[BEEK]

I would venture to say, that all us spec 1608's came with floating pins, only the 1438 had press fit pins here. this is not to say that there is an oddball out there, but the norm is what i said. i have disassembled 1000's of fiats. started when i was 15 and in 49

[anicoletti]

I really loved your article on the 80 mm pistons. My 124 has 1.8 engine 132AC1, piston MONDIAL 2575F. Would you have some article on the 84 mm pistons for this engine? Thank you very much.

There are two distinctly different TYPES of pistons available for the 80mm twincam variants, those with press fit pins and those that are full floating. Press fit is quite self explanatory, the gudgeon pin is pressed thru the small end of the connecting rod (after the rod is heated in an oven) and to do properly requires a small jig to mount the piston on so that the pin is correctly centralised. The swiveling action at the piston end happens inside the piston pin bosses, where the pin goes thru the connecting rod it is held firm. The piston does not need any clips to hold the gudgeon pin in place, so the piston bosses have no clip grooves. Full floating gudgeon pins fit thru a bush that is mounted into the small end of the connecting rod. the piston pin slides freely thru this bush, and swiveling action takes place at the small end and inside the piston bosses. To hold the pin in place there are circlips that fit into machined grooves inside the piston bosses.

THEN there are differences in the piston compression heights, as well as piston dome differences. The compression height is the distance from the piston pin centreline to the top of the piston (the crown) EXCLUDING any dome or dish ... this difference can have a marked affect of the resulting static compression value as well, as it alters the compression volume at TDC by a large number. So JUST concentrating on the dome height is a commonly made error. The piston domes also vary in their base diameters. This also affects the total volume that the dome occupies so thus alters the static compression ratio as well. Fiat made many changes during production of the 1438 and 1608 engines to the piston dome height, the piston domes base diameter and also the flycut diameter and depth... these ALL affect the resulting static compression ratio. You CAN'T pick this up by eye in a picture, you need to MEASURE it.

Here is a scan from the european 124 BC/BS part book, showing the full floating pin setup used in these motors.



1608 full floating pins, 125BC.000 engine as found in 124BC/BS in Europe. These pistons have a compression height of 39.25 and a 5mm or a 6.2mm dome, and there are two variations of valve flycut (narrow and wide inlet flycut) that I am aware of. Borgo and Mondial also made higher compression versions of the 1608 125BC piston with a 7.5mm dome height. The correct pistons for a 125BC.000 engine are scarce, but if you look hard enough you can find them. No need to have forged pistons made unless that's what you want to do.



I have a big stack of 80.0 standard size genuine Fiat pistons available (39.25CH 5mm dome narrow flycut) and one last set of 0.4 oversize genuine pistons (39.25CH 6.2mm dome wide flycut)


narrow and wide flycut comparison.


a picture of a Mondial piston with a 7.5mm dome, narrow flycut.


The Borgo 125BC.000 piston equivalent is a 66.6523 (39.25CH 5mm dome, narrow inlet flycut)


The Mondial piston equivalent is a 7080, 39.25CH 5mm dome , narrow flycut.


It's not just the dome height that gives the 125BC.000 variant it's 9.8:1 static compression value, the extra 1,5mm of compression height over the 1592 equivalent FFG piston and the press fit variants that is a big factor.

Press fit found in 125A.000 and 125B.000 as found in 125 sedan (A) or 125 Special (B) and North American variants. The pistons for a 125A or 125B variants are the same as the 1438cc (124AC.000) Press fit pin pistons come in four types. All have 37.75mm compression height, but four dome heights / types are available. A 3mm dome for USA spec engines, later european has a 4.2mm wide base dome, early european narrow base dome in both 6.2 and 6.45mm dome.

If your engine is press fit pins, then plenty of pistons are still available.

These are Mondial piston 6741, 37.75CH and have a 4.2mm dome, press fit pins

A Borgo 66.5946 is like this as well, 37.75CH and a 4.2 dome

the Kolben Schmidt equivalent is 91 528 600, 37.75CH and a 4.2mm dome


A Borgo 66.6705 looks similar to this but has a dome height of 3mm, 37.75mm CH, C/R is 8.5:1 USA market only.

These are Mondial piston 6710 and have a 37.75mm CH, 6,2mm dome, press fit pins


a Borgo 66.5872 piston looks like this, 37.75mm CH with a 6.45mm dome



Note the BIG difference in flycut size between these pistons and the factory 1608 from the 125.BC000 engine variant, also note the big difference in the base diameter of the two domes...despite being a couple of mm taller, both domes are approximately the same volume, resulting in similar static compression ratios, between 8.8 and 9:1 in a 1608.

1592 piston has a compression height of 37.75mm and a dome height of 4.5mm, they are full floating gudgeon pin. If these are used in a 125BC.000 engine (they will fit) the resulting compression ratio is much lower than the original 9.8:1 due to the much lower compression height, and the piston will sit approximately 1.5mm down the bore at TDC (known as MINUS DECK) and approx 8.8:1 is the result.

Note that Fiat only list 0.6 as the largest oversize for these engine, as beyond that size the wall thickness of the bore is marginal... yes they can be taken out to 81.0 mm (1mm over) and I have seen it done, but you had best be sonic checking the wall thickness of your particular block if you go this route.

If you want to work out the dome volume , the shape is known as a conical frustum, a good calculator can be found here... http://www.calculatorsoup.com/calculato ... rustum.php

I should hope I have a bit of experience with this stuff, I built my first twin cam when I was 14, I'm now 48...

SteveC

[anicoletti]

I really loved your article on the 80 mm pistons. My 124 has 1.8 engine 132AC1, piston MONDIAL 2575F. Would you have some article on the 84 mm pistons for this engine? Thank you very much.

There are two distinctly different TYPES of pistons available for the 80mm twincam variants, those with press fit pins and those that are full floating. Press fit is quite self explanatory, the gudgeon pin is pressed thru the small end of the connecting rod (after the rod is heated in an oven) and to do properly requires a small jig to mount the piston on so that the pin is correctly centralised. The swiveling action at the piston end happens inside the piston pin bosses, where the pin goes thru the connecting rod it is held firm. The piston does not need any clips to hold the gudgeon pin in place, so the piston bosses have no clip grooves. Full floating gudgeon pins fit thru a bush that is mounted into the small end of the connecting rod. the piston pin slides freely thru this bush, and swiveling action takes place at the small end and inside the piston bosses. To hold the pin in place there are circlips that fit into machined grooves inside the piston bosses.

THEN there are differences in the piston compression heights, as well as piston dome differences. The compression height is the distance from the piston pin centreline to the top of the piston (the crown) EXCLUDING any dome or dish ... this difference can have a marked affect of the resulting static compression value as well, as it alters the compression volume at TDC by a large number. So JUST concentrating on the dome height is a commonly made error. The piston domes also vary in their base diameters. This also affects the total volume that the dome occupies so thus alters the static compression ratio as well. Fiat made many changes during production of the 1438 and 1608 engines to the piston dome height, the piston domes base diameter and also the flycut diameter and depth... these ALL affect the resulting static compression ratio. You CAN'T pick this up by eye in a picture, you need to MEASURE it.

Here is a scan from the european 124 BC/BS part book, showing the full floating pin setup used in these motors.



1608 full floating pins, 125BC.000 engine as found in 124BC/BS in Europe. These pistons have a compression height of 39.25 and a 5mm or a 6.2mm dome, and there are two variations of valve flycut (narrow and wide inlet flycut) that I am aware of. Borgo and Mondial also made higher compression versions of the 1608 125BC piston with a 7.5mm dome height. The correct pistons for a 125BC.000 engine are scarce, but if you look hard enough you can find them. No need to have forged pistons made unless that's what you want to do.



I have a big stack of 80.0 standard size genuine Fiat pistons available (39.25CH 5mm dome narrow flycut) and one last set of 0.4 oversize genuine pistons (39.25CH 6.2mm dome wide flycut)


narrow and wide flycut comparison.


a picture of a Mondial piston with a 7.5mm dome, narrow flycut.


The Borgo 125BC.000 piston equivalent is a 66.6523 (39.25CH 5mm dome, narrow inlet flycut)


The Mondial piston equivalent is a 7080, 39.25CH 5mm dome , narrow flycut.


It's not just the dome height that gives the 125BC.000 variant it's 9.8:1 static compression value, the extra 1,5mm of compression height over the 1592 equivalent FFG piston and the press fit variants that is a big factor.

Press fit found in 125A.000 and 125B.000 as found in 125 sedan (A) or 125 Special (B) and North American variants. The pistons for a 125A or 125B variants are the same as the 1438cc (124AC.000) Press fit pin pistons come in four types. All have 37.75mm compression height, but four dome heights / types are available. A 3mm dome for USA spec engines, later european has a 4.2mm wide base dome, early european narrow base dome in both 6.2 and 6.45mm dome.

If your engine is press fit pins, then plenty of pistons are still available.

These are Mondial piston 6741, 37.75CH and have a 4.2mm dome, press fit pins

A Borgo 66.5946 is like this as well, 37.75CH and a 4.2 dome

the Kolben Schmidt equivalent is 91 528 600, 37.75CH and a 4.2mm dome


A Borgo 66.6705 looks similar to this but has a dome height of 3mm, 37.75mm CH, C/R is 8.5:1 USA market only.

These are Mondial piston 6710 and have a 37.75mm CH, 6,2mm dome, press fit pins


a Borgo 66.5872 piston looks like this, 37.75mm CH with a 6.45mm dome



Note the BIG difference in flycut size between these pistons and the factory 1608 from the 125.BC000 engine variant, also note the big difference in the base diameter of the two domes...despite being a couple of mm taller, both domes are approximately the same volume, resulting in similar static compression ratios, between 8.8 and 9:1 in a 1608.

1592 piston has a compression height of 37.75mm and a dome height of 4.5mm, they are full floating gudgeon pin. If these are used in a 125BC.000 engine (they will fit) the resulting compression ratio is much lower than the original 9.8:1 due to the much lower compression height, and the piston will sit approximately 1.5mm down the bore at TDC (known as MINUS DECK) and approx 8.8:1 is the result.

Note that Fiat only list 0.6 as the largest oversize for these engine, as beyond that size the wall thickness of the bore is marginal... yes they can be taken out to 81.0 mm (1mm over) and I have seen it done, but you had best be sonic checking the wall thickness of your particular block if you go this route.

If you want to work out the dome volume , the shape is known as a conical frustum, a good calculator can be found here... http://www.calculatorsoup.com/calculato ... rustum.php

I should hope I have a bit of experience with this stuff, I built my first twin cam when I was 14, I'm now 48...

SteveC