Offset boring 600/843/903/1050 engine blocks - precise details explained?

Frknfiat

Frknfiat

bad day on black top
Question:
One can see that there is a bigger cylinder casting gap between the two middle most cylinder bores of 600/843/903/1050 blocks - so, which of the remaining cylinder "pairs" are being off-set bored to increase the casting gap between the remaining 2 front cylinders and then the two rear cylinder pairs?
Are the outer most cylinders or the inner most cylinders pairs offset bored, only moving one bore ?
Or, are both front and rear most pairs of cylinders both equally offset bored from one another?
How much of an diameter offset is being used??
Neither PBS or Paul Vanderhayden posted on the internet what the precise procedure was, J. Edward says "its obvious" in his youtube video.


I'm sure there is honest confusion over what offset boring cylinders precisely means.

Drawings and pictures would be helpful.
 
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just get a stock 903 head gasket, and a 965/1050 head gasket and lay them on top of each other with the locating dowels to align them and "it's obvious"

I can't get two gaskets and do it for you as all the gaskets I have are wrapped up.

basically the two end cylinders get moved offset most, and the two inner cylinders are offset much less... a good machinist can set up to bore offset pretty easily...just give him the two head gaskets and he will figure it out. All he will be doing is following the factory stock offset bore.

To do this modification you really need to start with a bore that is very close to standard, you can not do this to a block that's already been bored more than about 65.2mm, otherwise the bores don't clean up properly at 67.2 / 67.4 mm. You can get the 965 (press fit gudgeon) style pistons all the way up to 68.2mm and 68.4mm (I have them in stock) but bores bigger than 68mm will require a special head gasket with a larger inside diameter to the fire ring, as the stock 965/1050 gasket is only just on 68mm to the inside of the fire ring

SteveC
 
Paul offset bored a 903 block for me for 67.2mm pistons. I bought a copper gasket from him that had offset bores and he said it could be used as a template for a machine shop. Ended up selling it because I didn't think I ever need it. The new Scuderia Topolino in Germany may have some.

I never did quite understand exactly what it meant. Paul does post from time to time on the Fiat 850 FB group and you might inquire there.
 
It would be wonderful to find one place in the internet that has complete information on the correct method for offset boring blocks - as I have heard many versions and most conflict with others, so clarity is the goal.
Please keep the postings coming!
 
I was thinking that a new thread topic could be be posted hosting copies of John Edwards rear engine hot rod tip “pamphlets”, as John Edwards built on many other mechanics/machinist who came before him - no disrespect - but passing on info to new owners keeps the Fiat hobby alive - and that IS the point.
 
Abarth: An Enthusiasts guide by Pat Braden & Greg Schmidt has some pretty good information on the process too.
 
Anyone know the piston to cylinder wall clearance dim for a 1050c build via offset bore, 74mm crank & 67.2mm piston's?
 
59-600 said:
Anyone know the piston to cylinder wall clearance dim for a 1050c build via offset bore, 74mm crank & 67.2mm piston's?
Click to expand...
It will depend on the actual pistons you have in your possession. They should come with some notes. Pistons can be sorted into classes like "A", "B" & "C" which will have different clearance specs. and the box or pistons will be identified what they are.

Not sure what you're doing but it may be a good idea to obtain a factory manual so you have full specs for your motor build. The one's I just looked for online are all in Italian, Spanish or German though.

Otherwise, if you post this question on the FB 850 group, Paul vander Heyden sometimes posts answers on engine questions. He built my 1050cc motor around a 903 block.

Curious if you're using a 1050cc A112 block or possibly something else?
 
myredracer said:
It will depend on the actual pistons you have in your possession. They should come with some notes. Pistons can be sorted into classes like "A", "B" & "C" which will have different clearance specs. and the box or pistons will be identified what they are.

Not sure what you're doing but it may be a good idea to obtain a factory manual so you have full specs for your motor build. The one's I just looked for online are all in Italian, Spanish or German though.

Otherwise, if you post this question on the FB 850 group, Paul vander Heyden sometimes posts answers on engine questions. He built my 1050cc motor around a 903 block.

Curious if you're using a 1050cc A112 block or possibly something else?
Click to expand...

Paul on the 850 FB group suggested the following "As far piston to wall clearance, I would use a total clearance of 0.0025” (0.063mm) for either forged or cast pistons".

Basing this on a 903 block also. I dropped off a copy of Abarth: an Enthusiast guide, an 850 manual, & Motori Fiat Abarth Bialbero 700, 750 e 850 cc derivati dalla with the shop.
 
I saw Paul's comment. Interesting that he said it will mean really thin cylinder walls. But he built a 1050cc motor for me around a 903 block and never once mentioned anything about that. If I ever had to rebuild my engine and the cylinder walls were worn, I'd use a spare block I have rather than use oversize A112 pistons.

BTW, are you align boring the main bearing saddles or regrinding the journals on the 74mm crank? The 903 and 1050 cranks have different journal diameters. An A112 crank can be reground down to 903 dimension which is the easier way to deal with the difference. You will need to notch the bottom of the cylinder walls to clear the crank journals and IIRC one of the lobes on the cam needs to be narrowed slightly for clearance.

Another question is what cylinder head do you plan to use? A 903 head will end up with way too high CR due to the increased stroke. I used an A112 head which has a larger combustion chamber. I can't recall exactly what's involved in modifying a 903 head to work. This photo is an A112 1050cc head. Wow, seller on ebay.it is asking 1550 Euro for it!! I paid 207 Euro on ebay.es for a new one but that was a few years ago. With a bit of math calcs., you can figure out what the cc of the chambers should be.

Several years ago Paul did a good write-up called "Preparing that 115 HP motor" which might help. It's still available on the net AFAIK.

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