Technical 650cc fits and clearances

[gordinir8]

Anyone has the manual of a 650cc engine? (i have the well known downloadable FIAT 126 600cc manual) Looking the piston/cylinder clearances section in order to measure my piston and cylinder bore. I can see some big gaps between the piston and cylinder walls as well as some piston seizure marks so i might need to replace with oversize pistons (currently have 77mm B pistons)
This is one of my last 2021 posts here so i wish everyone a happy new 2022 that i hope it will be better for all of us
Thomas

 

[the hobbler]

Anyone has the manual of a 650cc engine? (i have the well known downloadable FIAT 126 600cc manual) Looking the piston/cylinder clearances section in order to measure my piston and cylinder bore. I can see some big gaps between the piston and cylinder walls as well as some piston seizure marks so i might need to replace with oversize pistons (currently have 77mm B pistons)
This is one of my last 2021 posts here so i wish everyone a happy new 2022 that i hope it will be better for all of us
Thomas

Hi Thomas---652cc "B" pistons are:---76.390mm to 76.940mm---bore diameter:---77.010mm to 77.020mm
"C" pistons are:---76.940mm to 76.950mm---bore diameter:---77.020mm to 77.030mm
Piston clearance in bore is from 0.070mm to 0.090mm
Piston over sizes are (should!) available +0.2mm---+0.4mm---+0.6mm. I have found AUTODOC to often be a good source of oversize pistons at a sensible price--usually 'Nural' pistons (complete with Goertz rings).
Hope that this helps you
Tom

 

[the hobbler]

Hi Thomas---652cc "B" pistons are:---76.390mm to 76.940mm---bore diameter:---77.010mm to 77.020mm
"C" pistons are:---76.940mm to 76.950mm---bore diameter:---77.020mm to 77.030mm
Piston clearance in bore is from 0.070mm to 0.090mm
Piston over sizes are (should!) available +0.2mm---+0.4mm---+0.6mm. I have found AUTODOC to often be a good source of oversize pistons at a sensible price--usually 'Nural' pistons (complete with Goertz rings).
Hope that this helps you
Tom

Just looked on the AUTODOC website Thomas---they have both +0.4mm AND 0.6mm over-size in stock.They are selling at a discounted price at the moment; just over £69 each (instead of £85). The +0.4mm pistons will take your engine up to 659cc---with 1mm off the head and 0.5mm copper head gasket,that will give you about 9:1 compression ratio (depending on the eventual size of your combustion chamber) If you DO goto new pistons, only hand the barrels to the machine shop WITH the pistons so that they can match the pistons to the barrels.
You MIGHT find that CAR engine-shops have a problem holding an individual barrel in place to bore it (the first shop I tried had such a problem) because their machinery is designed to hold crankcases in place. I eventually found a machine-shop that did motor-bike barrels as well as cars---it was a motor-bile restorer who told me about the shop that I now use.
All the best in the New Year to you and all the other members of the Forum

 

[gordinir8]

Thanks Tom, one question though, B piston (the one I have), means it is the 2nd oversize?
Also due to thousands of motorbikes in Greece all machine shops can handle a single cylinder, no problem with that, Ordering from UK is out of a question since Brexit, Axel Gerstl has pistons but I'll check around, maybe I can find them here also. I will borrow from work telescopic gauges and measure the cylinders. I'll post the results here. Your help is always valuable.

Thomas

 

[the hobbler]

Thanks Tom, one question though, B piston (the one I have), means it is the 2nd oversize?
Also due to thousands of motorbikes in Greece all machine shops can handle a single cylinder, no problem with that, Ordering from UK is out of a question since Brexit, Axel Gerstl has pistons but I'll check around, maybe I can find them here also. I will borrow from work telescopic gauges and measure the cylinders. I'll post the results here. Your help is always valuable.

Thomas

Yes, a 'B' piston is the 2nd 'manufacture-tolerance' standard size that Fiat fitted----it is NOT an 'oversize'. AUTODOC is----GERMAN! You should have no problem ordering from them.

 

[gordinir8]

Got it, autodoc comes also in Greek form, I guess in all countries. Yes price is better that others. Also speaking of good prices Diangelo has a 20% discount until tomorrow 02/01/2022, bought yesterday my camshaft and some other stuff.