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I am rebuilding a 1275 Innocenti Engine.

The clearance in the Bores is quoted 3/100 for the pistons. (Nüral from Minispares)

My machineshop is now asking if this is right?

He would recomend 6-10/100.

What is your opinions?

Regards

Fabian

I would go with what the manufacturer suggests.

Fanfaniracing wrote: ↑Tue Jun 09, 2020 10:40 am I am rebuilding a 1275 Innocenti Engine.

The clearance in the Bores is quoted 3/100 for the pistons. (Nüral from Minispares)

My machineshop is now asking if this is right?

He would recomend 6-10/100.

What is your opinions?

3/100mm seems VERY little. Are you sure the manufacturers recommended clearance is not 3thou" (0.076mm) rather than 3/100mm?

I guess you are right Minikin

Here is the piston

The use of imperial figures always requires a lot of thinking for a "natural born metric person"

To be fair - a manufacturer stating the oversize in imperial, the diameter in metric and then the clearance in imperial (the last two on the same line) is a pretty poor practice

The second row of markings on the piston are all in Metric.

71.103mm diameter, Sp 0.030mm = Spiel (german for 'play' - i.e. clearance)

From the Nural data sheet for the STD piston:

87-524100-00
Piston diameter 70.595 mm measured 6.00mm from base of piston
Clearance 0.030 mm
Bore finished diameter: 70.625 mm


There's no clearance data given for the over-sizes, but we can do the same math:
Piston diameter 71.103 mm
Clearance 0.030mm
Bore finished diameter: 71.133 mm

Converted to imperial, that's
2.79933071" piston
2.80051181" bore
or approx 1.2 thou clearance

Seems a bit tight as the machine shop says - but that's what's in the data sheet.

Size of piston is on top of piston

Bore size is on piston box .

The AE / Nural slipper pistons generally require a slightly bigger bore than omegas

Just going mildly backwards (normal for me ) but if it was thou then that would of meant 30 thou which would sound interesting if it ever fired up

251 ENG wrote: ↑Tue Jun 09, 2020 6:50 pm Size of piston is on top of piston

Bore size is on piston box .

The AE / Nural slipper pistons generally require a slightly bigger bore than omegas

Cast or forged Omegas? Omega quote 4 thou when I checked(Same on every piston they make) which can easily be tightened up without issue.

Forged pistons have a bigger clearance.

The piston is smaller when cold , the bore size does no change

It's cast piston.
I would go with what the manufacturer suggests too.
It's 0.03 mm.

For fast road engine,
it may be prudent to run them with a lighter larger clearance, 0.038mm (0.0015").