How to measure bore size?

[fordconvert]

Can you get an idea of bore size with a caliper at the top?

If standard bore is 3.680
Would 30 over be 3.710? Or 3.683?

On one of my motors that I can read a 60 on the top of the piston I measure 3.738. My other motor which has no markings on the top measures 3.715.

Were/are there oversize pistons that were not marked?

 

[Bort62]

"30 over" refers to increasing the bore diameter by .030, or 30 thousandths. You should be able to measure this at the "top" of the bore or anywhere else in it. However, if the motor has a lot of miles on it, there may be a ridge at the top of the cylinder which would give you an inaccurately small reading.

a lot is going to depend on your caliper and your technique. a crappy set of calipers can't really be trusted past .0x

 

[fb71]

If standard bore is 3.680
Would 30 over be 3.710? Or 3.683?

On one of my motors that I can read a 60 on the top of the piston I measure 3.738. My other motor which has no markings on the top measures 3.715.

Were/are there oversize pistons that were not marked?

30 over equals 0.030", as stated by the previous post, so;

3.680 + 0.060 = 3.740
minus 0.002 for oil clearance = 3.738

So yes, that piston is 60 over, or + 0.060"

the 3.715 piston is most likely 40 over with some wear...

 

[Anonymous]

3.680 + 0.030 = 3.740

3.680 + 0.030 = 3.710

Math error

 

[fb71]

fixed! post-lunch brain-fade!!

 

[The Plankster Prankster]

a lot is going to depend on your caliper and your technique. a crappy set of calipers can't really be trusted past .0x

thats for darn sure - i was using one that belongs to napa to measure something in the store (to see if it would fit), and the caliper was worn about .020 on the entire front half of the jaw - always check their zero before trusting them

 

[matt1967]

I'd rather use a snap gauge, below the ridge line, and measure it with a micrometer, lots more accurate, if your determining if you need to rebuild anyway.

 

[fordconvert]

Has anyone ever seen oversize pistons that were not stamped as such on the top?

 

[Lazy JW]

Has anyone ever seen oversize pistons that were not stamped as such on the top?

Yes.

 

[fb71]

I rarely see them WITH markings. And I see LOTS of engines...

 

[fordconvert]

I thought I finally found a virgin for my next build up.