Block markings

[darcher121]

    The top face of the block is marked +1/64 which I assume means it has been bored out at some point in the past?
Also, adjacent to each bore is a diamond with a 1 in; what could this mean?

[Colin Reed]

Hi,

1/64 seems an odd size .o156 th
normally over size bores go in .010 th increments in imperial measurements 
unusual to stamp the block face

Colin
NZ

[JonE]

what do the bores measure now, and what size pistons are in?
I had stamps in the same place, for a (fully dated!) 1953 rebuild. It wasn't great...

[Bob Culver]

The original Austin branded pistons in my car, reconditoned about 1939, had the marking1/32

[Parazine]

It was common, in the pre-war era to bore blocks oversize by 64ths. 

By the end of World war 2, we had become Americanised to some extent; the Americans had "decimalised" the inch. Rulers became available with 1/10 inch divisions and the the thousandth became the standard base unit. Somewhere I have a pair of Vernier callipers, calibrated in 1/64" increments - very difficult to read!

My set of drills however, remains in 64th increments!

[Tony Betts]

as suggested austins pistons originally went up in 1/64ths.

its not until we got into post war aftermarket things changed to 10thou at a time.

the 1/64 stamp on the block would have been done by austins. it may be that the block didnt clean up at standard. and had to be taken out to 1/64. or the engine was used on a test bed or in a car on the test track. then rebored when it came back so it could be refitted and sold. so it left the factory at 1/64

sorry i dont know what the diamond stamp dictates, but again its an original factory stamp. it could be test department it has been through??

tony

[darcher121]

For interest, the pistons have the same markings.
It will need a rebore and new pistons as the pistons can be “wobbled” in the bore
with my thumb.
I will decide on a new size after careful measurement.

The crank seems remarkably unworn; no discernible wobble or end float, and the big ends are the same. But more investigation before I decide to leave it as it is as I can’t believe it won’t need at least big end bearings renewed.
   

[andrew34ruby]

  A30 piston.jpg (Size: 41.12 KB / Downloads: 175)
The number in a diamond. Pictured is a piston from a 1955 Austin A30. Standard size. In 1962 it was explained to me that pistons were made, then measured, and graded according to the exact measured size. The grade number was stamped on the piston in a diamond. Bores were similarly bored, measured, graded and stamped.

The pistons were then selected such that a number two piston goes in a number two bore etc.

Effectively this gave closer tolerances than they could manufacture.

So I'm guessing this was used in the 1930s too.

I've remembered all this, and kept the piston, since 1962. I was eight.

[darcher121]

Very interesting!
I will be keeping these original Austin pistons,
maybe as some kind of mantelpiece ornament
if management allows it.

[Chris Garner]

Just to expand a little more on Andrew's graded number in the diamond.

The Austin Service Journal, dated April 1934, states:

Each grade covers a limit in diameter of 0.0005" only. When built the same grade number is stamped both on the block face and the respective piston so ensuring correct clearance.
Where oversize, fractions such as +1/64, +1/32, + 1/16 etc will be stamped, subdivided into grades as per the standard sizes above. Where the + figure is a plain number. eg +5, this indicates the oversize in thousandth of an inch. +5 thus being + 0.005". All oversize stamping appear on both piston and cylinder block face.

Prior to '34 the nominal oversize figure of a bored out block was stamped on the valve cover facing.