Rebored or not...?

[Ian R]

Hi all, I'm trying the workout if my engine has ever been rebored. Its a 1958 BN4 100/6.
The markings on the piston tops are AEC800 and also has a number 3 inside a diamond.
The top of the engine block is also stamped with number 3 inside a diamond.
Any ideas..?

 

[Michael Oritt]

Measure the bores with a micrometer and compare the results to specifications.

 

[Ian R]

I don't have a micrometer, I'd hoped the markings would mean something..

 

[Duane_Rhynard]

Hi all, I'm trying the workout if my engine has ever been rebored. Its a 1958 BN4 100/6.
The markings on the piston tops are AEC800 and also has a number 3 inside a diamond.
The top of the engine block is also stamped with number 3 inside a diamond.
Any ideas..?

Ian,

I have a spare 100-6 block with the original 3.125" (79.375mm) bore, the pistons are also marked AEC800 and they have a "2" inside the diamond.

Thanks,
Duane

 

[Dandare]

The factory blocks had differing standard bore sizes due to machining tolerances so the blocks and pistons were graded and selectively assembled, hence the number in the diamond...they had to match.
Oversize pistons were not graded I believe, so if you have a diamond then it's standard bore IMO.

Danny: EDIT Could be wrong about O/S not being graded but would be stamped with the nominal oversize (in an ellipse) in any case.

 

[Keoke]

I don't have a micrometer, I'd hoped the markings would mean something..

YOU fellas got Harbor freight over there???----LOL

 

[Joe Schlosser]

really difficult to measure 20 thous or so in a worn bore with a plain micrometer. You can get maybe an idea if oversize but you need a bore gage

 

[roscoe]

If you want to do it yourself it can be done accurately, if you take multiple measurements and understand or learn how to properly use telescoping snap gages. As you can see they are inexpensive and if you have a decent vernier caliper you can use these in conjunction with tha or use a micrometer which tends to be a bit more accurate. If you go that route, do not start with the snap gage horizontal in the cylinder bore. Open the appropriate gage to a diameter larger than the bore and gently snug it to that length. Put it in the bore at a cocked angle, get it centered as best you can and pull the handle over center so the gage slides (retracts)to match the bore as it comes horizontal, go past center, remove the gage and measure with your caliper/ micrometer. Do this a billion times with each bore and you should get a very good average reading.

www.harborfreight.com/catalogsearch...f,EAFeatured+Weight,f,Sale+Rank,f&q=Hole+gage

 

[Ian R]

No sign of any ellipse markings. Your theory of the diamond marks make sense....