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Factory Cylinder Bore Oversize?

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AlaskaCJ2A View Drop Down
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    Posted: 22 June 2009 at 8:11pm
Was wondering if all factory new engines had standard cylinder bores, or if any came already oversized?  I was surprised after cleaning up my pistons to see them stamped .030, as this jeep has been in the family since new and there was no mention or evidence to suggest the engine had been removed or overhauled in the past.  This could have been the case of course, and I just may have been too young at the time (or not born yet!) to have been aware of it.  If all new engines came with standard bores then I have my answer, and more knowledge of the history of my jeep.  Thanks.
 
Tony
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Post Options Post Options   Thanks (0) Thanks(0)   Quote westforkboyd Quote  Post ReplyReply Direct Link To This Post Posted: 22 June 2009 at 9:01pm
Tony
 
As far as I know they all came standard bore. They did however make a final piston fit with sizes in .002" increments. It was coded A, B, C, D stamped on the deck by the valves. Who knows for sure though. After working in a factory for 30 yrs lotsa stuff goes on. What's your deck look like?...Mine has stamps all over it, different operator stamps, alignment markings, piston fit codes, and of course engine serial number. If yours is devoid of stampings perhaps it has been worked at some point.


Edited by westforkboyd - 22 June 2009 at 9:02pm
'48 CJ-2A #184135 Lefty
'49 CJ-2A #219719 Mule
'39 Ford 9N
'55 Oliver Super 55 Ollybelle
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AlaskaCJ2A Quote  Post ReplyReply Direct Link To This Post Posted: 22 June 2009 at 10:24pm
Greetings Allen,
 
So the A, B, C, D codes indicate how many increments of .002 that the cylinder was oversized beyond standard for correct piston fit during assembly?  Or were piston oversizes available in those increments?  I had thought that pistons were only available in .010 oversize increments.  My deck, from front to back 1-4 is stamped C, C, B, E between the bores and the valve seats.  Those are the only markings other than the serial # J230923.
 
I only have the #1 piston head bead blasted so far, and as I said it's stamped .030 while the #1 bore is stamped "C".  I'm waiting for the rain to stop before I can drag my blaster outside and do the others, but I have to assume that if one indicates .030 that the others will as well. 
 
I don't have the right tools big enough to measure these bore sizes, so I don't have a clue yet what to expect there, other than the bores look real good and show no signs of rusting, pitting, or unevenness - for sitting so long I'm impressed.  I'm going to order up a set of telescoping gages and outside mics of the correct size so I will know myself where everything is at prior to sending the block and crank out - and where it's all at once returned.   


Edited by AlaskaCJ2A - 22 June 2009 at 10:42pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bkreutz Quote  Post ReplyReply Direct Link To This Post Posted: 22 June 2009 at 10:35pm
There was a recent discussion about this, but I can't come up with the correct words in the search function to come up with the thread, I seem to remember (as Allen said) that the letters were for "fine" adjustments to piston clearance and didn't relate to overbore size IE: a .030 oversize piston with a "A" stamp would actually be .032. At least that's what I remember, hopefully someone will remember the thread and link it here.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote westforkboyd Quote  Post ReplyReply Direct Link To This Post Posted: 22 June 2009 at 11:33pm
There is some discussion of this in my project Mule thread
 
You might be correct in the oversize piston availability now days. The manual lists some more that were available in the '40s. Others know more about that than I. The fitting of pistons  during assembly and the coding is explained in the Mechanics manual page C-19. Rather than being available in oversize increments it is called "grading" of standard pistons. Probably dosen't have anything to do with your  .030" oversize.
 
Your block is stamped in the same location as mine for the assembly coding. Interesting but dosn't answer you question. I'd have to guess it was bored at some time. No big deal and if the pistons and cylinders are OK perhaps just new rings will be enough. How bad of a ridge does it have? That and cylinder taper will determine if a simple honing will work for you.
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'49 CJ-2A #219719 Mule
'39 Ford 9N
'55 Oliver Super 55 Ollybelle
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AlaskaCJ2A Quote  Post ReplyReply Direct Link To This Post Posted: 23 June 2009 at 7:07pm
I knew I should have gotten a Mechanic's Manual - been doing this entire resto with just the basic Parts and Service Manuals.  At any rate, I understand now what is meant by "grading" of standard pistons.  It's similar to what we did in aircraft engine overhaul shops during assembly, only we did it for the purpose of balance as opposed to fitment.  When matching piston and rod assemblies for opposing cylinders, we would pick through an assortment of standard pistons and rods to find those whose weight most closely matched the opposing set.  It was amazing to me the weight difference there was to be found between pistons and rods of the same part # - several grams difference was not unusual.  By selectively putting together matched weight pairs, we could get the balance of opposing cylinders down to a few tenths of a gram - it really made a huge difference in how smoothly our engines ran, and in their longevity.
 
So now I see that WO must have selectively picked through a pile of standard pistons - whose diameters differed by a few thousandths here or there - to find that particular one that would "fit" the bore in question most perfectly - then they would stamp the deck accordingly. Or perhaps the deck was stamped first, by the bore guy, to indicate the differences between each cylinder in a completed block, then when the block went to the assembly area, the assembly guy would know which piston to mate with it from his pile of possibilities.  Hmmmm...............makes one wonder about the quality control going on, as well as the tolerances allowed during the machining process.
 
At any rate, that is all null and void now, for when the bore is oversized at a later date the stamp codes are rendered meaningless.  Plus, I/we don't have the luxury of picking through a pile of pistons during assembly in order to find one whose diameter most closely fits our bores.  So then what does one do?  The block comes back from the machine shop and it is what it is - and the same for the pistons we've ordered - there is no selection process - what would be the procedure then if it is found that the fitment is out of tolerances - take the block back for two more thousandths removal?  Go to Shucks and buy a cylinder hone for a final reaming? 
 
This all makes me ponder once again how much I need to trust whatever machine shop I choose, and to monitor their work closely.  They might not think that a couple of thousandths here or there makes any difference, but if the pistons I've ordered don't fit right afterwards...........I guess it would be off to Shucks I go, eh?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bkreutz Quote  Post ReplyReply Direct Link To This Post Posted: 23 June 2009 at 10:41pm
The normal way during a rebuild is to send the pistons with the block to the machine shop so they can match the hole they bore to the piston on hand. I've always sent the pistons along with the block so they can get the tolerances right. If the machine shop says they don't need the piston, find another shop.LOL
Gale

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Post Options Post Options   Thanks (0) Thanks(0)   Quote flh canuck Quote  Post ReplyReply Direct Link To This Post Posted: 24 June 2009 at 4:41am
I think (or at least I hope) that most modern machine shops have equipment capable of machining to much closer tolerances that what was possible during the mass production of the 40's
Maybe I will just patch a rust hole or two, shouldn't cost much to fix up.....
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