PISTON questions....
I've begun the great quest of rebuilding my D2 engine. It is all down to the nuts and bolts. I've spent today examining the pistons for wear and defects and measuring with an outside mic for spec. My pistons are stamped with a "B" on the top. According to the RAVE, the specs for Grade B are 93.986 - 94.001 mm (3.7002 - 3.7008 in). WELL, I keep getting 3.7001. I've measure 10 times and I keep getting the same. This is a brand new mic, so I don't think it is out of calibration.... Is it possible these are grade A and mis stamped? It would seem odd that the pistons would wear exactly the same... NOTE... this is my first time doing the bottom end and to use a mic...

Another question,

If the bore is standard, 94 mm (3.7 in), and the grade A and B pistons have a range of 93.970 - 93.985 mm (3.6996 - 3.7002 in) to 93.986 - 94.001 mm (3.7002 - 3.7008 in) go in the same bore, does it really matter if it is 3.7001? The cylinders were recently replaced with top hat lines and they are to spec.

 

4 decimal places is such a small amount.
It's hard to get a consistent measurement that small, manually, even with a calibrated micrometer.
I can say that with some confidence too, as I am a QC lab tech, at a production machine shop, last 5 years.
I use calipers, micrometers, and some high tech measuring machines too.
I work with, and fill in for the guy that calibrates, or farms out, all the plant's 3k+ issued gages.
I say all this because I handle parts and calibrated gages 3 and sometimes 4 decimal places, daily, inspecting parts by the print.
So I get to see ALOT of mics, as well as multiple specs on automotive valves.
I can tell you confidentally that there is nothing to be concerned about. 4 decimal places is very difficult to measure accurately, consistantly, by hand.
You will get some slight varience across different micrometers at 4 decimal places even with calibration(a tolerance range).
I say this comparing it to having watched computers measure things more accurately, again, with that much variance, or more.
No way did Land Rover ever make a block and set of pistons that accurate. Including a perfect diameter.
Aluminum(piston)is softer than steel, but abrasive.
The pistons can wear 1-2 ten thousands Off the steel, through friction, like sand paper.
I have seen that happen with a basic steel outside diameter gage(A metal circle with a 17mm hole in it, roughly an inch thick, chamffered entry/exit...simple), for checking the diameter of Aluminum rods.
Within several dozen slides down 8' rods, the precision EDM machine cut steel hole...showed 1 ten thousandth of wear.
Imagine what happens at high heat, thermal expansion(why correct ring gap is important), different metals against each other.
Keep in mind that the 4.0/4.6 is a low revving, small block engine, that's going to redline a little past 5k.
It's not an Indy car engine.
Sorry to ramble on, I just wanted to add some first person witnessed facts to go with my opinion...which is;
All things considered, measure your rod and main bearings, the ring gap, make sure you don't have any grooves in cylinder walls that you can hang a fingernail on, that cylinders still have a cross hatch pattern from original hone, not a glazing(in general, I know you have new liners).
Even the LR RAVE service manual says Not to hone the cylinders, yet many people have without issues, for years, and honing takes away more material than 1-3 ten thousandths, and even though I'm talking about the cylinder vs the pistons, the gap between the them has enlarged more than LR's tolerance allows, again, without issue.
Honing Needs to be done on a machine with a stone hone for precision results too. By hand, with a drill, you'll never keep the speed or angle of the stroke consistent, and are putting waves in the cylinders.
A ball hone can take away alot of material too, depending on it's use, and diameter in relation to the cylinder's...yet LR engines have been reported here in the past to run fine after hand honed cylinders, which is no suprise.
You should make sure that cylinders and pistons are checked at 3 decimal places, in 3 places around the diameters.
That you consider the alternative, the cost of new pistons, that I'll bet vary in spec by that micrometer as much as the pistons you have now.
And if, IF, not, that the new pistons actually held that tight of a tolerance, that once you ran the engine, if you disassembled it, and measured the pistons or cylinders, that they individually, or combined, Would show wear in the 1 to 3 ten thousandths range.
Just my 2 cents...for whatever that's worth.

 

Thanks so much for the input! I figured, like most things, that there is some minor to very minor lee-way with the measurements. But it's good to hear from someone with some expertise. Being the first bottom end I've done, I wanted to start out as precise as possible. Thanks for letting me know there is a somewhat larger acceptable variance. Really appreciate the comment!

 

Yes sir.
I'm about the same stage of the build as you.
I have the crank and cam in and timed.
I need to gap all my rings next, then I can put the pistons and rods in.
I just need to have time when the mood strikes, maybe tomorrow, lol.
Good luck with it.

 

Likewise. Good luck! Hope to pick your brain another time. Thanks again

 

Oh, and if you don't have a ring filer, you'll probably need one.
Odds are all the new rings won't all be in spec, you usually have to file a few, but, you never know.

 

Reading my mind. I was just looking at that.

 

I reread my original post, and just to clarify, in case my wording was confusing.
Yes, the same person, with the same calibrated quality micrometer, should be able to accurately and consistantly measure the same part, with little variation at 4 decimal places.
It's what LR measured their's with, you measured yours with, I measure mine with, and everyone else, that'll you'll never accurately hold 4 decimal places.
So that Cannot be and is not a life or death tolerance.
The variance among micrometers, and an individual's physical hold and pressure on the micrometer, is too great among that many mics, parts, and people...more than the manual's 3 ten thousandths range.

 

Gotcha. That seems to check out with my measurements. I think a good test will be to get the pistons in and check clearance at that point. Side note (amateur hour) I had originally matched all piston components, only for the machine shop that did my liners to mix them up. But I hardly see how it matters if all the bores are the same and the bearings are replaced. Thoughts?

 

I would definately keep each piston on it's original rod, and each rod in it's original location.
The piston won't matter, as the liners are all new.
The rods that wear against each other in pairs are what is preferable to keep together.
Rod caps should always be stamped with a number before disassembly.
But, if they're mixed up, they're mixed up, measure all the bearings, and clearance between rods and journals, and do what you can.
It's not ideal, but as long as it's in spec, it's not catostrophic.
Just that parts wear to fit each other.