Piston Heights, worth cutting the block ?

Highest piston: .89mm - .0350"
Lowest piston: .81mm - .0320"
Difference: .08mm - .0030"

If I cut .11mm - .004" from the block, I can reach about .92mm for the lowest piston and yet keep the highest piston below the 1 hole head gasket range of .91mm - 1.00mm.

The question is, is it really worth the compression increase to cut a lousy .11mm - .004" to get the lowest pistons (2) within spec ?

For the curious, I have no idea why there are different piston heights. The rods were measured before installation, and the ceramic coated pistons are new.

I can think of 3 possible sources for differing piston heights, not in any particular order:

1) bent crankshaft
2) differing bearing thicknesses
3) uneven piston heights.

As I don't know the background of the engine, nor of the parts installed, and have insufficient experience to determine what is most likely in any case, these are just the possible areas of investigation.

If the con rod bearings are new and of OEM quality, then I would figure they are not likely.

You said the rods were measured.

The crank would have to be measured to rule it out.

That leaves the pistons. Were they measured before installations? Where did the coating come from?

Good luck,

PH

The local machine shop bored the block for new Nural .5os pistons, applied the ceramic coating to the top of the pistons, measured the imported maxpeedingrods (CR-VW-144C iirc) for twist, length and weight. Now that I think about it, they also ground the crank for undersize bearings too.

is .08mm even a significant number to worry about?

I build high performance, 4 stroke race engines for dirtbikes and I thought we had tight tolerances, but .08mm is tiny.

weve got ranges of .15mm- .18mm being acceptable, and even that is as thin as a zig zag rolling paper.

what are you building this engine for?? built bottom end, ceramic pistons, etc... "big" power?

It's called decking....to have the block cut is to have it decked...
Is the block flat and square?

According to my precision Starrett straight edge, the block is straight length wise and cross wise to at least a thou which is the thinnest feeler gage I have. I wonder how thick the piston top ceramic coating is, and if it's possible to be thick on one end and thinner on the other to account for a .002" difference or so ? As for the question of how significant .08mm difference is, I don't know that answer. Since the high and low piston heights and a block deck cut of .11mm (.004") will allow a one hole gasket, I don't know if it's worth that procedure or not. How much compression will be lost having two slightly lower (.003") pistons, and how out of balance (vibration) will there be ? Am I being too anal trying to get a square motor ? Why am I building this motor ? Mainly just because... and it will be put into my 93 eurovan once I'm happy with it.

Engines are pretty tolerant.....much more so than the "book" in my experience...
Had a Honda....the dealer messed up the cam, and as a result 2 pistons were damaged with about 70,000 miles....Honda changed the cam, and installed 2 oversized Pistons....engine ran strong a smooth even though that was a huge no-no by most accounts...was still going strong at about 300,000 when sold...
Would a perfectly flat, aka decked block be better, yes for the headgasket...usually it done if not perfect, just like the head, but lots of folks skip it on a head job...doing only the head to keep the block in the car...most are fine...

My honest guess....a few parts at the max limits, and a few parts at the minimum limits just happened to lined-up...maybe the coating is thicker...
I can tell you a modern 747 was said to vary in length even if all parts were perfect due to variations in parts, even if all were 100% legal..... difference was mathematically as much as foot, but reality is most were +/- a few inches in length....

ZippyNH said:

My honest guess....a few parts at the max limits, and a few parts at the minimum limits just happened to wind-up...maybe the coating is thicker...
I can tell you a modern 747 was said to vary in length even if all parts were perfect due to variations in parts, even if all were 100% legal..... difference was mathematically as much as foot, but reality is most were +/- a few inches in length....

Click to expand...

Correct. What you alluding to is called tolerance stack. Parts are made to a tolerance window and if all of your parts are say at the high end or the low end of the window the finished assembly can very dimensionally from one assembly to the next. Especially something as big as a large aircraft.

Was the crank checked for run out (bend)? If everything measures correctly I would guess the coating or perhaps the thickness of the bearings are the culprit.

Since you have the block out I think decking and line boring the crank journals if needed would be a good idea since I suspect you are going after serious power.

Probably the best reason for decking is reliability not HP. What it does is guaranties that the deck is perfectly flat, say within .0005" and that the deck is parallel to the crank journal bores.

Can't see a bent crank. Would've shown up when it was reground. Maybe he ground some offset into some of the journals? Could measure the stroke of the two that are farthest apart if you have the tooling.

Are the two highest at one end indicating the crank isn't parallel to the deck?

You are below minimum spec. The loss of compression ratio (CR) is rather dramatic. From the minimum piston projection, to the tallest, in a single head gasket can be a CR drop from 19.5 to 19.1. 1cc addition in the combustion bowl will drop the CR to 18.75. This is really close stuff.

Usually, what we find is the machine shop does not cut the head using the crank journals, which would be correct. They go off the flat of the bottom of the block. The most common fault we find when a block is decked like that is the #1 piston will stick out further. We like the measurements to be even and at least the minimum. Otherwise, the compression ratio (cr) is what you will have to deal with.

Fouillard13, There are three head gasket sizes for the ALH and AHU, with .91-1.2mm between them...the total range for three head gaskets is not more than .012". A smooth running engine is balancing the rotating assembly and accurate piston projection. Getting the squish right on a diesel that run 19.5:1 cr is not the same as running a single-cylinder gasoline engine.. not even close. Piston fit, compression ratio, fuel economy (who cares on the go-fast bike). But if you drop a three-hole head gasket, which we have seen done, into an engine that needs a 1-hole head gasket, you have effectively created almost the same CR drop as adding 1cc to the combustion chamber, but it's all in the wrong place. The fuel/ air mix that is in the area between the head and the rest of the piston is uncombusted fuel and poor economy.

I think you better go measure your zigzags again. Heh, that is not the usual 'measuring stick'. Maybe it's changed since my early hippie youth days, but I doubt it.

The total thickness of most of the ceramic coatings is .002". I doubt that is a major issue, as the method of application is virtually 'color cover coat'. You don't make it dripping wet.

The rods... good luck with that. The same thing is available from many ebay sources, local, quicker, cheaper. Better? don't know about that.

Why are we picky about rods? We like the rod we are running to have been assembled and honed with the same bolts you end up running in the engine. We see that as a big fault for most of 'those' rods.

If you are going to worry about piston projection being .003" off, you are really going to freak with tolerances for the rod. We have seen it all and ended up with some of those rods from what was supposed to be a quality vendor and yeah, you could say I should have checked closer, but I got complacent and blew up an complete big build in 5 miles. Not one of my better days...

Now, we check center-to-center and big end diameters real close. You really didn't say they checked the big end. That is something real important. Our rod big end tolerances are 2 tenths. Yeah, that makes a human hair look FAT... What have you got for a I.D. measurement? Taper? Out of round? Did they chamfer the inside corners of the rod cap mating surfaces?

I think you might take a look at your rods.

Sounds like Frank knows a bit more about tdi's than just head rebuilds!!!!!!!!!!

Franko6 said:

Now, we check center-to-center and big end diameters real close. You really didn't say they checked the big end. That is something real important. Our rod big end tolerances are 2 tenths. Yeah, that makes a human hair look FAT... What have you got for a I.D. measurement? Taper? Out of round? Did they chamfer the inside corners of the rod cap mating surfaces?

Click to expand...

Pretty sure you meant .0002 inch (2 ten thousands) not .2 inch. (2 tenths). not really a mistake, a machinist would know what you meant, but some folks might get confused.

Really great dissertation.................

I had a similar issue when I built my engine - you can read more about it in my "Where to Stop?" thread.

In summary, I started out with a measurement issue where I couldn't get repeatable measurements. After many hours of effectively doing a gauge R&R, I was able to get my measurement repeatibility down to ~0.0005" for the piston protrusion. This is critical and much more difficult than I anticipated. I'm no metrologist, but using precision measurement tools is not new to me either. Controlling all the variables is key (measurement location on top of the piston, direction you approach TDC, etc)

Second, I had a friend take the block to the machine shop to get the block decked and I think this induced a bit of a taper from one end to another. He snapped a picture where the machine cleaned up one end of the deck but not the other. They set it up on the oil pan rails like Frank mentioned above and used a machinist's level on the deck to insure it was square to the machine! I was completely dumbfounded by this practice. I was not present at the time so was at the mercy of others, but I was the one who ended up dealing with putting it all back together.

Number each rod and piston as well as the clocking of each. There are several assembly possibilities where you can mix/match the rod, piston, bore, direction combination to help "balance" out the height variables and get things even across all the cylinders.

Just for reference, on an ALH, every 0.001" of piston protrusion is roughly 0.1 points of compression ratio.

For what it's worth, I'd run as tight of a squeeze as I could to minimize squish volume at the top of the piston and maximize efficiency and power. If you end up a little tight (few thousandths) between the piston and head is better than a little loose in my opinion. Don't forget about valve protrusion from the head and piston pocket depth if you're planning on running an aftermarket cam with more lift/duration or not installing it "straight up" The OEM ALH cam is relatively forgiving.

Minigman, we are nearly overwhelmed with engine builds. I have 5 engine stands loaded. Could use another.

DannyBoy, I have been to school every day for the last ** years. It makes me sound old to say how long. Don't feel that way... But today is another day I learned something new in my trade and I try to do that every day. To share is the purpose of the Club.

Ol' Rattler... bad habits die hard. When I say tenths, I always mean ten thousandths(of an inch) I got a funny look when I actually said 15 ten thousandths to another machinist. He said,"YOU MEAN ONE PONT FIVE TENTHS?" I guess it's who you are talking to. Other then that, O'R, thanks.

FUB, we have a Comec surfacing machine in the shop. It is such a lovely machine. We have found that getting things flat is a big PITA, but worth it. Our irons on the Comec are flat and there isn't really any way to adjust it. Getting and keeping it perfectly flat is not going to happen, so we shim. We tell people to explain to their machinist what the problem is. I think I explain it fairly well... From what feedback I get, the machinists usually say something like,"You want me to do WHAT?!?" It's complicated, difficult and they don't want to do it.

I know I raise hackles on several people for my anal approach, but that is what this engine is all about. We have spent a lot of time getting things as right and reliable as we can. I get a lot of flak from 'experts'. Just as an example, porting and polish. When you are using ceramics in the port and head, finish really doesn't matter, because they are going to silicon blast the thing. The gains from containing the heat exceeds any advantage of the port finish, anyway.

Also, I got whacked for 'port match', but you can't see the reversion step in the pics and nobody ever asked, so I let them think whatever they want. But somehow the magic keeps happening. Basically, I love this stuff.

If you want to run your own CR Calculator, here is a real simple one from Summit:
https://www.summitracing.com/expertadviceandnews/calcsandtools/compression-calculator

It will blow you away what happens with a few thou.
Is that ok saying thou, Rattler? Heh...

Franko6 said:

It will blow you away what happens with a few thou.
Is that ok saying thou, Rattler? Heh...

Click to expand...

Well I guess it depends on if your audience's understands the tolerance window you are working with.
"Ah sir, we set all the clearances in you engine to the dimensions you specified and were at a loss as to why your engine has no oil pressure and the piston slap sounds like a Mariachi band."

Sure, as far as I'm concerned, no problem.

Dannyboy said:

Amen man. Every day is a school day in the shop, just when you though you say it all.ive never seen a engine run backwards until an old detroit came in. Turned engine off and started it again and it ran correct way

Click to expand...

Must have been an old 2 stroke

Reminds me of an old story from when I was young and raced motocross.

I had to be really careful when at the starting line on my old Husky 450 - it would occasionally start backwards (you had to start the bike at the line) and I learned really quickly after handlebars in the chest to feather the clutch on the starting line before the gates dropped to make sure it was going to pull forward

Dannyboy said:

Yep was an old 2 stroke, it wasnt difficult to do. Just turn it back on as engine was just before engine stopped turning. Smoked a bit

Click to expand...

Ha ha....I had this happen a couple times on an old 4-53T detroit in a truck.... a pile of smoke from the air filter, an un-godly clatter and no oil pressure thought I'd blown her up and had her towed home...next morning I tried starting it to see what it sounded like and it ran perfectly normaly.

wait. what? engine running backwards??? how does that even happen?

learn something new every day is right!

Had the same issue with my last ALH rebuild, machine shop cut the deck on an angle and caused staggered piston heights. Ended up with a 1 hole gasket which put it just under spec on the lowest piston. Engine runs fine but I was pretty pissed about that. I also had several fights on piston to bore clearance. Machine shop wanted a loose fit and VW specs otherwise.

fouillard13 said:

wait. what? engine running backwards??? how does that even happen?

learn something new every day is right!

Click to expand...

Certain types of two strokes. Never had an old Cox .049 powered model airplane?

I thought the 2 stroke Detroits had an exhaust valve and wouldn't run backwards. I'll have to look closer at that.

Apparently some 2 strokes don't really care about rotation direction. So I guess if you have say 16 speeds in reverse and one speed going forward with no oil pressure it's probably best to turn off the engine and restart it.

I think an exhaust valve or or intake ports could really care less about engine direction. Just that the pressure needed to operate is there at the right time.

To funny, though. I never knew either.

Back in my early days I had a Cummins 555, Detroit 8v92 and 6v92 all run backwards for a short time. Shut them off, restart, motor on with no problems.

Ol'Rattler said:

I think an exhaust valve or or intake ports could really care less about engine direction. Just that the pressure needed to operate is there at the right time.

To funny, though. I never knew either.

Click to expand...

It wouldn't care but if you have no fuel coming in thru the exhaust side then it won't run, unless I am missing something,

jptbay said:

Back in my early days I had a Cummins 555, Detroit 8v92 and 6v92 all run backwards for a short time. Shut them off, restart, motor on with no problems.

Click to expand...

Just about any driver at a truck driving school has been instructed in to be careful for backwards running motors....had an instructor tell me if it was a mechanical as opposed to an ECU run motor, it was a risk....these days the non electronic motors are getting rare...I've only driven 2....ecu controls just give engine designers so much more control over economy, emissions, and even drivability that they are a dying breed, largely due to the fact most electronic diesel motors got better mpg or more HP....and at 100,000+miles a year in many uses, it adds up fast!

That is a head scratcher. The start of injection timing would be completely wrong and the injector pump would be turning backwards tending to pump fuel to the tank instead of the injectors.

Ol'Rattler said:

That is a head scratcher. The start of injection timing would be completely wrong and the injector pump would be turning backwards tending to pump fuel to the tank instead of the injectors.

Click to expand...

Forgot about mechanical injection pumps and IDI injection, durr They also work backwards. The DB2 spins say clockwise on a GM and spins anti-clockwise on a Ford with no internal mods.

turbovan+tdi said:

It wouldn't care but if you have no fuel coming in thru the exhaust side then it won't run, unless I am missing something,

Click to expand...

2 cycles are often turbo scavenged, and the intakes are ports in the cylinder side, so on downstroke, the exhaust opens briefly to vent gases from the top, the side ports open and the turbo drives in air (as well as the rest of the exhaust out), piston comes up to compress, fuel injects, and *bang*. Timing backwards will be a train-wreck, which is why when this happens, things run with low power and like crap, but there is enough overlap to get it done. Just because it runs backwards does not imply that airflow reverses . . . . and as long as it has a turbo and not a belt driven super, this can happen. A supercharger, to steal a classic line from Spaceballs, will "go from suck to blow" and this is very unlikely to happen in that case . . .