A Boring Question

[Rabbit79]

I'll start out by saying that this is my first foray into re-building one of these VW engines. I've been a Ford V-8 guy up to this point in life so some of this German engineering is, literally, a little foreign to me  haha. Anyway here's what I've got....79 Rabbit, I was told by the guy I bought it from that the guy HE bought it from put a motor out of an 84 into it. So I have the 1.6 NA in there. Long story short the key-slot on the crank for the timing belt pulley wallowed out and I slipped time so now I'm in the midst of an engine re-build.
Now here's my question.....In reading through my trusty Bentley manual I see the part about the honing group code which is supposed to be stamped on the block. As best as I can make out from the picture it should be on the left front?, but I can't seem to find it anywhere on there. Now according to this there are 3 different dimensions that the cylinders were bored to originally. If I decide to bore it won't the machine shop need to know the original bore diameter as a baseline to start from? The rings and pistons I've looked at all say .020 and .040 over. Are these oversize based on original dimensions 1, 2 and 3, or is there just a fixed dimension that they want you to bore to for the oversize rings and pistons? I kind of have a hunch that if I buy a set of oversize pistons they'll just say "bore to dimension X", but, I've never had to get a block bored before, so this is new territory to me. Any help would be aprreciated. Thanks.

[Rabbit79]

That helps, but perhaps I should expand a bit. The Bentley manual lists 3 possible sizes for the original cylinder bore:
1) 3.0122
2) 3.0126
3) 3.0130
These are listed in section 9 of the engine chapter where they talk about the honing group codes. Now at the end of the engine chapter it just shows a bore diameter for all engines of 3.012. So if I were to order a set of oversize pistons that are, for example, .020 over, would I have to overbore from the basic 3.012 diameter, to 3.032, or do I have to take into account the more precise ten thousandth inch measurements. From what you say about a machinist balking at a .001 tolerance I would guess not, but I just want to make sure. 

[Rabbit79]

Well that sure makes it easy. Guess I should have thought of that before. Thanks for the assistance and for putting up with my dumb questions.

[ffgb]

My .020 oversized Mahle pistons had a size stamp on them, I think it was 76.98mm, under that was .03mm.  According to the Mahle, you add these two numbers together and that is what your finished/completed bore/hone equals to.  So the finished bore/hone size is 77.01mm.  My machinist said he bores the cylinders under the final size, then finish hones the cylinder to the final bore size.  I got lucky, because he said that the finished bore/finished just made it.  If the finished bore/hone didn't clean everything up, I would have had to buy the next oversized piston above .020.  My advise is to measure your worn out bore yourself or have your machinist measure them and then buy the recommended pistons, just in case your cylinders are too worn out and out of spec for the pistons you just purchased.  FYI, my cylinders were really worn out, I almost had to purchase 3rd oversized, and this is the first ever rebuild!

[sdwarf36]

I will NEVER EVER bore a cylinder until I have the pistons in hand. You can never trust that they are all the same/as marked/the last set in the country and UPS dropped them-etc. 
 SOP is to bore to .0025 under finished size + hone the rest. SOme shops (where I work included) have very good Sunnen honing equpiment thats just as quick + accurate to take it all out with a hone.
 If a customer insisted I bore his block to .001, I would set up the bore gauge to show him when he picked up the block AND put something on the work order that it was their idea-and witnessed the reading. Because what story do you hear? "Hey Joe-why did your motor blow up?" A: "I underestimated stacked tolerances" or B: "the machinist must have bored the block to tight."

 Yea-I've never heard A either.....

[ffgb]

So, with the Mahle pistons that I am putting in, do some individuals ask to have the finished bore .001mm or .001in. bigger than the piston size?  If it is .001mm, that is too tight!  If it is .001in, then that is .03mm.  In the Bentley, it is published that .07mm is the wear limit.  Wow, .04mm, or .0015in., isn't very much of a difference.  Is that why some individuals ask to have the finish bore smaller than what is recommended by the piston manufacture?

[sdwarf36]

Lib-one thing I think you are overthinking in the whole deal with clearance specs-with .001 being spec-and .003 being the wear limit, the main thing is -according to book- is once your cylinders have worn .002, its time to bore the thing-not as much as what your starting number is. If it starts at .002 and was now at .004 its the same thing-It not "OMG-.003! Its a door stop" By the time you've worn the cylinder a couple of thou, your ring end gap is goona be pretty big from wear anyways. (plus for every .001 of bore increase= .003 of more end gap.) That is what the engineers determined your motor is worn to a point needing service. A round ,straight cylinder is the importiant thing. If you keep everything sane, then you are doing what the engineers that came up with the spec had planned for-If you ever boost it before its warmed up-or get a hi egt-or overheat it that is what worries me.  I COULD be worring about nothing-Vw's cylinder walls are pretty thick-and you could be worrying about nothing because the pistons are full skirt and also thick-i mean what the worse it can do-have piston slap?  Like we could here that!

[theman53]

Yeah me too.
He said that he wanted me to come by when it was running. I asked why and he said I have never done a VW diesel, but I have been boring all sorts of diesels for 15 years and I haven't ever seen a .0011" spec. He then said that is what I finished it at and just think that there has to be more room for the piston to expand. Then I explained these pistons are 350 a set of only 4 and the material in them is probably good to go in handling it.

We shall soon see.

Machinists are like sound men *stereotype*. They are a completely different deal usually. Both equally picky just in different areas.

[Rabbit79]

OK from what I'm reading so far I'll be best advised to get my pistons first and take them in when I get it bored. On a sidenote that's no small thing cause the nearest machine shop that can handle this sort of thing is 130 miles away from my little town. My biggest mic reading on the cylinders is 3.022", (sorry metric system.....I just can't do you haha), about .010" over original. So according to that I'm safe to order .020" over pistons? My inside mic is a little hard to hold in the cylinders when measuring in the middle, but the top and bottom measurements are pretty accurate I think. One thing I did notice is the measurements get better from top to bottom on all cylinders (example: cyl #1 3.020 top, 3.0185 middle, 3.0175 bottom), is that normal?
Now on to next question. What are the best pistons and rings to get? Every place you look is screaming "our stuff is the best", which is to be expected of course, but that does make it kinda hard for a newbie to decide. I've read a few things that say to go with turbo diesel pistons because they're made from better material and will handle the heat better, and will work fine in a non-turbo motor. I know opinions will vary on this but I'd like to get a few baseline ideas to go on from guys who have more experience at this than I do. Also I want this motor to last and don't want to fill it up with a bunch of cheap Chinese junk. Sorry China if I'm being politically incorrect.......but I just don't trust your stuff.

[sdwarf36]

Yeah me too.
He said that he wanted me to come by when it was running. I asked why and he said I have never done a VW diesel, but I have been boring all sorts of diesels for 15 years and I haven't ever seen a .0011" spec. He then said that is what I finished it at and just think that there has to be more room for the piston to expand. Then I explained these pistons are 350 a set of only 4 and the material in them is probably good to go in handling it.

We shall soon see.

Machinists are like sound men *stereotype*. They are a completely different deal usually. Both equally picky just in different areas.

I understand-its all about comfort zone. Talk about swinging the other way-today I was building a 588 cu in. Chevy-that has NINE thousands clearance!  Same deal as too tight-I was questioning the boss-"are you SURE you want .009?" But that was with forged pistons and a 4.6" bore.

[sdwarf36]

OK from what I'm reading so far I'll be best advised to get my pistons first and take them in when I get it bored. On a sidenote that's no small thing cause the nearest machine shop that can handle this sort of thing is 130 miles away from my little town. My biggest mic reading on the cylinders is 3.022", (sorry metric system.....I just can't do you haha), about .010" over original. So according to that I'm safe to order .020" over pistons? My inside mic is a little hard to hold in the cylinders when measuring in the middle, but the top and bottom measurements are pretty accurate I think. One thing I did notice is the measurements get better from top to bottom on all cylinders (example: cyl #1 3.020 top, 3.0185 middle, 3.0175 bottom), is that normal?
Now on to next question. What are the best pistons and rings to get? Every place you look is screaming "our stuff is the best", which is to be expected of course, but that does make it kinda hard for a newbie to decide. I've read a few things that say to go with turbo diesel pistons because they're made from better material and will handle the heat better, and will work fine in a non-turbo motor. I know opinions will vary on this but I'd like to get a few baseline ideas to go on from guys who have more experience at this than I do. Also I want this motor to last and don't want to fill it up with a bunch of cheap Chinese junk. Sorry China if I'm being politically incorrect.......but I just don't trust your stuff.

 Yes-have pistons in hand.
 Wear at the top of the cylinder is normal-on what would be the front of the cylinder (while in car) .010 is alot-but my motor had .008 wear. .O2O pistons should be fine.
 Mahle makes the best piston-they are the largest piston maker in the world-and the original supplier to VW. Costly though. Ask your machinist where he can get parts. I was able to get a set of Topline pistons reasonable-no time on them yet to give any long term results-but they looked + measured up good.
 As far as I know, there is only one piston availible now for both applications (combined part #'s I'm sure)-the difference is the cut out for the oil squirter.

[Rabbit79]

Actually I measured both perpendicular and in line with the crank. I just averaged out the numbers for the purpose of that post.