Arco AAZ Pistons Does the Coating change the piston to bore spec?

[RockOcean]

I am rebuilding another AAZ engine again 10 years later and this time went with my local machine shop. They are telling me that they don't know if they need to adjust the .001 piston to bore spec because of the coating on the piston. I never had this asked last time I rebuilt one of these, also very questionable is that this machine shop said that they can hone to the 0.50mm oversized piston to bore spec instead of boring. Something I have never heard of before. Feeling a little uneasy about all this, what do ya'll think? Currently they say they are within a half thousand of spec but are unsure of what to do because the piston has a coating on it.

[fatmobile]

Common for cylinders to be honed to size instead of bored.
 Since sometimes there is so little to remove to get to the next size that a cutter will just chatter over it.

I'm not sure about the coating and how it effects the bore size.

[oblique]

I have a set of OE late AAZ pistons here that are coated..it looks factory and doesnt seem thick. I dont know what it is but its solid...held up just fine after 3 rounds of cleaning with oven cleaner and wire brushing..it was a veg motor.

Regardless I would measure from that.

In terms of your Arco's...has the shop confimed that they are understroked? Typically OE oversize pistons were about 0.1mm shorter per size to allow for decking the block...your .5mm pistons are the 2nd size (I believe)so pistons should be about 0.2mm shorter...just making sure you havent forgotten.

[RockOcean]

Great to know that honing to oversized is common. I had never heard of it before so it seemed questionable.

Yes I took measurements before disassembly so they can deck the block to put the piston protrusion in the middle of a 2 Notch gasket. I made the mistake of not doing this 10 years ago and had to find a 1 Notch Gasket which wasnt hard then but now it looks like they are pretty hard to come by.

[oblique]

Its been discussed here but I think most shops that are used to doing gas motors dont realize how tight thse old diesels are...I asked my guy to keep the 0.0012" factory spec clearance when I went .5mm over and he said "thou and a half scares me" and would not go tighter than that. Its 1.6na, Ive yet to assemble it. Also Arco pistons...he was impressed at their quality..said they all measured exactly the same.

[RockOcean]

My machine shop has bored the engine to what the Bentley manual lists as the second oversized bore. I have the .50mm Arco Pistons but they still want to confirm this is correct before giving it back to me. This is what they wrote and want to know:

ARCO PKVW407954DA 0.50 pistons skirt clearance dimension.  Manual lists as 3.150 as finished bore size for .50mm pistons which ideally would be .0012 skirt clearance.  These pistons with coating measure aprox. .0007 skirt clearance or .0005 (half a thou) tight.  Original pistons with good clean skirts indicated aprox. .0032 original skirt clearance (not a coated piston).  So the question is does ARCO have a listed clearance spec. or do you finish to a specific size?, and what size is that.  Or has anyone had experience with a specific clearance with this piston and accurate measuring.

[oblique]

Very strange that the 1.9td Arcos have coated skirts and the 1.6td do not. The coating on top of the piston does not take abrasion like the skirts would so I dont know why its there on the sides. I would call parts place and ask them about it, i think they still actively build 1.6 and 1.9td motors using the same pistons.

https://www.partsplaceinc.com/vw-aaz-piston-set-020-20614.html

I will likely be ordering a set as well as the block that i thought had stock bores has been bored up to 79.90 for some reason 

1.6td arco:

[RockOcean]

Thanks that's a great tip I sent them an email.

[RockOcean]

Well Parts Place Inc did not want to help me since I did not purchase the pistons from them. As soon as I told them I did not purchase from them they just ghosted me.


But I did finally get an answer from Arco here is what they said:


"Regarding PTVW407954DA AAZ/ABL/ADE, after checking with our Technical Dept., the recommended piston to bore clearance is 0.03mm when measured at 15.0mm above the skirt.  If you still have any query, kindly contact me at any time.  Thank you."

[oblique]

Too bad no insights on the coating, would be interesting to know why they put it on the skirts.

If it were me I would size to the coated skirt dimension nust to be safe. Will be interesting to see how it wears over time.