Nüral piston to cylinder clearance - clear up my confusion

[twentyeight]

My block is at another machine shop with the pistons ready to be bored .5mm over. They'll be measuring each piston individually and boring however much over I tell them. I have read a few times that the cylinder to piston clearance should be 0.0012" to 0.0015. I told the shop this and they said it would end up between 0.001 and 0.0015, which made me nervous it could possibly run too tight.

So after much searching I found a document on a Czech server detailing Nüral's pistons. The relevant page is 1248 (which is 1323 of the PDF). The pistons I have are etched 87-114907-90, which seem to be listed as 0.070mm clearance. That's .0028" which is significantly larger than the other numbers I've read.

Another thread talks about .5mm over pistons from IDParts, which show in the picture as being 87-114907-60. The -60 is the "version" according to Nüral. The document above lists them as requiring 0.05mm or 0.00196" clearance—*still not the 0.0012 talked about in the thread. Interestingly, this piston also 0.02mm larger than the -90 that I have.

I'd like to call the shop tomorrow to correct the specs - I'd really appreciate an explanation of typical TDI piston to cylinder wall clearance and what to tell the shop.

 

[Digital Corpus]

Stick to one unit system as much as possible or you'll confuse yourself more.

Have you finished balancing and matching pistons, rods, and wrist pins? If not, do this first before letting the machine shop have a go at the block.

The 80 mm Nural pistons I purchased were 79.96 mm. If this were a largely significant factor, the difference between 0.00196" & 0.0015", aka 0.00046" aka 46 millionths of an inch, would be brought up more regularly by forum members. Heck even my balancing of rotating and reciprocating parts to less than 1 gram out of 1.4 kilos was viewed as anal retentive.

Overbore to 0.5mm. Install pistons. Drive.

 

[twentyeight]

Thanks. I guess my problem isn't with 46 millionths but more between random-TDIClub-post's 0.03mm, Bentley's .04mm, and random-czech-sourced-but-legit-looking-Nüral-document's .07mm. It's 230% larger.

I also want to be specific in what I request because apparently I needed to tell the last guy not to deck my block on an ancient rotary broach and ruin it (turns out he had newer equipment but just didn't use it). Brilliant.

 

[mk3pd]

No experience with Nurals,but on Mahle and KS the requested bore size is laser etched on the piston crown,which always turns out to be 0,05-0,06mm clearance
Rings are also fit to requested bore,so they have always ended up with 0,30-0,35mm gap on top ring 0,40-0,45mm on second

0,03mm piston to wall sounds like too little on a piston with that much material in it

 

[twentyeight]

Yeah, now that I look they say 79.940 Sp 0.07 LNI. I assume the .07 is the clearance.

I guess I'll ask for 0.0025" and hope for the best. Thank you.

 

[Whitbread]

I have my machine shop aim for . 0015" with a torque plate on. Lots of happy motors running around and no oil burning.

 

[twentyeight]

If there's a torque plate in Phoenix I sure haven't found it.

Do you think .0025 is too loose?

 

[Whitbread]

If there's a torque plate in Phoenix I sure haven't found it.

Do you think .0025 is too loose?

I made my own. I think .0025" is way too lose and you'll burn oil.

 

[twentyeight]

Ok, are we talking

 cylinder_wall <- 0.0025" -> piston <- 0.0025" -> cylinder_wall?

or

 cylinder_wall <- 0.00125" -> piston <- 0.00125" -> cylinder_wall?

?

I've now found some people talking about 0.003"— 0.006"

Contacted Federal Mogul and they were no help at all. It took them a short while to even remember that they sold Nüral pistons.

So if VW (Bentley) says 0.0015", why does Nüral apparently say 0.0028?

 

[Whitbread]

I've built more motors than I can count and long, long ago I had a machine shop "do me the favor" of setting them a little loose. They felt .0015" was too tight and the motor would seize when run hard. So they bumped it to .0025"-.003" without telling me when I had 3 blocks to be machined at the same time. All 3 of those motors burnt 1qt per 2000 or less miles.

TRUST ME, tell your machinist to run .0015"-.0018" and your motor will run great and burn no oil. Use proper brad penn or joe gibbs break in oil, as soon as the motor hits operating temp and has no leaks, give it a good 75% flogging for a good 10 mins to seat the rings. Get the egt's over 1000F and the boost over 25. Heat and pressure seat rings; grandpa drive it and the rings won't seat.

 

[john.jackson9213]

First, I would listen to Whitbread.

Second, I have taken my car half way across the country to have Matt work on it, really!

Next, the B4 Bentley manual, vol. 1 says on page D 13-46: Piston and cylinder dimensions Engine code: 1Z
Honing dimension Piston Cylinder bore
2nd oversize mm 79.97 80.01

I have a set of .5 oversize Nueral pistons in my garage. Like DC's, they are marked 79.96 mm. My block will be finished honed to 80.00mm That is .04 mm clearance according to the Bentley. Converted to inches - I get: 0.001574803 inches.

Now, go back an look at what Whitbread told you to use as clearance. Notice he is right there at .0015-.0018 inches?

 

[Vince Waldon]

TRUST ME, tell your machinist to run .0015"-.0018" and your motor will run great and burn no oil.

Just my opinion, but 2X.

Have 'em aim for .0015... it will probably seem "tight" to them and they may balk... but many's a block been bored out to over half the wear limit right from the get-go by applying conventional gasser clearances to this engine design.

And if they really balk... or can't guarantee 0.0015" to 0.0018"... find a shop that can.

 

[turbocharged798]

I argued with my machine shop when they wanted to go way large on the bores. They told me bigger is better. IIRC I think they set mine at .005" and I wasn't happy about it but let them do it.

Engine doesn't burn oil but I do what sounds like piston slap when cold.

 

[Vince Waldon]

I argued with my machine shop when they wanted to go way large on the bores. They told me bigger is better. IIRC I think they set mine at .005"

Don't have my Bentley with me at the moment, but that's exactly what I meant by a machine shop boring out half the wear limit before you even get started.

 

[twentyeight]

Found it in another thread from a rather helpful user, but not strangely in my Bentley.

The wear limit for cylinder bores is 0.0039"

So I'm fairly assured that I want to shoot for 0.0015 to .0018, but I'm not less confused as to why Nüral (and others) seem to want .003" and even looser. Are they talking about crazy hot-running compound turbo applications, or is it something about how the clearance is measured? Bentley flatly states Piston Ø and Cylinder Bore Ø and the difference is always 0.04mm or 0.0015". I don't get where the other numbers are coming from.

 

[john.jackson9213]

Found it in another thread from a rather helpful user, but not strangely in my Bentley.

So I'm fairly assured that I want to shoot for 0.0015 to .0018, but I'm not less confused as to why Nüral (and others) seem to want .003" and even looser. Are they talking about crazy hot-running compound turbo applications, or is it something about how the clearance is measured? Bentley flatly states Piston Ø and Cylinder Bore Ø and the difference is always 0.04mm or 0.0015". I don't get where the other numbers are coming from.[/QUOTE

So have you found a shop that can finish hone your block to the correct dimension?

 

[twentyeight]

Yeah, the shop I have now seems very competent and will do whatever I say, I just need to give them the final word. I'm going to heed Whitbread's advice, but I am trying to understand it before I notify the shop. It seems some discuss piston to wall clearance, others state diameter differences, and then there are complications with the taper of the piston crown to skirt, etc.

It just bothers me that a piston manufacturer seemingly has a spec (but doesn't include it in-box) and I'm second-guessing it.

If I don't hear back from a PM by tomorrow I'll just tell them to hone it .0015-.0018" over and hope for the best.