cylinder bore tolerances?

Hello, I have a 1.9 ALH with about 90k on it and some serious engine damage and am trying to decide rather to hone and rebuild with stock rings or bore and get oversized pistons/rings. problem is, I cant find what the cylinder bore tolerances are.
basically, the engine was run on SVO and and gummed up the rings and eventually everything with sludge. not sure if it already had low compression, dont know the history of the car much, but I think so, which allowed excessive blow by to gum up the oil...
anyway, the engine now has groves in the #4 cylinder and even a little in #3. I ran a hone through the cylinders and they cleaned up nice but the grooves are still visible. maybe a couple thou deep. I think it will be ok to rebuild with a few shallow grooves but I want to make sure the bore is within spec. what is the outside tolerance on these bores?
Thanks!

Bore it, your are wasting your time if you don't. A hone just preps the surface.

that was my original thought. but after honing it and looking at the bores, I started to second guess. the grooves are so shallow you cant hardly catch a finger nail on them. maybe .001" - .002" deep... and only a few of them. I just dont want to got to the time and $$ to pull the engine and send it to a shop my drag racing buddy, who also drives a TDI, says to rebuild it as is. I just want some other opinions and the dang cylinder bore tolerances!

I've been tight on money before and honed and re-ringed an engine with less than perfect bores. Engine ran fine, a little oil use, a little extra blowby, a little piston slap.. And another 100k trouble free miles. A few light vertical scratches should be ok. Really best to measure with a proper bore gauge and get all technical, but sometimes you just need the car to run. I'd say go with what you've got. It can always be bored at some later date.

I did measure with bore gauge. thats why I'm looking for tol specs to compare my measurements too. all cylinders are between 3.1300 and 3.1320

If you still got grooves, bore it. Do it right or don't do it at all. You bought a cheap grease car that the owner messed up and sold to you dirt cheap as a non runner. Don't be a cheap skate. Cheap owners and TDI's make poor partners.

This probably doesn't need to be said, but I'm going to anyhow. Don't forget to treat this as a brand new engine and break it in accordingly.

Did you check the bores at the top, middle, and botom of the piston travel and also in various positions around the circumference of the bore (the the piston diameters too)? Later, when I am home and have access to my Bentley, I can give you the tolerances/specs for the cylinders and pistons for an ALH. And also any guidance from the manual on measuring them.

--Nate

PDJetta, that would be greatly appreciated! I did check top middle and bottom and at multiple diameter locations. It actually was running when I got it. wouldn't fire on #4 when cold, do to low compression from the stuck rings and grooves, but after a few min it would run smooth. I knew there were serious problems, after checking compression and listening to it run, and correctly guessed what damage had been done. wasn't surprised when I saw the scratched cylinders. car is totally worth rebuilding though.

OK, this is for the ALH. Here is the nominal piston sizes:

Stock Piston diameter is 79.47 mm, cylinder diameter is 79.51. 1st oversize piston diameter is 79.72 mm, cylinder diameter is 79.76 mm. 2nd piston oversize is 79.97 mm, cylinder size is 80.01 mm.

Note: Measure piston diameter about 6-10 mm from bottom of skirt and at right angle to piston pin.

Here is how to measure the bores in three positions (the three positions are 10 mm in from each end and in the middle):

• Measure bore at 3 locations in both directions - A - across engine and - B - in line with crankshaft. Deviation from nominal dimension: 0.10 mm maximum.

• Note: Do NOT measure the cylinder bores when the block is mounted on a repair stand using the holding fixture VW 540. Bending stresses resulting from the mass overhang will make incorrect measurements likely.

Note: the measured bore can not deviate more than 0.10 mm more or less than the nominal dimension given above for the cylinder sizes.

Here is the piston ring side clearances:

Compression ring #1 (top one) 0.06 to 0.09 mm (new), wear limit .25 mm.

Compression ring #2 0.05 to 0.08 mm (new), wear limit .25 mm.

Oil ring .03 to 0.06 mm (new), wear limit .15 mm.

Here is the piston ring end gap measurement (measure w/ ring 15 mm from top of cylinder, square in the bore):

Compression ring #1 (top one) 0.20 to 0.40 mm, wear limit 1.0 mm.

Compression ring #2 0.20 to 0.40 mm, wear limit 1.0 mm.

Oil ring .25 to 0.50 mm (new), wear limit 1.0 mm.

--Nate

i would bore it .5 over and be done with it. start fresh, rebuild it once nicely and never again... while you're at it, get the block shot-peened + baked and the deck skimmed. then paint it all new bearings, seals, nice fresh new bolts, the results are well worth it!

THanks Nate! thats exactly the info I wanted. the bore diameter measurements I took were at exactly those locations. seems to be standard for measuring bores. according to this data, my cylinders are well within nominal tol. per that data, the stock diameter is 3.1303 outer limit is 3.1342" and the largest I got was 3.1320". and odds are I had a bit of an angle on the bore gauge which would give a slightly larger than actual measurement... either way, I may just bore it to the next size up as I'm thinking I better tank it and have it cleaned up nice anyway... this data helps either way as I can give it to the machine shop if I bore it. and yes, it will get all new bearings and seal and cam followers and such.
Thanks!

Since this is the top result on Google regarding this topic, thought I'd post the service manual chart page on wear tolerances.

I'm running .004" piston-wall clearance, doesn't burn oil, blowby is managable.

I've been vexed by the problem when, at times, a single bore will cause the block to be considered for oversize pistons and rings. We have often honed a cylinder bore until the line you can catch a fingernail on is gone, only to see that .005-.010" is removed from the cylinder bore. At the larger measurement, there is enough metal removed to qualify for a 1st oversize piston. (.25mm).

That will cause excessive blowby and a build we have always decided was not worth doing, especially if you were going to suffer a quart of oil consumption in 1,000 miles.

We are now honing cylinder bores to a defined cylinder dimension that cleans up the bores for no taper or out-of-round, using piston ring sets that are varying sizes up to a .25mm oversize and having a graphite coating, somewhat similar to the graphite wear coating you will find on the stock pistons, applied to the entire skirt below the rings. This coating is applied to a thickness to make up the material removed from the cylinder bore, and can be effectively applied to a used piston, when properly cleaned. The material, with it's graphite composition, will wear to a perfect fit, maintain excellent cylinder wall clearances, improves oil retention on the skirt face and with rings that are slightly oversize, we can fit the piston rings for the correct end gap. The engine will be back to new clearances for the piston and the rings.


The process has it's limits. Although the coating, as advertised, can be applied to a .020" (.5mm, second oversize) thickness, we are going to not be so adventurous, and think if a cylinder is bored to .5mm, it should require new pistons. But some may want to give that a try, if they are on a tight budget. The process can be done in-frame, which reduces not only the cost of the parts, but also reduces the effort of removal, disassembly and reinstallation of the block into the frame. To improve the honing, a torque plate will improve the piston ring fit and life-expectancy of the job.

486,

Your .004" piston-wall clearance is at the VW maximum spec for clearance.

We have imported sets of 79.51mm rings that are intended for an Audi. The rings are .07mm(.00275") larger than our ALH/ AHU STD piston and rings, which are 79.44mm. That would cut the ring end gap clearance to a nominal .00125", or virtually EXACTLY the piston bore fit we aim for. We recommend end gap clearances of no less than .016", which you may have to slightly file the 79.51mm rings for a 'no blowby' fit.

If the job required opening up the cylinder more than what the 79.51 rings can do, we have the .25mm oversize rings, which will carry you to .010" of clearance between stock piston size and cylinder bore. Our coating will then, eliminate the piston slap.


When we work up a set of pistons, we not only clean the piston, but also balance weigh pairs, double check rod dimensions and professionally mark the pistons and rods, then install the rings. We need to know exactly how big the bore is.

It's always amazing to me, when we come up with new and innovative improvements for piston fit and efficient methods of engine overhaul...

Crickets.

Franko6 said:

It's always amazing when we come up with in and innovative improvements for piston fit and efficient methods of engine overhaul...

Crickets.

Click to expand...

Amazing? Yes, sir, that actually goes without saying lol.
Could be your audience is stunned into awed silence

It is interesting that you're using the coatings to make up for large clearance, sort of an elephant in the room kinda thing now that I think of it, knew about the coatings being available both for the skirt and for the crown, but never put the two together into using them to build up undersize pistons.

Only trouble I can picture is that I usually see the skirt coating worn down to aluminum on overhaul, though that might just be a lower grade coating meant for additional lubrication rather than buildup.

On mine, I'm just running them loose on purpose so I can do stupid things with less worries about thermal expansion.

I don’t know Frank06 or anything about him; but I am very pleased with his stage 2 alh cam. I was worried it might be too much, not the case at all. It’s not radical, basically it’s like stock but just pulls harder. Nice work Frank.

486,

The black skirt coating seen on OEM and just about every aftermarket piston we use, is a graphite wear coating. On some of the pistons, there is an oblong 'hole' in the coating that we don't agree with as for location, but it is designed as a measuring point for the piston skirt diameter, excluding the coating. The coating from the factory is usually .0015" - .002" increase in diameter. The entire purpose of the coating is to wear in for a 'more perfect fit'. As many pistons as we have seen, I can tell you that the wear coating usually has a wear pattern in the ALH engine, for example, that the very center of the skirt coating will wear a circle dead-center in the coating. The older AHU pistons are very similar in construction except there are no brass wrist pin bushings. The AHU pistons seem to continuously wear out the entire coating. I think the brass pin bushing has much to do with this patterning.

As for thermal dynamics of piston expansion, it's supposed to be a science, but I think it ends up being an art. To figure out the expansion properties just for a piston whose combustion bowl is off-center is a problem to be solved all in itself, let alone the heating of each cylinder wall and it's distortion due to less than perfect water flow. We can't possibly include enough information to correct every anomaly in this engine, so we build in compensation factors. That is the skirt coating as it is applied in our most commonly used Nural pistons. We simply expand on the coatings purpose.

I understand running a 'loose' piston to avoid the extreme heat cycling that can happen with a highly modified engine. But 'loose' is relative and the piston slap is wear to piston and cylinder wall. The tighter the fit, the less the piston rocks, the more accurately the rings seat, the longer the whole thing holds together...as long as it doesn't seize. I would say that below the threshold and limit of what the piston can do, below the point of seizing, there are control factors we can incorporate to improve overall performance. The wear coating as designed in the OEM pistons is a 'fudge factor' to allow for wear on a piston as it breaks in for a 'perfect fit'. The whole point of the graphite polymer; although it is soft, and very quickly wears away in an interference fit situation, it is also very slick, with a very low coefficient of drag.

A properly designed coating not only will support the improved fit, but will actually increase oil retention on the skirt, as the coating is not perfectly smooth, but has peaks and valleys. If you have kept up with some of the innovative cylinder wall finishing, like laser honing, the purpose is to remove the peaks and use the valleys for oil retention. There is a quantum improvement in break-in period and effectiveness with both laser and graphite polymer technologies.

The question is does it last? Based on our large sampling of blocks, we know that the original coatings do not completely wear off, but are performing exactly their purpose.

In our perspective, and although the company we are using, claims survivability of coatings up to .020" thick, or .5mm, that amounts to second oversize. We are not doing that. Our limit is to the first oversize; .010" or .25mm. Most of the coatings we are having applied are in the .003" -.006" range. This is not a 'single coating' system. If a thicker coating is necessary, a harder, less abradable substrate coating is applied.
There are two oversize ring sets we use to fit for end gap. One set is 79.51mm and the other is 79.75mm. The first ring set we have found that honing an out-of-round bore usually takes us to a very close ring end gap, without filing.

Here is the cool trick. We have done 'in frame' rebuilds, using the original pistons to revitalize engines for a savings for engine removal and piston replacement. It's a huge savings in time and money and the cost is 1/2 the price of a new set of pistons.

I am really enjoying the new technologies that exploding onto the scene.

Dieselmonkey02,

It's actually Franko6; not Frank Zero Six. But thank you for the compliment. The Stage II ALH and AHU cams are the 'most under-advertised' of our products. The cam has wide acceptability and we have engines up to 240hp using it. We really don't push them and the biggest advertisers are the people who own them. Thank you for your unsolicited response.

We have innovated for several items.

Our Molnar 'H-beam' connecting rods are the other big development we created. There are several innovations in the Molnar rods, but that is another story. It must be a good product. Two of our competitors have tried to go around us to get them. No Can Do.

Franko6 said:

As many pistons as we have seen, I can tell you that the wear coating usually has a wear pattern in the ALH engine, for example, that the very center of the skirt coating will wear a circle dead-center in the coating. The older AHU pistons are very similar in construction except there are no brass wrist pin bushings. The AHU pistons seem to continuously wear out the entire coating. I think the brass pin bushing has much to do with this patterning.

Click to expand...

The wear pattern is what tips me off to bent rods. Being a tremendous hack, I don't actually measure connecting rods, just look for wear patterns on the skirt and rod bearing. so long as the engine hasn't seen any damage shortly before disassembly these wear patterns will show any bend.
Very interesting on the difference made by the brass inclusions. Differential expansion is a powerful force.

Franko6 said:

As for thermal dynamics of piston expansion, it's supposed to be a science, but I think it ends up being an art. To figure out the expansion properties just for a piston whose combustion bowl is off-center is a problem to be solved all in itself, let alone the heating of each cylinder wall and it's distortion due to less than perfect water flow. We can't possibly include enough information to correct every anomaly in this engine, so we build in compensation factors. That is the skirt coating as it is applied in our most commonly used Nural pistons. We simply expand on the coatings purpose.
I understand running a 'loose' piston to avoid the extreme heat cycling that can happen with a highly modified engine. But 'loose' is relative and the piston slap is wear to piston and cylinder wall. The tighter the fit, the less the piston rocks, the more accurately the rings seat, the longer the whole thing holds together...as long as it doesn't seize. I would say that below the threshold and limit of what the piston can do, below the point of seizing, there are control factors we can incorporate to improve overall performance. The wear coating as designed in the OEM pistons is a 'fudge factor' to allow for wear on a piston as it breaks in for a 'perfect fit'. The whole point of the graphite polymer; although it is soft, and very quickly wears away in an interference fit situation, it is also very slick, with a very low coefficient of drag.
A properly designed coating not only will support the improved fit, but will actually increase oil retention on the skirt, as the coating is not perfectly smooth, but has peaks and valleys. If you have kept up with some of the innovative cylinder wall finishing, like laser honing, the purpose is to remove the peaks and use the valleys for oil retention. There is a quantum improvement in break-in period and effectiveness with both laser and graphite polymer technologies.

Click to expand...

Add to the offset bowl an oil cooling channel that may or may not be getting oil and you get a lot of chance for uneven expansion. Everyone tries their best to line up the nozzles, but they are in a pretty tricky spot to get perfect.
I really like the idea of mahle's monotherm pistons, being steel they don't expand nearly as harshly as aluminum. Pullers report that they tried to use 'normal' clearance (which to anyone else is slap worn out) and it would just beat the iron cylinder walls ragged, so they run them tight now. Too bad the offset bowl makes the friction welding method currently used impractical for us 8v owners, add on needing to make large runs to be reasonably priced and it's right out the window. I'm holding hope for further advancements in metal printing (which seem very close!) to make some steel pistons for oddball small TDIs.
So it seems the skirt coating is sort of like brake shoes. They wear quickly until making full contact with the drum, then wear almost stops completely.
I may well have to look into the skirt coating when I build another nutso engine, sounds like a very good solution.

Franko6 said:

Here is the cool trick. We have done 'in frame' rebuilds, using the original pistons to revitalize engines for a savings for engine removal and piston replacement. It's a huge savings in time and money and the cost is 1/2 the price of a new set of pistons.
I am really enjoying the new technologies that exploding onto the scene.

Click to expand...

I love seeing things like this, hate throwing away perfectly reconditionable parts. Just like spray welding the seal journals on industrial shafts. Do you use a portable rigid hone such as lisle's #15000? Old guys used to swear by such a hone for "proper" in-frames, and I use one, myself. Doesn't do quite the magic of the factory applied laser finishing though.

My apologies for the name discrepancy.

Dieselmonkey02, no problem, unless you are looking for Frank06 and can't find him...

486, we really do like the Mahle pistons, but don't be looking for them to make the ALH style piston anytime soon. When installing the steel pistons, and especially them not being forged, we would go with a very tight tolerance of .0005", as piston and cylinder wall expansion rate coincide. There is a possible 'mushroom piston' on the horizon for a small run. We will see what happens. The skirt coating is still a very good idea with a steel piston.

On another note, we really dislike the mis-advertisement from a competitor who brags about 'forged' pistons. The problem with a forged piston is that the clearances must be increased to compensate for the annealing of a forged piston. They will 'relax' and expand over the course of break-in and become larger. I don't use forged pistons and don't expect to. For the most part, people bandy around terms like 'billet' and 'forged' without knowing what they are talking about.

The steel pistons do still heat up faster than the block so the expansion doesn't exactly coincide, but the steel expands about half as much (inch per inch per degree) as aluminum does.

you can't do a forging with the oil channels, either
With guys making 1k-1200 hp on stock cast pistons in gen3 GM v8s I think the day of forged pistons being more durable isn't really accurate at this point. Casting technology isn't some guy with a ladle ramming up and pouring greensand moulds any more.

I'm not so sure what the coefficients, or how much the pistons grow, but there is still a large following for forged pistons. It is a problem, as the pistons are not stable as purchased.

The Lisle is a good deal and great for the money. We use an old Sunnen ($$$!) that I'm thinking Lisle copied the design. We built a depth stop for it to keep the stones out of the oil squirts. It's still a bit of a problem. It can twist your arm off. Work it slow with rough stones, then graduate to very fine (280) to finish. Keep it cool. We usually work to get the taper square and quit. Let it cool. Measure for coating.

We bend the oil squirts, but that's not for everybody. You have to break a few to know when and where to stop. Modified hone shoes get behind the squirts.

And we always torque plate... y'know, we don't make this easy, do we?

I was redirected to this page from one of my customers. In response to Cevans' post, #13, a page from the Bentley book, the information provided by Bentley for the ALH/ BEW pistons is WRONG! DO NOT DEPEND ON THE INFORMATION TO FIT PISTONS!

If you look in the Bentley book, for 1999- 2004 VW's, Page 13a-16, the pistons for the ALH/ BEW are listed as 79.47mm. I have seen pistons of that size, but they are an extreme rarity. Almost every ALH piston is 79.44mm, or 3.12755"; not 3.1287. That .0012" difference may not seem like much to you, but THAT IS THE CLEARANCE WE USE for boring piston/ cylinder wall clearance.

So, if your normal clearance of .0015" is used and you depend on the Bentley's incorrect information, your piston clearance ends up being .0027". I consider a cylinder bore with a clearance of .0035" WORN, and we have a special set of rings just for that situation.

When the cylinder bore reaches 3.130"- 3.132", we hone to square up the bore and use 79.51mm rings and a piston coating service that applies a .003"-.004" abradable graphite coating. Using the existing pistons, coated to the new 'oversize', and the 79.51" rings, we have honed to fit pistons without removing the block from the frame. It is the quickest, cheapest and most effective 'rebuild' we can figure out, with reliability to boot.

The method works in very limited situations, but when it does work, it is quite phenominal. We also will coat the pistons and go up to .010" thickness for the coating and use 1st oversize rings. Although the coating is supposed to be good up to .020 (.5mm or 2nd oversize..), I am not confident of that practice. I would install the .5mm O.S. pistons instead.

I did it again. Crickets...

If it makes you feel better, Frank, I found it very interesting to read. A bit over my head, but interesting.

Franko6 said:

I did it again. Crickets...

Click to expand...

This thread came up in my search about a situation i have. BEW broken ceramic debris from GP scratched the 4th cyl. now im weighing my options. ball hone was tried and still catch my nail. guess its time for larger parts in that hole.