S52 maximum total wear clearance between piston and cylinder (engine operated)

[Franky goes]

I'm checking the race engine to make sure I'm within tolerances and the folowing TIS page :
https://www.newtis.info/tisv2/a/en/e...s-pins/601cOvM
specifies a maximum 'wear' tolerance of 0.113 mm (4.45 thou).

The Piston running clearance is straight forward at 0.026 to 0.058 mm (.9 to 2.8 thou) measured at the widest part of the piston skirt. For racing the higher number makes more sense.

On the other end, the max total wear clearance concept escapes me. I looked everywhere and can't figure out what it means or what needs to be measured to figure it out. The 'Engine operated' is also unclear. Does it mean the engine is used or the measurement should compensate for operating temperature ?

Any help would be appreciated.

[tjm3]

What you're working with are 'new' specs and maximum cylinder wear before BMW recommends an oversize piston. Stock factory piston to cylinder clearance from .0009-.0028" would be normal tolerance. The basic standard for stock pistons with a high silicon content would be safe at .001-.0015" for street or track. If you move into forged pistons with low silicon content then machinework stuff starts to happen differently. If you hit the max wear at .0045" then crosshatch is about worn off and ring sealing becomes an issue. It's difficult to get the measurements accurately without a good micrometer to measure the piston skirt and to measure the bore to a ten-thousanth of an inch. The way it should work is measure the piston several times and average the result. The dial bore indicator is then set to that piston size measurement and placed in the cylinder bore which then shows the difference between piston size and bore size. A good bore gauge is marked in .0001" and easy to get measurements down to .00005". Just can't do it accurately with everyday type tools. The nice thing about forged pistons, if you went that route, is that you can measure the bore and have pistons made just a couple thousandths larger, then easily hone to size of .003-004" larger bore and not need to actually rebore the holes.

Do a youtube search for 'piston cylinder bore measurement' for some good videos.

[Franky goes]

Thanks for the explanation.
I indeed forgot to mention that my class does not allow changes to the engine so I'm with OEM high silicon aluminum castings.
Also left ou that I have the bore gauge an micrometers to measure in 1/10th of a thousand of an inch (0.0001).
I've been practicing and can consistently get within +/- 0.0002. So close enough for the task.

If I paraphrase to make sure I understand, you are saying that new engine tolerances for piston to bore clearance is 0.0009 to 0.0028 inches. When the engine is worn enough to increase that clearance to 0.00445 inches the systems stops functioning as designed ? Ring lands, end clearances, compression and bore lubrication all gets out of wack... ?

Thanks.

[tjm3]

Thanks for the explanation.
I indeed forgot to mention that my class does not allow changes to the engine so I'm with OEM high silicon aluminum castings.
Also left ou that I have the bore gauge an micrometers to measure in 1/10th of a thousand of an inch (0.0001).
I've been practicing and can consistently get within +/- 0.0002. So close enough for the task.

If I paraphrase to make sure I understand, you are saying that new engine tolerances for piston to bore clearance is 0.0009 to 0.0028 inches. When the engine is worn enough to increase that clearance to 0.00445 inches the systems stops functioning as designed ? Ring lands, end clearances, compression and bore lubrication all gets out of wack... ?

Thanks.

I think you have the right idea. If you're taking a class then they're most likely showing you the correct way to do things, and it appears you have access to the correct tooling. Taught engine machining and rebuilding for years. Obviously, if you were in business and rebuilding these engines then you are obligated to follow the factory specs. BMW wants to see those in order to maximize the lifetime of an engine. If you vary from the specs then end of life comes early but not necessarily at the expense of a good running engine. Use to be that if the bore got too big you could throw in a set of oversized rings and be good for a while. Any kludge like that will be at some cost. Compression and oil burn might be OK, but unknown for how long. The factory specs are for maximized engine life. A race engine that gets torn down every season might not matter too much. And, yes, as the engine gets worn out of spec the parts will begin to wear faster and unevenly. On the other hand, I've torn down some BMW engines with 250K+ that still have decent crosshatching in the cylinders and still within usable tolerances. Know that you can't go wrong with factory specs and any changes not covered by factory specs (forged pistons, low tension rings, cross hatch angle and finish, etc.) will affect performance somewhere down the line and at that point you become the test pilot.

I'm noticing lately that engine classes are leaving out the actual machine work and focusing a lot on accurate measurement. A lot of the newer engines can't be machined and reused without some sophisticated equipment to do alloys and coatings. Measure for tolerance and either reuse or discard. I also notice that some programs don't have instructors who know how to use the equipment and so never see a head or block machined. That's really a loss to the students. Look for programs that offer everything. If there's nothing to clean up after class then you're not getting introduced to the whole picture.

[Franky goes]

Thanks !

Actually everything is self thought.
I'm taking the measurements but having an experienced engine builder/racer help me with what can be reused and what should be refreshed. All the real work (Checking for parallelism of the main bore to the deck, 'magnafluxing' the block and rods, over boring if necessary, cross hatching, ring seat procedures) will be done by a professional shop. It will be like a double check of what I found. Cut once measure twice... I know my limits.
That's why my question was specifically about the measuring. I like to know what I will have to ask and use the right terminology to make sure the shop is cristal clear on the work to be done and to what tolerance.

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[Franky goes]

Here is an update.

I looked at the pistons and chose the one that looked the roughest (#4) in my case.
Cleaned it very good and measured at 9.5mm above the skirt. Right off the bat, the piston is out of specs.
Smaller tolerance is 3.3998 inches. My piston diameter is 3.3986. NewTis does not specify if that tolerance is new or used. I'm assuming it is the tolerance for a used piston. Could be wrong...

The NewTis maximum allowable 'out of round' and 'taper' are respectively 0.0002 & 0.0004 inch. This is specified for 'new' or 're-work'. Makes sense as those are very tight tolerances. Different sources quote 0.0005 and 0.001. Metric Mechanic specifies 0.0003 inch for both dimensions. A used engine will never have the tolerances of a new one so If I double those values to obtain a maximum for a used engine I get the maximum allowable tolerances of 0.0008 & 0.002 inch.

My measurements are 0.0115 & 0.0006 inch
Under that assumption my block's is out of specs for Out of Round and within for Taper.
As soon as I can have the blocked rechecked by a competent shop I will update. For now, pretty much everything points to a re-bore.

Thanks

[tjm3]

First thing, for a rebuild the absolute best and safest way to go is to rebore for the oversized pistons. For the S52 your choices are limited to BMW (very expensive), aftermarket (Mahle, most likely, less expensive but expensive anyway), nameless internet junk (much less expensive) and custom forged (about $1000 for the set).

Here is an update.

I looked at the pistons and chose the one that looked the roughest (#4) in my case.
Cleaned it very good and measured at 9.5mm above the skirt. Right off the bat, the piston is out of specs.
Smaller tolerance is 3.3998 inches. My piston diameter is 3.3986. NewTis does not specify if that tolerance is new or used. I'm assuming it is the tolerance for a used piston. Could be wrong...

At this point the important thing is to get a good overall picture. Measure all of the pistons, measure them 3, 4 or 5 times each and get an average of the overall numbers. The piston wear for that one
piston is well below the maximum piston to wall clearance of 0.0045" . Measure them all and get an average. Then measure the holes.


The NewTis maximum allowable 'out of round' and 'taper' are respectively 0.0002 & 0.0004 inch. This is specified for 'new' or 're-work'. Makes sense as those are very tight tolerances. Different sources quote 0.0005 and 0.001. Metric Mechanic specifies 0.0003 inch for both dimensions. A used engine will never have the tolerances of a new one so If I double those values to obtain a maximum for a used engine I get the maximum allowable tolerances of 0.0008 & 0.002 inch.

Your concern at this point (if you want to stick with the original pistons) should be 'can the cylinders be honed to within taper and out of round tolerance and still be within the piston to wall max clearance of .0045", which is a lot of room. You can't do the honing with a hand hone. It's needs to be done on a professional hone or you'll end up with something equivalent of a worm hole looking thing (by micrometer standards).
Also, depending on how the honing is turning out, the machinist can change the finish to allow the rings to seat better or quicker although there's some limitation there, too, depending on what ring and what material.

My measurements are 0.0115 & 0.0006 inch
Under that assumption my block's is out of specs for Out of Round and within for Taper.
As soon as I can have the blocked rechecked by a competent shop I will update. For now, pretty much everything points to a re-bore.

0.0115" out of round is probably not gonna cut it, BUT you should pick the worst cylinder, hone it to just barely in tolerance and check the piston to wall clearance to see where you're at. There's really nothing to lose. If you're over the limit after honing then you're still way under the first oversize of .020", unless you go custom. Once again, maximum life is gonna be with new pistons and a rebore. But who's to say whether the original parts machined into tolerance will get you close to maximum mileage life or a few thousand miles less with any other engine mods NOT considered.

Thanks

[gdavid]

A set of Mahle oversize pistons are for sale on ebay for about $710. I've been looking for stock bore and keep seeing them. I have no experience with the seller but that seems like as good of a deal as you will find and maintain class compliance.

If you do end up replacing, I'm still shopping for 2 used OE bore pistons.

[tjm3]

If real Mahle pistons are available at those prices then I would definitely consider that. The numbers they have advertised are a bit confusing, though. Mahle marks their piston diameters in mm but the oversize markings are in inches. The Ebay advertisers are showing the oversize in standard numbers with a metric sizing. Watch out and be aware.

[gdavid]

If real Mahle pistons are available at those prices then I would definitely consider that. The numbers they have advertised are a bit confusing, though. Mahle marks their piston diameters in mm but the oversize markings are in inches. The Ebay advertisers are showing the oversize in standard numbers with a metric sizing. Watch out and be aware.

Agreed, and please note that I don't have any experience with that seller, I just happen to notice the listings in my searches.

[Franky goes]

Thanks for the comments.
I will purchase oversize anyway because everything is at the tolerance limit or past it.
From Bimmerworld, I understand the Mahle oversize are in the $800 range and i try to support them because of all the help they provide me with the race car build.
They confirm the regular size are also available from Mahle but at a ridiculous cost north of $2,000 if I remember correctly.
The price difference is such that it would be about the same price, or cheaper to have the block rebored for the oversize pistons.
Only draw back is that options left after that are custom made pistons. Since I only rev the engine to 6,900 RPM. These should give me another 10 years of service.
Thanks

[Franky goes]

Thanks Tjm3 for the detailed information.
i was not aware of the option to hone only and keeping the standard (existing) pistons.
Good to know but in this case as you also mentioned, it will most likely not give enough life until the next rebuild.

Interestingly, I've been talking to Metric Mechanic to modify the oil galleries going to the cam journals.
They will also provide me with a tested used crankshaft.

They took the time to explain to me that engines need to be refreshed / rebuilt every 150 hours of racing.
I thought that was for hardcore racing at 7,200 RPM with modified everything, long duration cam profiles...
I took the time to inventory all the hours raced/tracked and I'm at 130 hours and 200,000 miles.
Looks like they were correct. From now on I will be logging racing/track time.

Not discussed in this thread I am also looking at the main bearings and they have also seen better days...