Piston To Wall Clearances...

[Mandic 0]

Hi,

 

I interested in how this can be determined.

 

To my knowledge cast. ie standard pistons have steel inserts to control excessive expansion and therefore keep the engine quiet when cold.

 

But when tuning the engine, producing more heat or trashing it on track for longer periods of time one should run a bit looser as the piston expands a bit more and there is a possibility of picking up. So the only downside would be a bit louder startup/cold running.

 

Now forged pistons on the other hand do not have steel inserts thus expand a lot when hot so again one should run a bit loose otherwise they would seize quite fast.

 

I was given info one should run something like 0.0063mm of clearance using stock pistons and 0.01-0.013mm using forged ones.

 

All good here, but what if one uses ceramic coat, does this coat help bringing the expansion rate back to normal? Can one run std. clearances (talking for Mi engine) when runing coated pistons?

 

And the last one, is there any formula from which one can calculate what clearance one should run for a given horsepower or application (track, occasional track, daily driver, ...)

 

 

If anyone of You (petert, puma, sorrento, matt, etc.) can please write some more regarding this subject.

 

 

Cheers

 

Ziga

[jacobs53 0]

if you look very closely at most stock pistons you will notice that the gudgeon pin boss is slightly off center to minimise piston slap, in a performance engine you want the boss to be central in the piston not offset, can't remember the reason why now, but i did a assignment on this..

 

sorry much help towards your answer

 

lee

[DrSeuss 0]

having the gudgeon pin off centre will press the piston into one of the sides of the cylinder. Thus stopping it flapping around (piston slap). But this also creates extra friction which is bad.

 

I'm guessing the piston clearances are a tolerance dictated by the material properties.

[petert 554]

The Mi16 strutted piston is now a very old design and not common in modern engines. The theory is that the steel strut will control the expansion of the aluminium, allowing a closer tolerance (0.00075-0.0015"). This eliminates piston slap when cold.

 

Modern cast pistons are made from a hypereutectic alloy (and coated) eliminating the need for the strut. The problem with struts is that they can come loose.

 

Forged pistons are different again, and depending on the alloy they're made from, will determine the clearance. I can't recall the exact alloy numbers, but typically an 83-86mm piston would require 0.003" for the shiney alloy, and maybe 0.004-0.0045" for the darker grey alloy. As there is no strut, the piston will expand considerably in size, thus they slap when cold. There is no hard and fast formula, but an engine that is going to worked very hard from the start will need more clearance than an engine that might be run in carefully, at the expense of bore life.

 

I'd recommend running more clearance with the std. Mi16 piston for track work. It's possible to run up to 0.003" clearance without dramas. I think a friend of mine even runs 0.004". This is quite good for two reasons. Firstly, it allows you to hone a typically worn liner (providing you can find someone to do it properly!). As the wear patch is normally 2-2.5 thou. deep, a good hone will clean them up. Secondly, the ring end gap will increase with an increase in bore diameter (as circum.= pi x dia.) This is desireable in an engine which is going to be worked very hard, 0.020" for a top ring being very acceptable for a N/A engine.

 

Sorry, as a child of the '70s, my brain works in inches for clearance and metric for diameter.

Edited February 2, 2006 by petert

[Mandic 0]

Ok, and what if I have my pistons ceramic coated (all over: crown, skirts, bell), does this make any difference, will the expansion rate stay the same, or should I go a bit tighter?

 

Thanks

 

Ziga

[petert 554]

Ok, and what if I have my pistons ceramic coated (all over: crown, skirts, bell), does this make any difference, will the expansion rate stay the same, or should I go a bit tighter?

 

I'm not sure why you'd ceramic coat the skirt? Normally you do the crown, after you've done the valve/piston clearances. Some pistons have an anti-friction coating on the sides which does increase the overall diameter of the piston slightly.

[Mandic 0]

Jup, I have coated the crown, but since I'm using std. cams there is no worry about piston/valve clearance.

 

The skirts are coated with anti- friction coating yes, and they said it does change overall dia. for 2/100mm or so. But first need to know what sort of clearance should I run anyway in order to hone the liner properly, so piston change in piston dia. due to coating is not important. It's even better as piston is slightly "bigger" and if one has a bit of a worn liner one can get away with it just fine.

 

I would really like to know about more about this, cos this is what ceramic coating is all about, to lower the piston temps, so according to this it shouldn't expand as much...

 

Cheers

 

Ziga

[Mandic 0]

Anyone else having any thoughts about this?

 

 

Cheers

 

Ziga

[Mandic 0]

Anyone?

[pacey205 2]

Coating the whole piston wont change the thermal expansion rate of the metal it will just lower the temperatures seen by the metal so it will expand by less, relative to a non-coated piston.

 

You could work out the expansion with a wee bit of maths but youd need to know the temperatures that the metal was seeing with the ceramic coating which would be pretty hard to determine unless you had some sort of testbed setup.

 

Michael

[petert 554]

I can't see it making any difference. Once the engine is up to operating oil temp., say 90-110 deg. C, the piston diameter will stabilize. I can't recall the co-efficient of expansion for aluminium, but the percentage difference in that range will be insignificant. Taking the engine up to 130-140 deg. C however, is a different matter.

Edited February 9, 2006 by petert

[Mandic 0]

OK, just needed confirmation, if someone else is thinking the same way,

 

My thoughts are the same, once the piston or better yet the whole system reaches its normal operating temperature it makes no diference.

 

My oppinion is the coating is there just to protect sudden temperature rise due to pinking or any other simliar malfunction, so I'll just follow those 0,0025" of clearance.

 

Thanks Peter!

 

Cheers

 

Ziga

[DrSeuss 0]

Insulation reduces the thermal gradient across its thickness. But ultimately only slows the transfer of energy. So once the engine is up to temperature the piston will be at normal operating temperature.

[Mandic 0]

Well it's possible that the amount of energy that the coat of ceramic is capable to give away (or radiate) to liner or oil is greater than the one it recieves (at certain point of operation, ie. slow driving) so the piston does not expand as much as it should.

 

Space shuttle has such (black) ceramic tiles to help radiate the heat away at re-entry and white ones which reflect solar radiation when in orbit. And if it rotates those tiles are capable to.

 

If You lie You hand to caramic tile You'll notice that its thermal conductivity is very high and it seems "cold", even if its temperature is the same as surrounding's.

 

Cheers

 

Ziga