Are clearances actually tighter now?

[modok]

Compared to the average car engine of 30 years ago,
Do the newest engines have tighter bearing and piston clearances? And If true, is this possible because of more precise machine work?

pistons yes, hyper-eutectic alloys and better skirt designs. Certainly pistons are better and cheaper than ever before, thanks to high speed machining with PCD inserts.
bearings, overall....no Some do run tighter but many don't.

there have always been some engine built to very tight specs and others not.
In some cases it seems to have little relation to how much people like them or don't.

The average engine today.... Not really any more precise than was achieved 50-60 years ago in better examples.
Better designs, better oil, better fuel, better fuel systems, better air filters, better materials.....is what makes them live so much longer.

[jed]

Compared to the average car engine of 30 years ago,
Do the newest engines have tighter bearing and piston clearances? And If true, is this possible because of more presice machine work?

Not necessarily tighter but, probably closer limits.

I didn't understand your post first time I read it. But I think I do now and agree but think clearances are .0005/.0008 tighter
and that's due to oil viscosity. Also I have read that manafactures want to minimize engine noise and vibration and tighter
clearances help with that.
As far as factory published clearances I put no faith in them. Why, because I have torn down new or nearly new engines
and they were not with in published clearances they were always looser.

[hoodeng]

The fact you find looser clearances in tear downs of new parts is because of tolerance stack up. That is when mating parts have a higher figure on the o/d of a shaft as in lets say, nom 1.000"ø +.0002" -.0002" and its mating bush bore i/d of 1.001" +.0002"-.0002" the design clearance for this component assy is lets say .0005" to .0015" ,any combination of shafts and bushes within spec can only be minimum clearance of .0006" and max .0014", all within spec.

When we individually fit parts we may choose to close down on the clearances, that's our call, even the manufacturer may want tighter clearances but they have to be realistic. They could invest in expensive machinery that will hold much tighter tolerances but for what benefit? If they machine a cylinder head and guide clearances fall between .0008" to .0015" and work perfectly well and give design life service at the higher clearances, why chase the minimum at a cost premium?

Holding .0002" in manufacturing is pretty slack to these days standards, i only used it as an example. We easily hone rods, bores, guides etc to this tolerance.

Cheers.

[Baprace]

Indeed they do. I'm planning an 2nd gen LT1 build around the tighter ideology. It's a drag race only engine with stock 3.48 stroke, 4.03 bore, .001-.0015" rod and main clearances, 6.125" rods, 1.125" pistons, 1/1/3mm rings, coated bearings, skirts, and pins, and 0w-8 oil.

RDY4WAR , if this engine was a circle track engine with preheated oil your clearances would probably live, in a drag race engine clearance is your friend, your engine will make more power with a cool engine temperature, if I was building that engine I would open the clearances. You run the risk of spinning a bearing and /or scuffing a piston, I don't have a problem with your 0w-8 oil <? but your engine needs lubrication. Not trying to criticize your build and I'm sure the clearances in Jason Line's engines are pretty close to what you are thinking, BUT these engines have many years of development behind them and are worth $150,000 - $200,000 and up , just the machining of these engine are an amazing procedure with the cost of machines to do the work PLUS the programming cost's. I'm not criticizing your build or your expertize on engines building, I'm just saying it's past my pay grade. Good luck to you.

[RDY4WAR]

I know that plan is extremely optimistic and realistically I'd end up closer to .002" after a few rounds with the machinist.

[David Redszus]

Modern engines are built with more accuracy.

Engine designers consider dynamic clearances which includes factors such as component flex, thermal expansion, stacking tolerances, and a lot more. That is quite different than merely considering cold static clearances.

While clearances are up to the engine designer and are engine specific, accuracy of dimension and tolerance consistency are not. Modern materials and machining methods allow for a level of precision never before attainable.

[4vpc]

Aluminium blocks have made cold clearances smaller than they once were with cast iron.
Improved piston design has enabled the OEMs to tighten clearances there, sadly it's often lost with dumb and lazy after market ones. Go read an internet available workshop manual on an S2000 engine, Honda were running bore clearances on their forged pistons that are much smaller than cast pistons were a decade or so earlier.

[peejay]

Aluminium blocks have made cold clearances smaller than they once were with cast iron.
Improved piston design has enabled the OEMs to tighten clearances there, sadly it's often lost with dumb and lazy after market ones. Go read an internet available workshop manual on an S2000 engine, Honda were running bore clearances on their forged pistons that are much smaller than cast pistons were a decade or so earlier.

That's a bad example, given that S2000s were growing a reputation for galling the pistons when used at track days. I believe they fixed this issue for the 2.2l version.

[ptuomov]

On piston to bore walls clearance, it’s not just a question what tolerances you can hold but also what is the right clearance. The Mahle piston book has a lot of graphs of what piston to wall clearances for to frictions, noise, wear, etc. Horses for the courses.

In the most recent round, we ended up choosing 0.0026”/0.066mm piston to bore clearance on a 3.937”/100mm bore and got 0.0026”-0.0028” across the eight cylinders. 8000 rpm engine, forged aluminum pistons with Mahle alloy similar to 4032, 1mm/1mm/2mm rings, aluminum block plated with Nikasil. (Stock crank, played stock block, etc. 1987 Porsche 928 S4.) The Nikasil likes to hold oil which is a good thing for the bore walls. The ring end gaps are set to 0.024" top and 2nd, went generous there because it’s a pump gas turbo engine that may knock and get hot. Since we used bearings originally intended for Ford Cosworth YB turbo, we went with rod bearing clearances that have worked well in those engines. That’s 0.0020”/0.050mm on a 2.047”/52mm shaft, which is way looser than a modern engine but also justified by the fact that the cams/lifters require us to run thick oil.

[modok]

Tighter is not always better.
In the 60's, 70's a lot of bearing clearances and skirt clearances were aimed near the minimum,
because the engineers had a general expectation that they would wear looser, because that's what used to happen.


But all along, as materials, and lubricants and filters, and fuel improved, it's been happening less and less.
Clearances do not always wear looser. under some conditions they can get tighter over time.
if the rate of wear making it looser is NOT greater than the embedded material, oxide films, and housing collapse cause it to be tighter, then...it actually stays the same.

[pdq67]

Back years ago when I worked at WA Shaeffer Pen, Ft. Madison, IA, an old Master Die Maker showed me that if I knew what I was feeling, that no more than my thumb nail could feel .0002" of an inch.

And he had his Starrett mic in his hand with me feeling a .0002" high light on a ball point pen barrel.

pdq67