600+ pages of bearing sizes and clearances, from ACL, who make stuff for just about everyone.

General information

http://www.acl.com.au/web/acl00056.nsf/a...37?OpenDocument

The good guts

http://www.acl.com.au/web/aclwebsite.nsf...cb?OpenDocument

My question is a little off topic, but I don't know of better place in the formum to ask it. Are clearances now as tight as they can get for any of the bearing in an ICE? It seems that the machining has gotten to the point where clearances that account for thermal expansion are easily within the capabilities of mass production.

Always have been.

GM, and Rolls Royce used "select" fit for more than half a century.

Acknowledging that there are tolerances in manufacturing, manufacturers used to pick/make not quite to spec equipment, and assemble it to get the clearances that they were after.

There were bearings over and under dimension, 1/4 thou, 1/2 thou, 0.75, 1, 1.5 and 2, so the machining of the crank and bores could me matched exactly to the right bearing or piston to get exactly the clearance that was specified...and engine could have 8 different over/undersizes, and be "perfect".

Makes remanufacturing an engine difficult, when you have to "fit" parts together to get the best, or the best compromise.

The old ways delivered precision fit, at loose tolerances.

Japanese expect that parts are manufactured such that every piston can go into every block, way more expensive set-up,and it works most of the time in modern times.

I'd be surprised if clearances were that much better controlled than in the past...the SBC in my brother's 1969 Holden Premier still called for 5W-20, even 'thought there was no such available in Oz at the time.

From this perspective, the idea that tolerances are getting "tighter" has to end. You just aren't allowed to break the laws of physics.

Unless we find a way to formulate alloys that allow for compensatory thermal expansion that maintains a fixed distance between rings/races, then bearing "fit" cannot get any tighter.

If this is the case, bearing tolerances cannot continue to be used as an excuse for lower KV oils. I can accept that thinner can be "better", but I am having difficulty accepting that bearing clearances has anything to do with the "thin is in" trend.

Clearances and tolerances are two different things.

Agreed. But in the context of this forum, where "tolerances" is often used incorrectly, I tried to use the word as it is often used here. The two words, although not strictly interchangeable, are related. As tolerances decrease, or become tighter (as often referenced here), the range of clearances decreases (not the clearance, but the range). As the range decreases, the target specification for clearances can be decreased. In other words, if the tolerances are made tighter, the engines can be built tighter (smaller clearances).

Tolerance is a quality control description that uses dimensions to set limits.
Clearance is a dimension, the range of which can be set by tolerances.

X2. No significant reduction in clearances, just tolerances are better managed.

Where, here in Bitog and similar sites, or in automotive industry publications?

I agree, even with laser machining you will reach a limit on toleraces.

As for clearances I do think the clearances have decreased somewhat, but not to the point that lower KV oils could be used as a sole reason,

I think there have been three factors for the use of lower KV oils:

1. government mandates for better mpg,

2. better surface finishing resulting in less contact friction,

3. and the fact the lubrcants industry has met the challenge with lubricants that have better AW components, better friction modifiers, better anti-oxidants, better HTHS, and better base oils.


As a reminder, my dad's 1951 Buick straight-eight engine used 20W20 until it had to be rebuilt at 175,000 miles due to a failure unrelated to lubrication. And at that time, the science of oil formulation was only beginning; the quality of base oils was realtively poor, and additive technology was still in its infancy.

The core of my question is related to the fact that the bearing must be able to contract and expand with temperature change. How much tighter can a bearing really get (clearances) and still be able to survive a Montana winter. It seems that tighter clearances will result in damaged bearings upon cool down.

Or, am I wrong. Are there alloys or bearing designs that can compensate for temperature effects on clearances.

Similarly, even if a bearing can be designed to compensate for temperature, at some point internal fluid friction would have to get fairly large. I am guessing that we still have wiggle room from a fluid friction point of view.