FredR

If different bearing sizes are required to ensure reliable operation I find it somewhat outrageous that you cannot purchase the sizes your engine needs given it is possible to measure such. I also find it rather strange that bearing manufacturers can fabricate shells in each of three different sizes but Porsche could not grind the crank to suit a specific shell size. As with most things there will doubtless be a reason for this but I sure do not understand this.

My grey matter tells me better to have too much clearance rather than too little if at all in doubt. Are saying that if a blue size smallest] journal is fitted with a red size shell bearing [largest ID] the motor will fail even with a 20W50 lube?

ptuomov

Whether more than stock clearance is needed depends on whether the stress comes from higher than stock gas loads (compression) or higher than stock rpms (tension). In a turbo car that doesn't go much past 6000 rpm, a relatively small clearance will live very happily. In a high-rpm normally aspirated car, a relatively large clearance is better. That's my opinion anyway.

I think the historical origin of "tolerance groups" was that they'd make a bunch of parts which would all be slightly different size within the manufacturing spec and then sort them on size within the spec into tolerance groups.

GregBBRD

The different size bearings gave the crank manufacturer a little more lead way when grinding the crankshaft and gave Porsche the ability to "dial in" the exact correct bearing clearance, without a bunch of measuring on the assembly line. Match red with red, yellow with yellow, or blue with blue....idiot proof.



I'm not sure what other manufacturers do to deal with tolerances. I know the Clevite bearings I use on my custom crankshafts come in standard, +.001', and -.001". Mixing and matching these bearings allows me about .0005" of adjustment range, in theory. In reality, the amount one can adjust is closer to .00025, by measuring the different shells and picking out the ones that are slightly different than what is written on the box.

Red bearing on a blue crankshaft? The math tells you that if you have a blue crank and you put a red bearing on it, you then have .0016" additional clearance, over what you would have with the blue bearings.

You might want to switch to a straight 60 weight oil, if you do this.

This whole bearing thing is amazingly simple....if you don't have the tools required to measure this stuff accurately, either buy the tools (and learn how to use them) or hire someone to do this for you. No one should be assembling an engine without knowing the exact bearing clearance!

I once had a friend that put an entire engine together without a torque wrench....said he was so good he could tell what torque things were at, by feel. Amazingly enough, it ran....for almost an entire minute!

FredR

Greg,

Much appreciate you sharing your insights in this thread- fascinating reading- you have filled some of the gaps in my knowledge base regarding this subject matter. To be clear, the "bee in my bonnet" is that one seemingly cannot purchase the appropriate distribution of sizes one needs having determined the dimensional status of the requisite crank. It is one thing blue printing a motor but quite another if the build is within tolerance range and the motor can self destruct. My take on what you have written is that if the bearing is one size [colour code] adrift then the motor will be OK but less than optimal. Two sizes adrit it will self destruct.

This should be no surprise as one presumes that Porsche traditionally build "closer to the wall" than mass producers such as Ford, Chevrolet etc. Modern production techniques presumably have allowed the mass producers to catch up in this regard and build motors with much tighter tolerances. I view engine building as quite a fine art and most certainly something not to be taken lightly.

Strosek Ultra

Here is a summary of technical specifications including bearing clearances for the 928.
Åke
http://v12.dyndns.org/Porsche/928/Te...ch%20Specs.pdf

The Forgotten On

That is why I started this whole thread in the first place. The WSM ( at least my copy ) only states how to measure the bearings using plastigauge.

I wanted something much more precise, which is why I invested in a set of Japanese caliper mics and a Swiss vernier inside micrometer, all accurate to within .001mm. I want to build it once, and build it right and never have to do it again, well, within reason anyway .

Plus having an easy to find thread won't hurt for people to reference in the future

Thank you for all of this info Greg, it has evaded me for a long time.

Strosek Ultra

If you need to acquire measuring tools I got this offer for a complete set of micrometers from Goodson this morning.
Åke
https://mailchi.mp/goodson/goodson-t...s?e=56c6dbcfce