Carpokes Porsche Forum

I did not record the measurements. I plan to remove the pan and check them again. I am going to shoot for your recommended clearance of 0.0025".

The clearance differences are curious. I am looking at WKD 423 520, 1st edition. Con rod bearing/crankshaft Radial Play 0.027 - 0.069. The upper end of 0.069 is very close to your recommendation. I am going to dig out the big manual and see what it says. I am probably worrying about nothing.

I believe I deferred to (and undoubtedly was more confident in due to MM/chris mentioning similar numbers) ACL's guidelines from their catalog, which are...kinda one-size fits all, but also in agreement with .0025 :')

ACL Race Series performance engine bearings can be assembled
with .00075-.001” per 1” of journal diameter (0.020-0.025mm per
25mm of journal diameter) plus .0005” (0.013mm).

What’s the gold standard method to measure bearing clearance?

ID micrometer or bore gauge for the ID, micrometer for the crank. with practice should be sufficiently consistent.

Obviously the ceiling is the limit for how much those cost, but I wouldn't expect to spend less than a $2-300 on both tools combined. Not unreasonable to shop around for vintage mitu/starrett gear either, but make sure they're in good shape!


edit: defer to MM's much more thorough answer

cp99 wrote: Fri Dec 08, 2023 5:49 am What’s the gold standard method to measure bearing clearance?

Note: This adds some detail to fourOfour's previous post.

This is how we did it back in the shop. This assumes you've already measured the crank to be straight (+/-.0005), and you've confirmed the big end dimension of each rod to be consistent in size/diameter. BTW, each rod (new or used) is to be measured for size, length, and twist, then corrected beforehand. I've corrected many a new high dollar rod.

- Measure each rod bearing thickness using a Stirrett ball anvil micrometer ($300+) to confirm consistency.

- Make sure the crankshaft, rods, and measuring tools are at a consistent or room temperature. Too cold will result in a small reading, too warm will read large. Since measurements are made in .0001 increments, temperature differences in the components being measured are important.

- For measuring rod oil clearance, insert bearings and torque the rod bolts in a proper rod vise ($100+) with the proper lubrication and using the bolt stretch method if that spec is provided. ($100+ for the bolt stretch tool). For Porsche rods, use the old nuts for this process, then new nuts for final engine assembly.

- Reach into your box of Starrett micrometers ($2000.) and select the 2-3" mic.

- Measure each crank journal in several places while minimizing the warmth from your hands being passed to the mic. Document each measurement for file. Lock the mic.

- Set up your Starrett dial bore gauge ($500 or so) zeroed on the journal measurement from your mic.

- Measure the ID of the rod big end with the dial bore gauge and document the readings which show (in .0001" increments) the measured difference between the crank journal and the torqued rod with bearing. This Incremental difference is your oil clearance.

- Correcting the clearance can be done two ways: Either mix and match OS or US bearings, or go back and resize the rod big end to open or close up clearance; all while keeping the big end within spec. We typically addressed the rod to avoid waiting for alternate bearings and the added expense.

Both quality measuring tools and an experienced hand are vital to get reliable and accurate results. Our shop had a "do it right" mentality, whereas many engine shops subscribe to the "do it right NOW" strategy. Yes, it's difficult to do it right in your home workshop.

MM

Thank you Michael. That’s why I need to send you my motor

cp99 wrote: Fri Dec 08, 2023 3:21 pm Thank you Michael. That’s why I need to send you my motor

I've been retired for many years now. I still do a few cylinder heads, but not any engines. I'm here to give a little back to the community for all the Porsche blessings I've received over the years, and to keep my toes dipped in the P-water.

I'll be car chief for a Fabcar 944GTR next year. I'm looking forward to the challenge and to be back at the track. There's several other interesting cars under the tent: 997 Cup (Daytona), another 997 and 996 Cup, 944 Turbo Cup, Mustang GT-R, others.
MM