Checking Connecting Rod Bearing Clearances With Plastigage

[Gordo2]

I used Plastigage to check the rod bearing clearances in preparation for reassembly of my 3.2L engine.

Background

For starters, I'm reasonably confident that my crankshaft is within spec. I have receipts from the previous owner who had the crank magna-fluxed and micro polished when he rebuilt the engine ~ 30k miles ago. The bearings were standard (stamped STD) but the receipts/documentation didn't include any journal measurements. As such, I have purchased some micrometers to confirm the journal measurements.

Meanwhile, I decided to use Plastigage to confirm that my bearing clearances are within spec. I toyed with the idea of buying a cheap dial bore gauge (can be found for ~ $50) to measure ID of the rods & mains, but decided against it. I don't like to buy limited use tools and also prefer to buy good quality tools when I find they are essential for the job (a good bore gauge is $$$).

Of note, I also had my rods reconditioned prior to checking clearances.

Rod Bearing Clearance Check Using Plastigage

I purchased 2 strips of Sealed Power SPG-1 (green) Plastigage at a local Napa store (~$3.00 ea). Each package contains one 12" length the Plastigage green thread. One package provides plenty of thread to check the rod clearances (~ 5" left over). I purchased two so I had enough to check the crankshaft main journals and also because its cheap and I thought I might re-check a few times if I found some odd clearance readings.



I initially attempted to conduct the clearance check with the crankshaft laying flat on my workbench, but quickly decided to re-mount the crank on the flywheel. It's nearly impossible to put the rod together and torque the rod bolts with the crankshaft laying on the workbench.

The Plastigage package has metric scales on one side and a inch/standard scale on the other side. The scale is repeats itself across the length of the package. After removing the Plastigage thread from the package, I cut one of the metric scales off to use as the visual reference.



I used a pair of scissors to cut a small strip of the Plastigage to fit across the journals.

Since the crankshaft was upright, I applied a bit of grease to the connecting rod journal to hold the Plastigage in place as I connected and torqued the rod bolts.

Note on using grease to hold the Plastigage in place:
There's loads of conflicting info regarding using grease for this purpose - few sites recommend it, many don't. I used the grease since I couldn't figure out a better way hold the Plastigage in place with the crankshaft upright. Here's the thread that convinced me that the grease would not affect the measurements: Genuine Plastigauge - How It Works

I used a very small dab of white lithium grease to hold the Plastigage thread in place - it's what I had on hand and I figured the lithium grease would clean up easily:



I assembled the rods on the crankshaft with Plastigage in place, and checked each journal one at a time.

Note on Smearing the Plastigage:
I was very concerned about the rod moving while I assembled and torqued the rod bolts; I read that this would smear the Plastigage. It was nearly impossible to keep the rods steady as you installed the caps and finger tightened the rod bolts - no worries, you would really have to press the rod against the Plastigage to affect it. I also found that I could keep the rods pretty stationary once I started to torque the rod bolts.



Although I have ARP rod bolts, I decided to torque the bolts to ARP's recommended spec of 40 ft-lbs as opposed to tightening to specified stretch. I have a stretch gauge that I will use for final installation of the rods. Meanwhile, I figured the torque method would be easier, faster and accurate enough for the purpose of confirming rod bearing clearances.

I was happy to find that each bearing clearance came out at the same width - wider than the 0.051mm but smaller than the 0.038mm reference. I estimate this to be ~ 0.045mm, well within the rod bearing clearance spec (0.030mm to 0.088mm).



While I was at it, I figured I would go ahead and try my Summit Racing rod stretch gauge to see what kind of stretch 40 ft-lbs produced.



ARP instructions call for stretch of 0.0100" to 0.0105". If I'm reading my gauge correctly, this one came out to 0.0040" (the gauge was zero'd before I torqued to 40 ft-lbs).

The Summit Racing tool seems OK for the price, but probably more difficult to use than the ARP gauge. Topic for another day and another thread.


Happy holidays folks - Gordo


(My other obsession - homebrewing. A cool Kolsch to wrap up the clearance check and good findings)

[AlfonsoR]

Of course you know that the torque method assumes you have the same exact identical friction factor for all the threads and nut faces. Which is to say a bad assumption. So why not take the time to measure the way you are going to put it together? Otherwise it seems a waste of time.

Also, you didn't mention but you should make sure that the plasti gage is being placed in a consistent location in relation to each cap and it normally should be placed (measured) at the center of the cap.

[Gordo2]

Originally Posted by AlfonsoR

...why not take the time to measure the way you are going to put it together? Otherwise it seems a waste of time...

Seriously - are you trying to make this more difficult than required?

I fail to see how torquing to stretch length (which would probably occur somewhere around 50 ft-lbs, vice the 40 ft-lbs that I torqued to) vs. ARP's recommended torque is going to have an appreciable/noticeable impact on the Plastigauge bearing clearance reading?

After all, Plastigage is an "indicator" - not a precise measurement (hence my eyeball, thumb in the air reading of 0.045mm - comfortably in the ballpark of the specs...).

By all means - if you want some practice at using a stretch gauge, I would recommend going this route, otherwise I believe stretching the rod bolts for the purpose of a Plastigage reading is simply a waste of time.

Gordo

[swade]

Nice write-up Gordo

[patkeefe]

Nice job.
The Summit tool is unwieldy, at best. It is really good for cam timing, however.
As soon as you get the slightest stretch in the bolt, the caps are as together as they can be, as far as checking the clearances.

[Gordo2]

Originally Posted by patkeefe

The Summit tool is unwieldy, at best.

I think you hit the nail on the head: Unwieldy - "that's a fun word to say..." - normally not what you're looking for in most tools though.

Good to hear I may find the Summit Racing stretch gauge more accommodating for other jobs - as I can already tell it's going to be less than optimal for it's intended purpose.

Also, I agree with your perspective regarding bearing clearances and their relationship to rod and cap clamping force - at some point of torque or initial stretch, I think the bearing clearances will remain pretty consistent.

--------------------------------

Back to the project at hand - I went ahead and checked the main bearing clearances with Plastigage this afternoon.

First, I placed the bearing halves in the case saddles and laid in the crankshaft:


Note - though the picture doesn't show it, I placed the # 8 bearing on the crankshaft before I proceeded.

Although I didn't need to use grease to keep the plastigage on top of the crankshaft to check the main bearing clearances - I went ahead and did it simply to maintain process consistency:



It felt good to see the case halves coming back together - even though it was only temporary:



I torqued each through bolt & nut to spec.

Note on using through-bolts or through-bolts + case perimeter bolts/nuts for main bearing clearance checks

I researched but didn't find any definitive guidance. As such, I used my own judgment and decided that the 13 case through-bolts/nuts + the one 15mm nut in the chain housing recess, would sufficiently torque & clamp the mains to allow an accurate Plastigage main bearing clearance check. In other words, I fastened the bolts and nuts that are directly adjacent to the crankshaft and didn't fasten any of the case perimeter bolts.

The Outcome

Once everything was torqued to spec, I removed the bolts & nuts and broke the case halves back apart (the case halves were stuck together again and needed to be whacked with a rubber mallet to separate). Once apart, I was happy to see all of the Plastigage that remained stuck to the crankshaft main journals appear to have a consistent width.



Each main bearing Plastigage thread flattened out to a width slightly smaller than the 0.051mm Plastigage reference. I estimate this to be ~ .060mm - which is midrange of the spec (0.030mm to 0.088mm).

Side Note on Molykote 321 dry film lubricant on bearing halves

I re-used my main and rod bearings and applied Molykote 321 to them to reduce friction during break in: Molykote 321 on Main & Rod Bearings

I wiped the bearing faces off with dry Kimwipes to reduce excess/surface coating before I installed the bearings in the rods and case halves. The Kimwipes looked dirty (like they wiped away a layer of graphite) on the first pass, but each consecutive wiping produced less discoloration. I repeatedly wiped the bearing surfaces until there was minimal coating removal. At this point, the bearings retained the coloration of the Molykote and had a very smooth and shiny surface.

I found an unintended benefit of the dry filmed bearings during the Plastigage clearance checks - the Plastigage didn't stick to the bearings at all. I still had to scrub the Plastigage residue off of the crankshaft journals (using Kimwipes that were wet with carb cleaner), but the bearing halves cleaned up with a quick wipe down.




All good.

I feel better about moving ahead with the engine reassembly knowing I've done everything reasonable as a DIY'r to confirm my crankshaft bearings have the appropriate clearances.

Added benefit - this also served as a good, relaxed pre-assembly check, allowing me to confirm that I have all the right tools and parts on hand before I start into the final assembly with the sealant set-up time clock ticking away.

Gordo

[porterdog]

Neat; thanks for the post!

[boyt911sc]

Gordo,

Well written and pictures are great. Were you using ARP M-10 bolts? The ARP stretch method clearly stated to use 50 ft-lbs. torque to get the desired stretch value. Was it a typo (40 ft-lbs.)? Thanks.

Tony

[Jcslocum]

The 9mm rod bolts use 40 ft lbs. the 10mm the 50.

[Gordo2]

Originally Posted by boyt911sc

Gordo,

Well written and pictures are great. Were you using ARP M-10 bolts? The ARP stretch method clearly stated to use 50 ft-lbs. torque to get the desired stretch value. Was it a typo (40 ft-lbs.)? Thanks.

Tony

Jon is correct - I used ARP 204-6005 9mm rod bolts, which are correct for the 3.2L engine.

SC's use the ARP 204-6001 10mm rod bolts, which have a 50 ft-lb torque spec.

Instructions for ARP 204-6005 rod bolts from ARP's website:

[brighton911]

Thanks for a great writeup Gordo, perfect timing for me as I will be there next week. I do have a question though. You stated the ARP bolts stretch standard was:

"ARP instructions call for stretch of 0.0100mm to 0.0105. If I'm reading my gauge correctly, this one came out to 0.0040 (the gauge was zero'd before I torqued to 40 ft-lbs)."

I'm guessing you meant to say .0100 to .0105 thou. Your gauge showed 4 thou stretch which seems low for 40 lbs of torque. What torque number gave you the desired stretch?

Dave

[Gordo2]

Originally Posted by brighton911

...I do have a question though. You stated the ARP bolts stretch standard was: "ARP instructions call for stretch of 0.0100mm to 0.0105..."

I'm guessing you meant to say .0100 to .0105 thou. Your gauge showed 4 thou stretch which seems low for 40 lbs of torque. What torque number gave you the desired stretch?
Dave

Dave,

You are correct; the ARP measurements are in inches, not mm as I previously posted (I edited & corrected the above post). Thanks for the point out - had to laugh, I recall loosing points on tests when I made that kind of mistake in college.

I agree, 0.004" doesn't seem like much stretch for 40 ft-lbs of torque. I haven't installed the rods yet. When I do, I will install the bolts to ARP's spec for stretch length and follow up with a torque reading to see what it takes to produce the recommended stretch. Will update this post when I install.

For ref, here's a related link to some more measurements & observations I recently made on the ARP rod bolts:

Measuring ARP Rod Bolt Stretch - Micrometer vs. Stretch Gauge

Thanks,

Gordo

[afterburn 549]

With Plasti you are making a LOT of assumptions......Did you measure the load side? (of the crank?)
It could be egg shaped and you would never ever know........just sayin.......with out measuring a couple spots 90 deg apart.

[wprater]

whoa. this is tough. I've smeared 3 of the 5 I've tried so far. and getting the rod cap off when you loosen is a pain too. had to tap with rubber hammer.

not sure I'm smearing them on way on or off.

[JonT]

I recently went through this. It took a few tries but position the rod on the crank so you can access the rod bolts with the socket and if you are using one, the stretch gauge with minimal rotation of the rod on the crank--this is your starting point. You may have to make a quarter turn to be able to get the stretch guage on each side properly. Turn the rod slowly around the crank when repositioning. I used a little assembly lube to hold the plastiguage on the crank as well. To remove the cap I had a little square hammer to remove the cap with light taps. You'll get a better feel the more times you do it. Patience helps!

[gtc]

I needed a helper for this job, to hold things steady and keep the rod from moving.