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Rod bearing clearance changing after fitment to crank.

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Can measured rod bearing clearances change after the rod has been fitted to the crank, then removed and re-measured?

I was told this by a friend helping me with my engine build. I didnt believe him and checked for myself, and i consitently measured about 0.0005 increase in clearance, after the rods were fitted to the crank, then removed for measurement. On all 4 rods that i have.

My friends explanation was that the bearings need to be seated correctly into the rod, and simply torqing the rod (and bearing) up on a bench without the crank doesnt properly seat them, even though the rod does have some crush on the bearing when torqued up.

I actually took my crank to him to have him polish it down to get me my desired clearance, but he questioned me, and found that i hadnt done this when measuring, and told me to do it before touching the crank. He was correct and the measured clearances changed by half a thou.

I dont believe Andre mentions anything about this in his engine building courses and bearing clearance webinars. (Please correct me if I am wrong here, it has been a while since i watched them) If this is so, then inexperienced builders following the coarse, like myself, could be getting varied and inconsistent results when building an engine.

I checked the rod bearing clearance using the micrometer and dial indicator bore gauge method Andre explains in the engine building coarse and also the bearing webinars.

First, I cleaned everything, fitted bearings to the rods, and torqued them up to spec without the crank. Then used micrometer and bore gauge to measure clearance. All clearances were 0.0004 to 0.0006. This is using brand new crank, which measures in tolerance. Brand new rods. And brand new ACL bearings. I then repeated the test using brand new ACL +1 bearings and measured 0.0014 to 0.0016. (At approx 18°c) Then i went to my friend to polish the crank. He said, put the rods on the crank, with lots of engine oil, spin them around, have a feel, then remove them, and re-do my testing. I did not believe him, and thought it was an old wives tale sort of thing.

But, curiosity got to me, and I did as he said, and afterwards my measurements had all incresed to 0.0018 to 0.0019. To be scientific, I re-measured the clearance before putting the rods on the crank on the same day that i put the rods on the crank and pulled them off to re-measure. My measurements before fitting to crank were consistent with my previos measurements at 0.0015. I did this because in earlier aspects of the build (like piston to bore measuring and ring end gapping) i was going around in circles with changes in temperature affecting my measurements. So i intentionally did it all on the same day, with a temp of 18°c, to take the temp variations out of the equation. (That is about as warm as it gets here this time of year.)

TBH, I find that VERY difficult to credit - but I have been wrong before and will again, especially if I misunderstand what you're saying.

If I read it correctly, you torqued the big end with the bearings in place to check their ID or did you just run them down until they were tight, and did you hold the rods in hand or use a rod vice/couple of alloy plates in a vice? You them fitted the rods to the crank and torqued them down fully and, when removed and checked, the clearances seemed to have changed?

My first thought was that thermal expansion rates may have caused an apparent variation, but you specifically mention the temperature was the same -was there any chance the rod assemblies or measurement tools could have been in the sun, or a cold drawer, or that you handled them more in one case? In metrology class a couple of degrees variation from holding the slip blocks more than a few seconds would affect measurements, but that was to 0.0001mm or finer, so would have been more noticable.

When you did the measurements, were the differences across the rod or in line with it, or both?

Going to follow this with interest - comments, folks?

I thought it was wrong too, and i did not believe the guy telling me, so i tested it for myself, hoping to prove him wrong. But he was right. I did it with all 4 rods, and each rod had the measurement grow by 0.0004 to 0.0005.

I was very anal about temperature, as temperature changes has caused me a lot of trouble already with taking measurements with checking piston/bore and setting ring gaps. So much so that i dont do any work until afternoon now, as mornings are too cold and cause me to pull my hair out. I used a "Milwaukee" laser temp gauge on the parts, and also a no name HVAC probe to measure ambient air. My workshop is enclosed. And i have even built a tempory dust free assembly room inside my workshop ($59 Bunnings gazebo wrapped in painters plastic and packing tape)

In the intitial measurement, everything was clean. I used a red "Proform" aluminium rod vice to hold the rod while I torqued it, so i could get a nice smooth swing on the wrench. I used a near new "Eastwood" 1/2 digital torque wrench to measure and torque them. I used ARP bolt lube on the ARP 625+ bolts, i put a fresh dab under the head and on the thread everytime i torque the bolts, and just wipe off whatever oozes out. And i torqued them to 55ft pound each time. Rod/Bolt manufacturer call for between 55 to 60 ft pound. I opted for 55ft/lb for all of my testing, but I will be stretch gauging them for the proper assembly.

When the rods were put on the crank, the crank was actually already chucked up in a lathe, about to be polished. This allowed me to torque the rods smoothly on the crank. And it allowed me to spin the rods about a bit by hand to get a feel. The guy who told me this is a bit of an old school type builder, and to him, "feeling" the clearance on the rods is equally important as physically measuring them. You wont believe me when i tell you this, but he was able spin the rods on the crank by hand, and able to pretty accurately tell me the clearance just by feel, like within a few tenths just by how it felt and spun. He then gave me a cheeky grin when i checked them, telling me the "feel" is just as important as the measurement. Like some kind of engine builfing Yoda.

My initial measurements were checked against the measurements i took before putting the rods on the crank. And they were consistent. The measurements after the rod was put on the crank, tourqued, then removed and re-tourqed, then re measured were the ones that had the expanding clearance.

I used a basically brand new set of micrometers and bore guage. The mics are "Measuremax" brand. And the bore gauge is "Fowler" brand. Both semi respectable brands of tools. Ie, not the cheapest, but not the most expensive. I even used a "Moore & Wright" micrometer stand to hold the micrometer while zero-ing out the bore gauge. (Again, being conscious of temperature changes) The mic consistently zeros +/- 0.0001 (ie within a vernier mark) on its setting standard that is included with the mics. Not that mic accuracy matters much with a comparitive measurement like a bore gauge clearence measurement.

Now that setup all sounds pretty respectable, and i dont think there is any faults with my equipment. But I am not a qualified engineer by any means. This is my first engine build. I am a very technical type of guy, but this is all pretty new to me. So there may be fault with my methodology or technique. Hence why I am asking. My friend helping me with the build swears that it is a thing, he is a pretty well versed after hours subaru builder, and his day job is with building industrial compressors and turbines, so he knows his way around engineering.

I would like to hear feedback from anyone willing to test this experiment next time they do a build. It doesnt take long to do, and if it does infact mean the difference of half a thou in clearance measurment, then it is well worth doing. If it doesnt change things, then i would still like to hear feedback, as it means i am doing something wrong.

You do seem to have covered al the things I can think of at this time of the morning, good thread topic.

I'm not too sure how fitting the rod to the crank is going to affect anything in terms of the bearing clearance. The bearings do 'seat' to the rod, however it's the crush provided by the rod and cap on the bearings that achieves this. Remember that there is no metal to metal contact between the crank journal and the bearing shell when the rod is installed - The bearing is protected by a film of oil and hence I can't really see how installing the rod onto the crank is likely to affect anything. I'd guess based on your results that perhaps you'd have seen the same change if you'd simply torqued the cap, removed it, and torqued it again. When you use the proform conrod vise (I use the same item), are you clamping the rod just by the body?

I agree whole heartedly that the feel of engine components is very important and definitely something you develop with experience. This is one reason why I don't personally like to use proper assembly lubricants on the bearing shells as it is so viscous that it affects the feel you get when you spin the crank in the block.

Well. I torqued it properly before even putting it on the crank. I torqued it the same while on the crank. And i torqued it the same after removing from the crank to re-check.

So all torque was the same.

When i use the rod vise, i hold the thrust sides of the big end of the rod. Leaving the removable cap hanging out just past the reach of the rod vice. I torqued it evenly and in a few steps progresing up to the final torque value. I guess because im holding the big end, then perhaps the vice stops it from spreading properly?

It would be interesting for you to do a test next time you assemble an engine. Just for curiosity. It really doesnt take that much extra time.

Interesting finding there Bram. I'll have a look into it on our upcoming builds and see if I can replicate what you've found. To put it simply though, the technique I've used to measure clearances has always been what I've presented in the course. I've never gone through installing the rods onto the crank prior to measuring clearances.

Ok. Thanks. It may well be a problem with my technique.

One thing i will touch on. Your previous comment that the bearing should never touch the crank. Yes this is correct. But that doesnt mean there is no forces acting between the crank and bearing. If this was the case, then why would we even bother to upgrade rods. Hydraulics can transmit immense forces, without ever having any direct metal to metal contact. That is the same as saying "car tyres have air inside them, and they support the car, therefore my car is wieght-less."

Even though the bearing doesnt touch the crank directly. There is an incompressable fluid in there. So when you spin the rod around by hand, the force you exert on the rod, goes to the bearing, to the oil, to the crank. So it could simply be the resistence the oil exerts back onto the bearing, that is enough to seat it.

My measurements only changed by 0.0004 - 0.0005. So each bearing shell would only have to settle 0.0002 to give that difference.

You're correct that the oil film can transmit force into the bearing shell but this is with the rod just torqued onto the crank journal and being spun by hand - There's no real compressive force being applied under this condition, and certainly nothing like the engine actually running. I'd be surprised if the forces involved under these conditions were higher than those involved with the crush applied to the bearing shells by the rod cap. I will however investigate and see what I find.