Need some advice on S52 rod clearances

[jph]

So I'll try to make this as brief as I can, but there's a lot of info so bear with me. I am in the process of rebuilding an S52 for boost and had VAC perform all my machine work on the block, head, and crank. I have mic'd everything out and have come up with very loose rod bearing clearances on TWO different sets of bearings. Here is a list of the parts being used:

-S52 crank (magnafluxed, polished, and balanced by VAC)
-Molnar rods (brand new)
-1st set of bearings were VAC coated
-2nd set of bearings were King coated tri-metal

Here's the story about how I got here…

I measured each crank journal with a micrometer and got readings from 1.7708-1.7709" (spec is 1.7707-1.7713"). I installed the VAC bearings into the rods and torqued the caps per Molnar specs and using a dial bore gauge found clearances ranging from .0025-.00275". At this point I assumed the bearings themselves were out of tolerance so I removed the bearings from the rods and re-torqued the caps to take measurements of the rod's big end bore without bearings installed. All six were 1.8901" (spec is 1.8898-1.8904"). What I measured was dead in the middle of acceptable big end diameter.

So at this point I have been able to rule out the crank and the rods as the issue since both measured in spec according to BMW TIS. I then looked at the bearings as the culprit and began researching alternative manufacturers. Using the bearing shell thickness from King's website and my previously taken measurements of the crank and rods I came to the conclusion that these new bearings would put me right where I wanted to be for oil clearance.

Fast forward a few weeks and the bearings arrive. Today I got them in the rods and torqued the caps down to spec and took readings again with a dial bore gauge and got clearances ranging from .00275-.0035". This time they were even BIGGER than the VAC bearings I originally had measured.

Factory rod bearing clearance is .0008-.0022". I'm at a loss here and quite frankly I am scared to run bearing clearances in excess of .0025", and especially scared to run clearances with such a large gap in measurements across different rods. Any suggestions?

[varg]

How did you zero your bore gauge?

[jph]

How did you zero your bore gauge?

Set the micrometer to the journal measurement and zeroed the gauge to the micrometer. I have been through this bearing measuring process a number of times at this point and everything is done in a 72 degree apartment and triple checked or more for each step.

[varg]

Set the micrometer to the journal measurement and zeroed the gauge to the micrometer. I have been through this bearing measuring process a number of times at this point and everything is done in a 72 degree apartment and triple checked or more for each step.

Well, you've got me then. Either a tool or the bearings are wrong, and you already knew that.

[AndrewBird]

Do you have a calibration ring for your bore mic? Usually they come with a precision ground ring to set your zero with. Setting it off a micrometer might work, but you could get some variation.

Also, what type of bore gauge? I've found that this style of gauge is a lot more accurate to use than a dial gauge that is a little more subjective in how it sits in the bore.

[jph]

Do you have a calibration ring for your bore mic? Usually they come with a precision ground ring to set your zero with. Setting it off a micrometer might work, but you could get some variation.

Also, what type of bore gauge? I've found that this style of gauge is a lot more accurate to use than a dial gauge that is a little more subjective in how it sits in the bore.

Thanks for the feedback. The dial bore gauge didn't come with a zeroing ring. Don't think it would be much help in this situation since I'm basically measuring the difference from one value (crank's rod journal) to another (rod's big end with bearings installed and caps torqued to spec). That difference is my oil clearance. I zero the dial bore gauge to the micrometer set up to the exact measurement of the crank's rod journal. I then transfer the "zero'd" gauge to the rod and measure the difference.

As far as the alternate tool you mentioned I'm not sure it would be right for this application since the bearings are manufactured with a small amount of eccentricity making them their tightest at the 12 and 6 positions and opening up a very small amount as you get closer to the parting line where the cap meets the rod.

EDIT: This is the exact bore gauge I'm using: http://images.nationaltoolwarehouse....72-646-400.jpg

[digger]

i get a stock bearing from KS or whoever and measure it, double check with plasti gauge?

[jph]

i get a stock bearing from KS or whoever and measure it

I have been tossing that idea around too. I can get a complete set of KS rod bearings for about $55 shipped from ECS tuning. I also found that ACL makes a .025mm (.001") oversized bearing which would bring my clearances from where they are now to pretty much right where I want them to be (~.0018"). The ACL's are about $100 for the set.

At this point I'm already $300+ into rod bearings that are out of spec for me and obviously non-returnable. If my crank was on the higher side of "standard" these sets would likely be fine.

[jeffnhiscars]

Since VAC did all your machine work and sold you at least one set of bearings, what do they have to say about this ?

[jph]

Since VAC did all your machine work and sold you at least one set of bearings, what do they have to say about this ?

Hi Jeff! Since VAC didn't check oil clearances as this was not part of my work request I cannot rightfully ask them to remedy the issue. I asked them to measure the parts I supplied (rods and crank) for sizing and i ordered bearings accordingly. The crank specs out to the lower end of STD and the rod big ends measure out to dead in the middle of spec. So when VAC tells me everything is STD that is in fact correct based on my findings. I'm at the point now where I'm leaning more toward ordering the .001" oversize ACL bearings and giving it another go. I have torqued and removed the rod bolts now a total of three times plus what I am assuming was one time when VAC measured. I'm looking into now adding a bolt stretch gauge to my growing collection of engine assembly tools. I don't know whether that's a good thing or not

[jeffnhiscars]

You would think with the hard parts bang on spec that the clearances would fall in line and if you are using tty's I would think all that torquing will cause a problem no ?

[jph]

You would think with the hard parts bang on spec that the clearances would fall in line and if you are using tty's I would think all that torquing wI'll one a problem no ?

The bolts were supplied with the rods and they are manufactured by ARP. They are not TTY like the factory rod bolts. I actually contacted Tom Molnar the rod manufacturer to buy a complete new set and he talked me out of it saying the bolts would be fine for many torque cycles as long as they were torqued using the specified lubricant and 25 ft/lbs and then 55 degrees. A bolt stretch gauge is relatively inexpensive and truly is the right way to ensure proper bolt loading and in this case I get the benefit of monitoring bolt health as I see this process leading me to go through X number of torque cycles to find a bearing that provides the oil clearance I need.

EDIT: Looks like the ARP stretch gauge will set me back another $175 or so. Might have to put this on hold for now. The cheaper brands will less resolution are about $60.

[AndrewBird]

I would plastiguage them and see with you come up with. I having a feeling something is wrong with your measurements, either with your method or the gauges themselves.

[TobyB]

Yeah, something's not adding up.

Set the micrometer to the journal measurement and zeroed the gauge to the micrometer

seems to cover it all, though, unless something really really weird is going on with your tools.

I too would plastigauge, then try the larger shells. Have you tried measuring a shell
clamped inside a rod? It's hard to do without damaging the shell, but it might tell you something...

Keep us updated.

t

[jph]

Yeah, something's not adding up.

seems to cover it all, though, unless something really really weird is going on with your tools.

I too would plastigauge, then try the larger shells. Have you tried measuring a shell
clamped inside a rod? It's hard to do without damaging the shell, but it might tell you something...

Keep us updated.

t

Not sure what you mean by a shell clamped inside a rod? If you mean installed in the rods and the caps torqued to spec and then the inner bore with bearings installed measured then that's what I have been doing all along.

I ordered a set of King .001" oversize rod bearings to match the set I have here that are standard size. I am going to mix and match the STD and .001" bearings to get to the exact clearance I want. Each bearing shell is .0005" thicker so I should be able to get right down to where I want +/- .0002". My goal is to be at .0018-.0020".

EDIT: I have to assume I am getting loose clearances because the crank journals have been polished down to the extreme low end of the STD spectrum. My smallest rod journal is 1.7707" and low side of spec is 1.7707". Largest journal on my crank is 1.7708".